Star-Delta Starter

Introduction:

Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. Two methods are used for reduction of starting voltage are star delta starting and auto transformer stating.

Working Principal of Star-Delta Starter:

• This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the motor runs normally.

• Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.
• Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW). The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical reduced voltage starters that can be applied.
• The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line.
• There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.

Star-delta Starter Consists following units:

1)     Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).

2)     Time relay (pull-in delayed) 1 No.

3)     Three-pole thermal over current release 1No.

4)      Fuse elements or automatic cut-outs for the main circuit 3 Nos.

5)     Fuse element or automatic cut-out for the control circuit 1No.

Power Circuit of Star Delta Starter:

• The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit.
• The main contactor connects the reference source voltage R, Y, B to the primary terminal of the motor U1, V1, W1.
• In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed initially, and then after a period of time, the star contactor is opened, and then the delta contactor (KM2) is closed. The control of the contactors is by the timer (K1T) built into the starter. The Star and Delta are electrically interlocked and preferably mechanically interlocked as well. In effect, there are four states:

                                                              

• The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequence during the initial run of the motor from standstill. This provides one third of DOL current to the motor, thus reducing the high inrush current inherent with large capacity motors at startup.
• Controlling the interchanging star connection and delta connection of an AC induction motor is achieved by means of a star delta or wye delta control circuit. The control circuit consists of push button switches, auxiliary contacts and a timer.

Control Circuit of Star-Delta Starter (Open Transition):

                         

• The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT) circuit.
• When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.
• When Star Auxiliary Contactor (1)( which is placed on Main Contactor coil circuit )became NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s  Main and Auxiliary Contactor Change its Position from NO To NC. This sequence happens in a friction of time.
• After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch.
• When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact KT (3) become NC to NO.
• Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time  Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and  Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.
• A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta contactor coil is active.
• The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch break the control circuit and the motor when necessary. The thermal overload contact is a protective device which automatically opens the STOP Control circuit in case when motor overload current is detected by the thermal overload relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by the thermal overload relay.
• At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.

What is Open or Closed Transition Starting

(1)   Open Transition Starters.

• Discuss mention above is called open transition switching because there is an open state between the star state and the delta state.
• In open transition the power is disconnected from the motor while the winding are reconfigured via external switching.
• When a motor is driven by the supply, either at full speed or at part speed, there is a rotating magnetic field in the stator. This field is rotating at line frequency. The flux from the stator field induces a current in the rotor and this in turn results in a rotor magnetic field.
• When the motor is disconnected from the supply (open transition) there is a spinning rotor within the stator and the rotor has a magnetic field. Due to the low impedance of the rotor circuit, the time constant is quite long and the action of the spinning rotor field within the stator is that of a generator which generates voltage at a frequency determined by the speed of the rotor. When the motor is reconnected to the supply, it is reclosing onto an unsynchronized generator and this result in a very high current and torque transient. The magnitude of the transient is dependent on the phase relationship between the generated voltage and the line voltage at the point of closure can be much higher than DOL current and torque and can result in electrical and mechanical damage.
• Open transition starting is the easiest to implement in terms or cost and circuitry and if the timing of the changeover is good, this method can work well. In practice though it is difficult to set the necessary timing to operate correctly and disconnection/reconnection of the supply can cause significant voltage/current transients.
• In Open transition there are Four states:

1. OFF State: All Contactors are open.
2. Star State: The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The motor is connected in star and will produce one third of DOL torque at one third of DOL current.
3. Open State: This type of operation is called open transition switching because there is an open state between the star state and the delta state. The Main contractor is closed and the Delta and Star contactors are open. There is voltage on one end of the motor windings, but the other end is open so no current can flow. The motor has a spinning rotor and behaves like a generator.
4. Delta State: The Main and the Delta contactors are closed. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available

(2)   Closed Transition Star/Delta Starter.

• There is a technique to reduce the magnitude of the switching transients. This requires the use of a fourth contactor and a set of three resistors. The resistors must be sized such that considerable current is able to flow in the motor windings while they are in circuit.
• The auxiliary contactor and resistors are connected across the delta contactor. In operation, just before the star contactor opens, the auxiliary contactor closes resulting in current flow via the resistors into the star connection. Once the star contactor opens, current is able to flow round through the motor windings to the supply via the resistors. These resistors are then shorted by the delta contactor. If the resistance of the resistors is too high, they will not swamp the voltage generated by the motor and will serve no purpose.
• In closed transition the power is maintained to the motor at all time. This is achieved by introducing resistors to take up the current flow during the winding changeover. A fourth contractor is required to place the resistor in circuit before opening the star contactor and then removing the resistors once the delta contactor is closed. These resistors need to be sized to carry the motor current. In addition to requiring more switching devices, the control circuit is more complicated due to the need to carry out resistor switching
• In Close transition there are Four states:

1. OFF State. All Contactors are open
2. Star State. The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The motor is connected in star and will produce one third of DOL torque at one third of DOL current.
3. Star Transition State. The motor is connected in star and the resistors are connected across the delta contactor via the aux [KM4] contactor.
4. Closed Transition State. The Main [KM3] contactor is closed and the Delta [KM2] and Star [KM1] contactors are open. Current flows through the motor windings and the transition resistors via KM4.
5. Delta State. The Main and the Delta contactors are closed. The transition resistors are shorted out. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available.

Effect of Transient in Starter (Open Transient starter)

• It is Important the pause between star contactor switch off and Delta contactor switch is on correct. This is because Star contactor must be reliably disconnected before Delta contactor is activated. It is also important that the switch over pause is not too long.
• For 415v Star Connection voltage is effectively reduced to 58% or 240v. The equivalent of 33% that is obtained with Direct Online (DOL) starting.
• If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected in Delta mode.
• When connected to Delta configuration the phase voltage increases by a ratio of V3 or 173%. The phase currents increase by the same ratio. The line current increases three times its value in star connection.
• During transition period of switchover the motor must be free running with little deceleration. While this is happening “Coasting” it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to or subtract from the applied line voltage. This is known as transient current. Only lasting a few milliseconds it causes voltage surges and spikes. Known as a changeover transient.

 Size of each part of Star-Delta starter

(1)   Size of Over Load Relay:

• For a star-delta starter there is a possibility to place the overload protection in two positions, in the line or in the windings.
• Overload Relay in Line:
• In the line is the same as just putting the overload before the motor as with a DOL starter.
• The rating of Overload (In Line) = FLC of Motor.
• Disadvantage: If the overload is set to FLC, then it is not protecting the motor while it is in delta (setting is x1.732 too high).
• Overload Relay in Winding:
• In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The overload then always measures the current inside the windings.
• The setting of Overload Relay (In Winding) =0.58 X FLC (line current).
• Disadvantage: We must use separate short circuit and overload protections.

(2)   Size of Main and Delta Contractor:

• There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor.
• Size of Main Contactor= IFL x 0.58

(3)   Size of Star Contractor:

• The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is 1/ √3= (58%) of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.
• Size of Star Contactor= IFL x 0.33

Motor Starting Characteristics of Star-Delta Starter:

• Available starting current: 33% Full Load Current.
• Peak starting current: 1.3 to 2.6 Full Load Current.
• Peak starting torque: 33% Full Load Torque.

Advantages of Star-Delta starter:

• The operation of the star-delta method is simple and rugged
• It is relatively cheap compared to other reduced voltage methods.
• Good Torque/Current Performance.
• It draws 2 times starting current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter:

• Low Starting Torque (Torque = (Square of Voltage) is also reduce).
• Break In Supply – Possible Transients
• Six Terminal Motor Required (Delta Connected).
• It requires 2 set of cables from starter to motor.
• It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses are produced while changing from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs.
• In this method of starting initially motor is connected in star and then after change over the motor is connected in delta. The delta of motor is formed in starter and not on motor terminals.
• High transmission and current peaks: When starting up pumps and fans for example, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approx. 80-85 % of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks. In some cases the current peak can reach a value that is even bigger than for a D.O.L start.
• Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
• Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The initial connection should be in the star pattern that results in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of the current at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque .
• The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed which make transient Sparks.

Features of star-delta starting

• For low- to high-power three-phase motors.
• Reduced starting current
• Six connection cables
• Reduced starting torque
• Current peak on changeover from star to delta
• Mechanical load on changeover from star to delta

 Application of Star-Delta Starter:

•  The star-delta method is usually only applied to low to medium voltage and light starting Torque motors.
• The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start. This starting method only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.
  

Direct On Line Starter

Introduction:

• Different starting methods are employed for starting induction motors because Induction Motor draws more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters.
• The simplest form of motor starter for the induction motor is the Direct On Line starter. The DOL starter consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.
• Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating.

Principle of DOL:

•  To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.
• As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load.
• The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.
• Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.
• The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the acceleration torque and the load inertia.
• DOL starting have a maximum start current and maximum start torque. This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users of the supply line, always experience a voltage drop when starting a motor. But if this motor is not a high power one it does not affect much.

Parts of DOL Starters:

(1)   Contactors & Coil.

• Magnetic contactors are electromagnetically operated switches that provide a safe and convenient means for connecting and interrupting branch circuits.
• Magnetic motor controllers use electromagnetic energy for closing switches. The electromagnet consists of a coil of wire placed on an iron core. When a current flow through the coil, the iron of the magnet becomes magnetized, attracting an iron bar called the armature. An interruption of the current flow through the coil of wire causes the armature to drop out due to the presence of an air gap in the magnetic circuit.

• Line-voltage magnetic motor starters are electromechanical devices that provide a safe, convenient, and economical means of starting and stopping motors, and have the advantage of being controlled remotely. The great bulk of motor controllers sold are of this type.
• Contactors are mainly used to control machinery which uses electric motors. It consists of a coil which connects to a voltage source. Very often for Single phase Motors, 230V coils are used and for three phase motors, 415V coils are used. The contactor has three main NO contacts and lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the control circuit. A contact is conducting metal parts which completes or interrupt an electrical circuit.
• NO-normally open
• NC-normally closed

(2)   Over Load Relay (Overload protection).

• Overload protection for an electric motor is necessary to prevent burnout and to ensure maximum operating life.
• Under any condition of overload, a motor draws excessive current that causes overheating. Since motor winding insulation deteriorates due to overheating, there are established limits on motor operating temperatures to protect a motor from overheating. Overload relays are employed on a motor control to limit the amount of current drawn.
• The overload relay does not provide short circuit protection. This is the function of over current protective equipment like fuses and circuit breakers, generally located in the disconnecting switch enclosure.
• The ideal and easiest way for overload protection for a motor is an element with current-sensing properties very similar to the heating curve of the motor which would act to open the motor circuit when full-load current is exceeded. The operation of the protective device should be such that the motor is allowed to carry harmless over-loads but is quickly removed from the line when an overload has persisted too long.
• Normally fuses are not designed to provide overload protection. Fuse is protecting against short circuits (over current protection). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishing between this temporary and harmless inrush current and a damaging overload. Selection of Fuse is depend on motor full-load current, would “blow” every time the motor is started. On the other hand, if a fuse were chosen large enough to pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occur later.
• The overload relay is the heart of motor protection. It has inverse-trip-time characteristics, permitting it to hold in during the accelerating period (when inrush current is drawn), yet providing protection on small overloads above the full-load current when the motor is running. Overload relays are renewable and can withstand repeated trip and reset cycles without need of replacement. Overload relays cannot, however, take the place of over current protection equipment.

• The overload relay consists of a current-sensing unit connected in the line to the motor, plus a mechanism, actuated by the sensing unit, which serves, directly or indirectly, to break the circuit.
• Overload relays can be classified as being thermal, magnetic, or electronic.

1. Thermal Relay: As the name implies, thermal overload relays rely on the rising temperatures caused by the overload current to trip the overload mechanism. Thermal overload relays can be further subdivided into two types: melting alloy and bimetallic.
2. Magnetic Relay: Magnetic overload relays react only to current excesses and are not affected by temperature.
3. Electronic Relay: Electronic or solid-state overload relays, provide the combination of high-speed trip, adjustability, and ease of installation. They can be ideal in many precise applications.

Wiring of DOL Starter:

(1)   Main Contact:

• Contactor is connecting among Supply Voltage, Relay Coil and Thermal Overload Relay.
• L1 of Contactor Connect (NO) to R Phase through MCCB
• L2 of Contactor Connect (NO) to Y Phase through MCCB
• L3 of Contactor Connect (NO) to B Phase through MCCB.
• NO Contact (-||-):
• (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)
• Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is connected to Common wire of Start/Stop Button.
• NC Contact (-|/|-):
• (95-96) is a normally closed NC contact (opens when the thermal overloads trip if associated with the overload block)

(2)   Relay Coil Connection:

• A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermal over Load Relay’s NC Connection (95).

(3)   Thermal Overload Relay Connection:

• T1,T2,T3 are connect to Thermal Overload Relay
• Overload Relay is Connecting between Main Contactor and Motor
• NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and Common Connection of Start/Stop Button.

Wiring Diagram of DOL Starter:

Working of DOL Starter:

• The main heart of DOL starter is Relay Coil. Normally it gets one phase constant from incoming supply Voltage (A1).when Coil gets second Phase relay coil energizes and Magnet of Contactor produce electromagnetic field and due to this Plunger of Contactor will move and Main Contactor of starter will closed and Auxiliary will change its position NO become NC and NC become (shown Red Line in Diagram)  .
• Pushing Start Button:
• When We Push the start Button Relay Coil will get second phase from Supply Phase-Main contactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2).Now Coil energizes and Magnetic field produce by Magnet and Plunger of Contactor move. Main Contactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) from NO to NC .
• Release Start Button:
• Relay coil gets supply even though we release Start button. When We release Start Push Button Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) – Auxiliary contactor (54)-Stop Button-96-95-Relay coil (shown Red / Blue Lines in Diagram).
• In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95.
• Pushing Stop Button:
• When we push Stop Button Control circuit of Starter will be break at stop button and Supply of Relay coil is broken, Plunger moves and close contact of Main Contactor becomes Open, Supply of Motor is disconnected.

 

 Motor Starting Characteristics on DOL Starter:

• Available starting current:    100%.
• Peak starting current:           6 to 8 Full Load Current.
• Peak starting torque:            100%

Advantages of DOL Starter:

1. Most Economical and Cheapest Starter
2. Simple to establish, operate and maintain
3. Simple Control Circuitry
4. Easy to understand and trouble‐shoot.
5. It provides 100% torque at the time of starting.
6. Only one set of cable is required from starter to motor.
7. Motor is connected in delta at motor terminals.

Disadvantages of DOL Starter:

1.  It does not reduce the starting current of the motor.
2. High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).
3. Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.
4. Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current affecting other customers connected to the same lines and therefore not suitable for higher size squirrel cage motors
5. High starting Torque: Unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipments, etc. leading to premature failure and plant downtimes.

Features of DOL starting

• For low- and medium-power three-phase motors
• Three connection lines (circuit layout: star or delta)
• High starting torque
• Very high mechanical load
• High current peaks
• Voltage dips
• Simple switching devices

DOL is Suitable for:

• A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10 kW.
• DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts.

DOL is not suitable for:

• The peak starting current would result in a serious voltage drop on the supply system
• The equipment being driven cannot tolerate the effects of very high peak torque loadings
• The safety or comfort of those using the equipment may be compromised by sudden starting as, for example, with escalators and lifts.

Effects of High Voltage Transmission Lines on Humans and Plants

 Introduction:

By increasing population of the world, towns are expanding, many buildings construct near high voltage overhead power transmission lines. The increase of power demand has increased the need for transmitting huge amount of power over long distances. Large transmission lines configurations with high voltage and current levels generate large values of electric and magnetic fields stresses which affect the human being and the nearby objects located at ground surfaces. This needs to be investigating the effects of electromagnetic fields near the transmission lines on human health.

The electricity system produces extremely low frequency electromagnetic field which comes under Non ionizing radiations which can cause health effects. Apart from human effect, the electrostatic coupling & electromagnetic interference of high voltage transmission lines have impact on plants and telecommunication equipments mainly operating in frequency range below UHF.

IS Power Line EMF safe? This is the controversy Discussion directly eludes on Government Regulation policy and Power Company. There are lots of supporting documents and research paper in favor and criticize this arguments.

What is The Electric and Magnetic fields:

• Electric and magnetic fields, often referred to as electromagnetic fields or EMF, occur naturally and as a result of the Power generation, Power Transmission, Power distribution and use of electric power.
• EMF is fields of force and is created by electric voltage and current. They occur around electrical devices or whenever power lines are energized.
• Electric fields are due to voltage so they are present in electrical appliances and cords whenever the electric cord to an appliance is plugged into an outlet (even if the appliance is turned off).
• Electric fields (E) exist whenever a (+) or (-) electrical charge is present. They exert forces on other charges within the field. Any electrical wire that is charged will produce an electric field (i.e. Electric field produces charging of bodies, discharge currents, biological effects and sparks). This field exists even when there is no current flowing. The higher the voltage, the stronger is electric field at any given distance from the wire.
• The strength of the electric field is typically measured in volts per meter (V/m) or in kilovolts per meter (kV/m). Electric fields are weakened by objects like trees, buildings, and vehicles. Burying power lines can eliminate human exposure to electric fields from this source.
• Magnetic fields result from the motion of the electric charge or current, such as when there is current flowing through a power line or when an appliance is plugged in and turned on. Appliances which are plugged in but not turned on do not produce magnetic fields.
• Magnetic field lines run in circles around the conductor (i.e. produces magnetic induction on objects and induced currents inside human and animal (or any other conducting) bodies causing possible health effects and a multitude of interference problems). The higher the current, the greater the strength of the magnetic field.
• Magnetic fields are typically measured in tesla (T) or more commonly, in gauss (G) and milli gauss (mG). One tesla equals 10,000 gauss and one gauss equals 1,000 milli gauss.
• The strength of an EMF decreases significantly with increasing distance from the source.
• The Strength of an electric field is proportional to the voltage of the source. Thus, the electric fields beneath high voltage transmission lines far exceed those below the lower voltage distribution lines. The magnetic field strength, by contrast, is proportional to the current in the lines, so that a low voltage distribution line with a high current load may produce a magnetic field that is as high as those produced by some high voltage transmission lines.
• In fact, electric distribution systems account for a far higher proportion of the population’s exposure to magnetic fields than the larger and more visible high voltage transmission lines.
• Electrical field: the part of the EMF that can easily be shielded.
• Magnetic field: part of the EMF that can penetrate stone, steel and human flesh. In fact, when it comes to magnetic fields, human flesh and bone has the same penetrability as air!
• Both fields are invisible and perfectly silent: People who live in an area with electric power, some level of artificial EMF is surrounding them.
• The magnetic field strength produced from a transmission line is proportional to: load current, phase to phase spacing, and the inverse square of the distance from the line.
• Many previous works studied the effect of different parameters on the produced magnetic field such as: the distance from the line, the conductor height, line shielding and transmission line configuration and compaction.

Electric and Magnetic Field (EMF) Effects

• Extremely high voltages in EHV lines cause electrostatic effects, where as short circuit currents & line loading currents are responsible for electromagnetic effects. The effect of these electrostatic fields is seen prominent with living things like humans, plants, animals along with vehicles, fences & buried pipes under & close to these lines.

 1)   EMF Effects Human beings:

• The human body is a composed of some biological materials like blood, bone, brain, lungs, muscle, skin etc. The permeability of human body is equals to permeability of air but within a human body has different electromagnetic values at a certain frequency for different material.
• The human body contains free electric charges (largely in ion-rich fluids such as blood and lymph) that move in response to forces exerted by charges on and currents flowing in nearby power lines. The processes that produce these body currents are called electric and magnetic induction.
• In electric induction, charges on a power line attract or repel free charges within the body. Since body fluids are good conductors of electricity, charges in the body move to its surface under the influence of this electric force. For example, a positively charged overhead transmission line induces negative charges to flow to the surfaces on the upper part of the body. Since the charge on power lines alternates from positive to negative many times each second, the charges induced on the body surface alternate also. Negative charges induced on the upper part of the body one instant flow into the lower part of the body the next instant. Thus, power-frequency electric fields induce currents in the body (Eddy Current) as well as charges on its surface.

• The currents induced in the body by magnetic fields are greatest near the periphery of the body and smallest at the center of the body.
• It is believed that, the magnetic field might induce a voltage in the tissue of human body which causes a current to flow through it due to its conductivity of around them.
• The magnetic field has influence on tissues in the human body. These influences may be beneficial or harmful depending upon its nature.
• The magnitude of surface charge and internal body currents that are induced by any given source of power-frequency fields depends on many factors. These include the magnitude of the charges and currents in the source, the distance of the body from the source, the presence of other objects that might shield or concentrate the field, and body posture, shape, and orientation. For this reason the surface charges and currents which a given field induces are very different for different Human and animals.
• When a person who is isolated from ground by some insulating material comes in close proximity to an overhead transmission line, an electrostatic field is set in the body of human being, having a resistance of about 2000 ohms.
• When the same person touches a grounded object, it will discharge through his body causing a large amount of discharge current to flow through the body. Discharge currents from 50-60 Hz electromagnetic fields are weaker than natural currents in the body, such as those from the electrical activity of the brain and heart.
• For human beings the limit for undisturbed field is 15 kV/m, R.M.S., to experience possible shock. When designing a transmission lines this limit is not crossed, in addition to this proper care has been taken in order to keep minimum clearance between transmission lines.
• According to research and publications put out by the World Health Organization(WHO), EMF such as those from power lines, can also cause:

• Short term Health Problem

1. Headaches.
2. Fatigue
3.  Anxiety
4.  Insomnia
5.  Prickling and/or burning skin
6.  Rashes
7.  Muscle pain

•  Long term Health Problem:

• Following  serious health Problems may be arise due to EMF effects on human Body.

        (1) Risk of damaging DNA.

• Our body acts like an energy wave broadcaster and receiver, incorporating and responding to EMFs. In fact, scientific research has demonstrated that every cell in your body may have its own EMF, helping to regulate important functions and keep you healthy.
• Strong, artificial EMFs like those from power lines can scramble and interfere with your body’s natural EMF, harming everything from your sleep cycles and stress levels to your immune response and DNA!

      (2) Risk of Cancer

• After hundreds of international studies, the evidence linking EMFs to cancers and other health problems is loud and clear. High Voltage power lines are the most obvious and dangerous culprits, but the same EMFs exist in gradually decreasing levels all along the grid, from substations to transformers to homes.

       (3) Risk of Leukemia:

• Researchers found that children living within 650 feet of power lines had a 70% greater risk for leukemia than children living 2,000 feet away or more.(As per British Medical Journal, June, 2005).

        (4) Risk of Neurodegenerative disease:

• “Several studies have identified occupational exposure to extremely low-frequency electromagnetic fields (EMF) as a potential risk factor for neuro degenerative disease.”(As per Epidemiology, 2003 Jul; 14(4):413-9).

       (5) Risk of Miscarriage:

• There is “strong prospective evidence that prenatal maximum magnetic field exposure above a certain level (possibly around 16 mG) may be associated with miscarriage risk.” (As per Epidemiology, 2002 Jan; 13(1):9-20)

2)   EMF Effects on Animals

• Many researchers are studying the effect of Electrostatic field on animals. In order to do so they keeps the cages of animals under high Electrostatic field of about 30 kV/m. The results of these Experiments are shocking as animals (are kept below high Electrostatic field their body acquires a charge & when they try to drink water, a spark usually jumps from their nose to the grounded Pipe) like hens are unable to pick up grain because of chattering of their beaks which also affects their growth.

3)   EMF Effects on Plant Life

• Most of the areas in agricultural and forest lands where high power transmission lines pass. The voltage level of high power transmission Lines are 400KV, 230KV, 110KV, 66KV etc. The electromagnetic field from high power transmission lines affects the growth of plants.
• Gradually increases or decreases and reaches to maximum current or minimum current and thereafter it starts to fall down to lowest current or raises to maximum current or a constant current. Again the current, it evinces with little fluctuations till the next day morning.
• Current in Power transmission lines varies according to Load (it depending upon the amount of electricity consumed by the consumers). Hence the effect of EMF (due to current flowing in the power lines) upon the growth of plants under the high power transmission lines remains unaltered throughout the year.
• From various practically study it was found that the response of the crop to EMF from 110 KV and 230 KV Power lines showed variations among themselves. Based on the results the growth characteristics like shoot length, root length, leaf area, leaf fresh weight, specific leaf weight, shoot/root ratio, total biomass content and total water content of the four crop plants were reduced significantly over the control plants.
• Similar trend were observed in the biochemical characteristics like chlorophyll.
• Reduced growth and physiological parameter was primarily due to the effect of reduced cell division and cell enlargement. Further the growth was stunted which may be due to poor action of hormones responsible for cell division and cell enlargement.
• The bio-chemical changes produced in this plant due to EMF stress quite obvious and it affects the production leading to economic loss.
• It is concluded that the reduced growth parameter shown in the crop plants would indicates that the EMF has exerted a stress on that plants and this EMF stress was quite obvious and it affects the production leading to economic loss. So further research activities are needed to safe guard plants from EMF stress.

4)   EMF Effects on Vehicles parked near Line

• When a vehicle is parked under high voltage transmission line an electrostatic field is developed in it. When a person who is grounded touches it a discharge current flows through the human being. In order to avoid this parking lots are located below the transmission lines the recommended clearance is 17 m for 345 kV and 20 m for 400 kV lines.

5)   EMF Effects  on Pipe Line/Fence/Cables:

• A fence, irrigation pipe, pipeline, electrical distribution line forms a conducting loops when it is grounded at both ends. The earth forms the other portion of the loop. The magnetic field from a transmission line can induce a current to flow in such a loop if it is oriented parallel to the line. If only one end of the fence is grounded, then an induced voltage appears across the open end of the loop. The possibility for a shock exists if a person closes the loop at the open end by contacting both the ground and the conductor.
• For fences, buried cables, and pipe lines proper care has been taken to prevent them from charging due to Electrostatic field. When using pipelines which are more than 3 km in length & 15 cm in Diameter they must be buried at least 30 laterally from the line center.

6)   EMF Effects on Maintenance Worker:

• For providing continuous and uninterrupted supply of electric power to consumers maintenance operations of power lines are often performed with systems energized or live.
•  This is live line maintenance or hot line maintenance. The electric fields and magnetic fields associated with these power lines may affect the health of live line workers. Its electric field and current densities affect the health of humans and cause several diseases by affecting majority parts of the human body. These electric field and current densities affects humans of all stages and causes short term diseases in them and sometimes death also.

Contradiction of EMF Effect on Human Health:

• There are two reasons why electromagnetic fields associated with power systems could pose no threat to human health.
• First, The EMF from power lines and appliances are of extremely low frequency and low energy. They are non-ionizing and are markedly different in frequency from ionizing radiation such as X-rays and gamma rays. As a comparison, transmission lines have a low frequency of 60Hz while television transmitters have higher frequencies in the 55 to 890 MHZ range. Microwaves have even higher frequencies, 1,000 MHZ and above. Ionizing radiation, such as X-rays and gamma rays, has frequencies above 1015 Hz. The energy from higher-frequency fields is absorbed more readily by biological material.  Microwaves can be absorbed by water in body tissues and cause heating which can be harmful, depending upon the degree of heating that occurs. X-rays have so much energy that they can ionize (form charged particles) and break up molecules of genetic material (DNA) and no genetic material, leading to cell death or mutation. In contrast, extremely low frequency EMF does not have enough energy to heat body tissues or cause ionization.
• Second, all cells in the body maintain large natural electric fields across their outer membranes. These naturally occurring fields are at least 100 times more intense than those that can be induced by exposure to common power-frequency fields. However, despite the low energy of power-frequency fields and the very small perturbations that they make to the natural fields within the body.
• When an external agent such as an ELF fields lightly perturbs a process in the cell, other processes may compensate for it so that there is no overall disturbance to the organism. Some perturbations may be within the ranges of disturbances that a system can experience and still function properly.
• During Research on health effects of electric and magnetic fields, it has come forward that electric field intensity exposure of about 1-10 mv/m in tissue interact with cells but not proved to be harmful. But strong fields cause harmful effects when their magnitude exceeds  stimulation thresholds for neural tissues (central nervous system and brain), muscle and heart

Surface Current Density(mA/m2)	Health Effect
<1	Absence of any established effects.
1 To 10	Minor biological effects.
10 To 100	Well established effects(a) Visual effect. (b) Possible nervous system effect
100 To 1000	Changes in central nervous System
>1000	Ventricular Fibrillation (Heart Condition 0. Health hazards.

• In India it is stipulated that electric field intensity should not exceed 4.16 kV/m and magnetic field intensity should not exceed 100μT in public areas.
• Even when effect is demonstrated consistently on the cellular level in laboratory experiments, it is hard to predict whether and how they will affect the whole organism. Processes at the individual cell level are integrated through complex mechanisms in the animal.

Mitigation of EMF Effect of Transmission Line:

1)    Line shielding:

• There are two basic 60-Hz magnetic field mitigation (reduction) methods: passive and active.
• Passive magnetic field mitigation includes rigid magnetic shielding with ferromagnetic and highly conductive materials, and the use of passive shield wires installed near transmission lines that generate opposing cancellation fields from electromagnetic induction.
• Active magnetic field mitigation uses electronic feedback to sense a varying 60-Hz magnetic field, then generates a proportionally opposing (nulling) cancellation field within a defined area (room or building) surrounded by cancellation coils. Ideally, when the two opposing 180-degree out- of-phase magnetic fields of equal magnitude intersect, the resultant magnetic field is completely cancelled (nullified). This technology has been successfully applied in both residential and commercial environments to mitigate magnetic fields from overhead transmission and distribution lines, and underground residential distribution (URD) lines.

2)    Line Configuration and Compaction

• Line compaction means that, bringing the conductors close together keeping the minimum (safe) phase-to-phase spacing constant. Keeping all the parameters the same and the only variable is the phase-to- phase spacing. The magnetic field is proportional to the dimensions of the phase-to-phase spacing.
• Other studies showed that, increasing the distance between phases by increasing the height of the central phase conductor above the level of the other phase conductors leads to the reduction of the peak value of the magnetic field.
• Reducing the phase-to-phase distance, leads to the decrease of the magnetic field. This reduction between phases is limited by the electrical insulation level between phases.
• (A) For single circuit lines, compaction causes a great reduction to the maximum magnetic field values. This reduction of magnetic field allows for lower conductor heights above the ground. This leads to transmit the same power on shorter towers. This gives a great reduction of the tower cost.
• (B) For double circuit lines, some studies showed that, the use of optimum phase arrangement causes a drastic reduction to the maximum magnetic field values for both conventional and compact lines i.e. with vertical conductor

3)    Grounding:

• Induced currents are always present in electric fields under transmission lines and will be present. However, there must be a policy to ground metal objects, such as fences, that are located on the right-of-way. The grounding eliminates these objects as sources of induced current and voltage shocks. Multiple grounding points are used to provide redundant paths for induced current flow and mitigate nuisance shocks.

4)    Providing Right of Way(R.O.W):

• Overhead transmission systems required strips of land to be designed as right-of-ways (R.O.W.). These strips of land are usually evaluated to decrease the effects of the energized line including magnetic and electric field effects.

5)    Maintaining Proper Clearance:

• Unlike fences or buildings, mobile objects such as vehicles and farm machinery cannot be grounded permanently. Limiting the possibility of induced currents from such objects to persons is accomplished by maintaining proper clearances for above-ground conductors tend to limit field strengths to levels that do not represent a hazard or nuisance.
•  Limiting access area by increasing conductor clearances in areas where large vehicles could be present.

 Conclusion:

Based on the review and analysis and other research projects it is of the opinion that there is no conclusive and convincing evidence that exposure to extremely low frequency EMF emanated from nearby high voltage Transmission lines is causally associated with an increased incidence of cancer or other detrimental health effects in humans. Even if it is assumed that there is an increased risk of cancer as implied in some epidemiological studies, the empirical relative risk appears to be fairly small in magnitude and the observed association appears to be tenuous. Although the possibility is still remain about the verse effect on health by EMF.

References:

• SSGBCOE&T, Electronics and Communication Engineering-Girish Kulkarni1, Dr.W.Z.Gandhare
• Pharmacology, School of Medicine, Chung-Ang University, Seoul, Korea-Sung-Hyuk Yim, Ji-Hoon Jeong.
• Electrical Engineering Department, Shoubra, Benha University, Cairo, Egypt- Nagat Mohamed Kamel Abdel-Gawad.
• Madurai Kamaraj University-S. Somasekaran.
• Electrical Engineering Department at King Fahd University of Petroleum & Minerals- J. M. Bakhashwain, M. H. Shwehdi, U. M. Johar and A. A. AL-Naim.
• Dept. of Electrical Engineering. College of Engineering – University of Tikrit-Iraq- Ghanim Thiab Hasan, Kamil Jadu Ali, Mahmood Ali Ahmed.

 

Dark and Bright side of CFL Bulbs (Is it dangerous to our Health?)

Introduction:

  We are becoming more conscious about climate change and many governments in the world are looking for different ways to reduce greenhouse gases and to reduce consumption of fossil fuels. One of the simplest solution for this is aggressively adopting CFL which is phasing out energy inefficient light. Compact fluorescent lights (CFL), heavily promoted for their energy saving properties and quickly pushing traditional incandescent bulbs out of the market. They are now inexpensive, payback in electricity savings is nearly immediate, and there is that side benefit of reducing power plant emissions. CFL bulbs use approximately 75% less energy than incandescent light bulbs and last longer. At first glance this seems like a good way to conserve energy and to protect our environment. However,

Many environmentally conscious people think they are doing a great thing by using compact fluorescent light bulbs – CFLs. We see them advertised everywhere, even our most trusted environmental news sources tells us we should be using them. Production of traditional incandescent light bulbs may be phased out completely by the year 2014.

Unfortunately most people are unaware of and not many are talking about the fact that although CFL bulbs reduce energy and greenhouse gases, they put our health at an even greater risk than incandescent bulbs. They are energy efficient but not environmentally friendly. There are a number of serious problems associated with CFL bulbs that need to be considered and corrected. These include mercury content, emission of UV radiation, emission of radio frequency radiation, and generation of dirty electricity. There is the additional concern that these lights are making some people ill. This includes those who suffer from migraines, skin problems, epilepsy, and electrical sensitivity.

Governments are mandating CFL use and banning incandescent light bulbs. Media, industry, and governments have “screwed” the benefits of CFL bulbs into the deepest sockets of our mind. We are neutrally try to highlight dark and bright side of CFL by some fact and supporting arguments.

 Bright Side of CFL:

There are some advantages of CFL over incandescent light bulbs

(1)  Compact fluorescent lamps are four time more efficient than traditional light bulbs. (13 Watt CFL would give off as much light as a 60 Watt incandescent).

(2)  CFL bulbs use approximately 75% less energy than incandescent light bulbs.

(3)  CFL has long life compared to incandescent light bulbs.

(4)  Compact fluorescent light bulbs are easily shrinking power bill and carbon footprint.

(5)  CFL reduces greenhouse gas emissions and other pollutants created by fossil-fuel power plants.

(6)  Price so CFL is less compared to incandescent light bulbs.

Dark Side of CFL (Health problems created by CFL)

CFL’s save energy. Saving energy is good for the planet and may retard global warming. Sounds good although CFL have some seriously negative effects.

(1)  Mercury emissions by CFL:

• One of the negatives side of CFLs that it contain mercury so it must be disposed of properly in order to prevent contamination of our environment, our landfills and our water supply
• Mercury is an essential ingredient for most energy efficient lighting products, including CFLs. It is the mercury that excites phosphors in a CFL, causing them to glow and give light. When electric current passes through mercury vapor, the mercury emits ultraviolet energy. When this ultraviolet energy passes through the phosphor coating, it produces light very efficiently. Because mercury is consumed during lamp operation, a certain amount is necessary to produce light and achieve long lamp life.
• Mercury can be added to the CFL in two ways. Some manufacturers use liquid mercury, which is less expensive and more difficult to accurately dose. Uses amalgam, a small “pill” which is a solid state form of mercury and other elements. Amalgam is much easier and more accurate to dose. This is the only manufacturer using 100 percent amalgam in its CFL products.
• Airborne mercury poses a very low risk of exposure. However, when mercury emissions deposit into lakes and oceans, they can transform into a highly toxic form that builds up in fish. Fish consumption is the most common pathway for human exposure to mercury. Pregnant women and young children are most vulnerable to the effects of this type of mercury exposure. However, the most people are not exposed to harmful levels of mercury through fish consumption.
• Mercury is an element found naturally in the environment. Mercury emissions in the air can come from both natural and man-made sources. Utility power plants (mainly coal-fired) are the primary man-made source, as mercury that naturally exists in coal is released into the air when coal is burned to make electricity. Coal-fired power generation accounts for roughly 40% of the mercury emissions.
• Health problems associated with mercury depend on how much has entered your body, how it entered your body, how long you have been exposed to it, and how your body responds to the mercury. Children are more susceptible to mercury poisoning than adults. Exposure to small amounts of mercury over a long period, and brief contact with high levels of mercury may cause adverse health effects. Symptoms depend on the length or level of exposure.
• Mercury is a powerful neurotoxin that can cause serious damage to the all the tissues and organs in the body as well as the central nervous system and endocrine system and it disrupts functioning of crucial neurotransmitters in the brain. It is one of the most toxic substances on the planet and has been linked to a variety of serious health conditions like autism, memory problems, infertility, depression, thyroid disorders, Alzheimer’s, adrenal disorders, anxiety, Parkinson’s and MS to name a few. It is especially toxic to children, pregnant women and small pets.
• While the mercury is contained in the light bulb there is no risk, however if you drop the bulb on the floor of your home, then you are exposed to dangerous mercury vapors. Many are reporting that it is quite easy to break CFL light bulbs as you are screwing it in the socket. Additionally, when we toss them in the garbage and they are picked up by the garbage company, they are getting broken all over the city and in the landfills. This means that our air and soil is being contaminated with mercury across our cities.

Arguments to oppose this Drawback

• The amount of mercury in the most popular and widely used CFLs is minimal, ranging between 2.3 mg and 3.5 mg. That is lower than other CFLs on the market, which generally contain approximately 5 mg, roughly the equivalent of the tip of a ballpoint pen.
• By comparison, older home thermometers contain 500 milligrams of mercury and many manual thermostats contain up to 3000 milligrams. It would take between 100 and 665 CFLs to equal those amounts.
• CFLs are safe to use in your home. No mercury is released when the bulbs are in use and they pose no danger to you or your family when used properly.
• CFLs are responsible for less mercury than standard incandescent light bulbs, and actually work to prevent mercury from entering our air, where it most affects our health. The highest source of mercury in our air comes from burning fossil fuels such as coal, the most common fuel used to produce electricity. A CFL uses 75% less energy than an incandescent light bulb and lasts up to 13 times longer. 70% of power plants are coal fired and thus burn fossil fuel to produce energy. These power plants will emit 10 mg of mercury to produce the electricity to run an incandescent bulb compared to only 2.4 mg of mercury to run a CFL for the same time. Coal-fired power generation accounts for roughly 40% of the mercury emissions.

 (2)  Compact fluorescent bulbs and Migraine

• In the past, some people reported headaches or eye strain when using fluorescent lighting. Some could see a flicker in the lighting, caused by lower frequencies and magnetic ballasts. The newer CFLs use higher frequencies and electronic ballasts, which mean the human eye, cannot detect any change in the light frequency. There is also less of a ‘hum’ in the newer lights. The ‘hum’ in older lights may have caused headaches.
• The flickering of fluorescent bulbs is a known migraine trigger. Compact fluorescent bulbs have made great strides in reducing the flickering that is common in this class of light bulbs. Despite this, many individuals are finding that compact fluorescent bulbs cause migraine headaches. CFL bulb manufacturers have denied any link between the bulbs and increased headache problems. Currently, there is little research to support the link between migraine and CFL use; however, personal, anecdotal evidence demonstrates that many migraines cannot tolerate the new lights. Migraine is not just a headache. Migraine disease is a neurological condition that not only causes pain but can impact motor function, sensory function, vision, memory, and speech
• Individuals who have problems with fluorescent bulbs can try the following tips to lessen the impact of a CFL on migraine disease:

 Arguments to oppose this Drawback:

1. Use the newest compact fluorescent bulbs available.
2. Sit as far from the bulbs as possible
3. If flickering is interfering with TV or computer monitor use, try repositioning the light to see if the flickering effect on the screen lessens.
4. Try eye glasses or contacts that block out UV radiation.
5. Use halogen or LED lighting
6. Try double walled bulbs or a light diffuser.

 (3)  Compact fluorescent bulbs and Lupus

• Compact fluorescent bulbs can produce more ultraviolet light and have a different light spectrum than incandescent bulbs. This makes compact fluorescent bulbs problematic for people with Lupus or other light sensitive skin conditions. Individuals with light sensitivity should monitor the effect of compact fluorescent bulbs on their health.

Arguments to eliminate this Drawback:

1. Keep at least 1 foot between yourself and the compact fluorescent bulb.
2. Try a light cover or diffuser over the light.
3. Investigate the amount of ultraviolet light produced by different brands of CFLs.
4. Use halogen or LED lighting.

(4)  Ultra violet light emissions from CFL

• Ultra violet light is responsible for skin cancer. It can also be a problem for individuals with ultra violet sensitive conditions such as Lupus. One would think that staying inside would keep a person safe from this harmful radiation.

Arguments to eliminate this Drawback:

1. This is not completely true. Fluorescent lights put off UV light. While this exposure is much smaller than that of sunlight, it is important to keep it in mind. The current guideline limit  is 30 J m-2 for the eye and skin, which is equivalent to a constant irradiance of 1 mW m-2 effective for 30,000 seconds or 8 hours, a normal working day. At close proximity (2 cm or ¾ inch), the exposure limit would be exceeded in less than 10 minutes by about 20% of the CFLs tested.
2. About half of the CFLs exceeded the exposure limit at this distance after 30 minutes. If the distance is increased to about 8″ only around 8% of the CFL bulbs exceed this limit.
3.  Also, encapsulated bulbs that have a globe of glass around the CFL itself emit less UV radiation than the traditional bulbs.
4. Do not use compact fluorescent bulbs for close up work or lighting.
5. Purchase double walled CFLs that are encapsulated.

(5)  Spectral distributions by CFL:

• Natural daylight provides the only true full spectrum lighting. Incandescent light is closer in spectral distribution to natural daylight; fluorescent light is far different which accounts for its negative effects on the human body. There are thousands, of well documented scientific photo biological studies indicating the negative effects of fluorescent lighting.
• The effects of different light sources on the body have been researched at a long list of prestigious institutions including MIT, and Harvard University. The latest research is being done on how different colors of light (spectral distributions) affect the body’s circadian rhythms. Researchers used to think of the eye as the main organ for vision but because of the recent discovery of additional nerve connections, it is now understood that light mediates and controls a number of biochemical processes in the human body, including the production of important hormones through control of the light/dark cycle (circadian rhythms) – the body’s biological clock.
• Fluorescent light gives off a very much distorted spectrum which is very different from the natural daylight in which our bodies have evolved. Fluorescent light disrupts our circadian rhythms – our body’s regulator mechanism – and in doing so studies have shown negative health effects from minor annoyances such as headaches, eyestrain, fatigue, and weight gain, to serious effects such as insomnia and sleep disturbances, an increased risk of cancer, and a suppressed immune system.

(6)  Emission of UV Radiation by CFL:

• Fluorescent light bulbs contain mercury, which emits UV radiation when it is electrically excited. This UV radiation then interacts with the chemicals on the inside of the bulb to generate light. According to Philippe Laroche, Media Relations Officer for Health Canada, compact fluorescent light bulbs, unlike tube fluorescent bulbs, do not have prismatic diffusers to filter UV radiation. “Therefore, there may be skin sensitivity issues, especially in people with certain skin diseases.”
• Interestingly, the British Dermatological Association has spoken out against CFL bulbs because their patients have adverse reactions to them. They are asking the UK government to allow people with skin problems to continue using incandescent light bulbs once the ban for energy inefficient bulbs becomes law.

Arguments to eliminate this Drawback:

1.  Not all CFL are the same. GE produces a low-UV bulb called Safe-T-Guard (registered Trade mark) for dark rooms. So the technology to produce safer bulbs is available and should be required for all bulbs.

 (7)  Emission of Radio Frequency Radiation by CFL:

• CFLs emit radio frequency radiation at levels that may interfere with various types of wireless technology.

Arguments to eliminate this Drawback:

1. GE has started to put on General Electric acknowledges this and notice on the back of product packaging for all GE electronically ballasted CFLs:“This product complies with Part 18 of the FCC Rules, but may cause interference to radios, televisions, wireless telephones, and remote controls. Avoid placing this product near these devices. If interference occurs, move the product away from the device or plug either into a different outlet. Do not install this product near maritime safety equipment or other critical navigation or communication equipment operating between 0.45-30MHz. “

 (8)  Poor Power Quality Produced by CFL:

• CFL is affecting Quality of Electrical Power. There is a deviation in the magnitude and frequency of the sinusoidal waveform.
• Fluorescent lamps will only run on alternating current. They also need a pulse of high voltage and heated filaments at either end to start the electrical discharge that lights them. After that, the current must be limited externally, otherwise too much would flow and they would burn out. In a traditional fluorescent strip light, this is accomplished by the starter switch and the choke (a coil of wire wound around an iron core). Once started, the current flows through the tube as a smooth sine wave at mains frequency, which is 50Hz (cycles per second).This makes the light flash on and off with each half cycle (i.e. 100 or 120 times a second) and some people, such as epileptics and migraine sufferers find this disturbing.
• CFLs produce transients that contribute to poor power quality on electrical wires. According to General Electric (GE) their typical electronically-ballasted CFL operate in the 24-100 kHz frequency range. This range is within the radio frequency band of the electromagnetic spectrum and is classified as Intermediate Frequency (IF) by the World Health Organization. There is concern about electromagnetic interference (EMI) associated with IF and recently studies have shown that IFs are biologically active and can have adverse health effects.

Arguments to eliminate this Drawback:

1. Not all CFL are the same some generate more dirty electricity than others. In a recent study the values for dirty electricity ranged from 47 to 1450 GS units compared with a background value (with lights off) between 54-58 GS units. Clearly technology exists to produce CFL that do not generate dirty electricity.
2. However, almost all CFLs use electronic control gear. This usually incorporates a switched-mode power supply in the base of the lamp itself. It rectifies the AC from the mains to convert it to DC and then chops it electronically into a series of sharp rectangular alternating pulses, which then light the lamp. However, the new frequency, which is usually about 40 kHz (40,000 cycles per second), is so high and the gaps between pulses are so short that the relatively slow response of the phosphors can fill them easily. Consequently, these lamps do not flash.

Required Vigilant Awareness while using of CFL:

Although CFLs are considered safe to use, here are some steps you can take to further protect you and your family:

• Always handle CFLs carefully when installing and removing them.
• Buy CFLs that are marked low UV.
• Buy CFLs that have a glass cover already added, which will help further filter out UV radiation.
• Use additional glass, plastic or fabric materials in your lighting fixtures to act as UV filters.
• Increase the distance you are from the CFL, as this will reduce the level of UV exposure.
• All ENERGY STAR® qualified CFLs have less than 5 milligrams of mercury (some manufacturers are able to produce CFLs that have only about 1 milligram of mercury). Avoid purchasing non-ENERGY STAR® CFLs, as they may have much higher levels of mercury in them.
• As of September 2008, all ENERGY STAR® qualified CFLs are required to list their mercury content on the packaging. This information is not required on non-ENERGY STAR® CFL packaging.
• A CFL is a sealed unit, and no mercury is released when it is in use or as long as it is intact. Some mercury is released when a bulb breaks, and appropriate clean-up guidance should be followed.
• If the bulb breaks, make sure to clean it up properly.  Also, check your local regulations to make sure that you won’t break any laws while disposing of the bulb.
• Look for recycling programs online, through local stores, or through the light bulb manufacturers.  Make an informed choice.  If CFLs concern you or if you have health problems do to them, switch to a LED or incandescent bulb

 Disposal of CFL Bulbs:

• If CFL breaks- carefully sweep up all the fragments, wipe the area with a wet towel, and dispose of all fragments, including the used towel, in a sealed plastic bag.
• Follow all disposal instructions. If possible, open windows to allow the room to ventilate. Do NOT use a vacuum. Place all fragments in a sealed plastic bag and follow disposal instructions.
• Due to the mercury in fluorescent bulbs, they require special disposal methods. When these bulbs are sent to a traditional landfill, the bulbs often will break and will then emit mercury gas that is harmful to workers and to the environment. The health threat to workers is especially large at transfer stations where large quantities of light bulbs may be crushed in a single location. Due to the dangers associated with mercury “ten states and multiple local jurisdictions prohibit the disposal of mercury containing products, including CFLs and other mercury containing lamps, in solid waste.”
• Require CFLs to go through special CFL recycling programs or for individuals to dispose of CFLs at hazardous waste collection centers.

 CONCLUSION

If we can afford the discomfort of higher electrical bills, it is OK to go back to incandescent. The Earth will be fine, it just goes through cyclical warming and cooling’s, and we humans might not have as much impact on it as we give ourselves credit for. The heat generated by incandescent is not always wasted either. In colder months the heat reduces the amount of energy drawn from household heating. In the next some year the prices of LED lighting will start to come down, and new LED lighting fixtures will be introduced. The CFLs will begin to be phased out, leaving behind a long term problem of mercury disposal, remediation, and a so far untold toll on human health

Instead of promoting compact fluorescent light bulbs governments around the world should be insisting that manufactures produces light bulbs that are electromagnetically clean and contain no toxic chemicals.   Some of these are available (LED) but are not yet affordable. With a growing number of people developing electro hypersensitivity we have a serious emerging and newly identified health risk that is likely to get worse until regulations restricting our exposure to electromagnetic pollutants are enforced. Also, with improper disposal of these bulbs we are creating a mercury-time bomb. Since everyone uses light bulbs and since the energy inefficient incandescent light bulbs are being phased out in many countries by this is an area that requires immediate attention. “Try a CFL, but use and dispose it very carefully”

Analysis the Truth behind Household Power Savers

Introduction:

A House hold power saving devices has recently received a lot of attention from both consumers and manufacturers. It is generally used in residential homes to save energy and to reduce electricity bills. It is a small device which is to be plugged in any of the AC sockets in the house (Mostly near Energy Meter). Moreover, some of the companies claim that their power savers save up to 40% of the energy.

          Many people believe that the claims made by the power saver manufacturing companies are false. Almost all people who buy power savers do it to reduce their electricity bills. Many people who have used these power savers said that they could reduce their electricity bills with the devices; however the reduction was not as much as they had expected. Moreover, they could not figure out if the reduction in electricity bills was due to the power savers or because of their efforts to reduce their electrical usage. There have been several serious discussions about the genuineness of the device. In This Note, We will try to find the real truth behind these power savers which claim to save as much as 40% of energy.

Working Principle of Power Saver as per Manufacture:

• A Power Saver is a device which plugs in to power socket. Apparently just by keeping the device connected it will immediately reduce your power consumption. Typical claims are savings between 25% and 40%.
• It is known that the electricity that comes to our homes is not stable in nature. There are many fluctuations, raise and falls, and surges/Spikes in this current. This unstable current cannot be used by any of the household appliances. Moreover, the fluctuating current wastes the electric current from the circuit by converting electrical energy into heat energy. This heat energy not only gets wasted to the atmosphere, but also harms the appliances and wiring circuit.

• Power Saver stores the electricity inside of it using a system of capacitorsand they release it in a smoother way to normal without the spikes. The systems also automatically remove carbon from the circuit which also encourages a smoother electrical flow. This means that we will have less power spikes. More of the electricity flowing around circuit can be used to power appliances than before.
  • Basically it is claimed that Power savers work on the principle of surge protection technology. Power savers work on straightening this unstable electric current to provide a smooth and constant output. The fluctuation in voltage is unpredictable and cannot be controlled. However, the power savers utilize current fluctuation to provide a usable power by acting like a filter and allowing only smooth current to pass through the circuit. Power savers use capacitors for this purpose. When there is a surge of current in the circuit, the capacitor of the power saver stores the excess current and releases it when there is a sudden drop. Thus only smooth output current comes out of the device.
  • Moreover, a power saver also removes any type of carbon in the system, which facilitates further smoother flow. The main advantage of power savers is not that they provide a backup system in times of low current, but that it protects the household appliances. It is known that a sudden rise in the power can destroy the electrical appliance. Thus, the power saver not only protects the appliance but also increases its life. Moreover, they also reduce the energy consumption and thus the electricity bills.
  • The amount of power saved by a power saver depends on the number of appliances on the circuit. Also, the system takes at least a week to adapt itself fully to the circuit, before it starts showing its peak performance. The maximum amount of voltage savings will be seen in areas where in the current fluctuation is the highest.

House Hold Power Saver Scam Review:

• Power Factor Correction for residential customers (home owners) is a scam? At most, each unit is worth as an investment. Power factor correction does make sense for some commercial / industrial customers.
• Many Companies promoting and advertise that their Power Saver unit are able to save domestic residential power consumption by employing an “active power factor correction” method on the supply line. The concept seems pretty impressive as the concept is true and legally accepted. But practically, we will find that it’s not feasible.
• To support above statement First we need to understand three terms.

1. Type of Electrical Load of House,
2. Basic Power Terminology (KW, KVA, KVAR).
3. Electrical Tariff method of Electricity Company for Household Consumer and Industrial Consumer.

• There are basically two kinds of load that exists in every house: one that is resistive like incandescent lamps, heaters etc. and the other that’s capacitive or inductive like ACs, refrigerators, computers, etc.
• The power factor of a Resistive Load like toaster or ordinary incandescent light bulb is 1 (one). Devices with coils or capacitors (like pumps, fans and florescent light bulb ballasts)-Reactive Load have power factors less than one. When the power factor is less than 1, the current and voltage are out of phase. This is due to energy being stored and released into inductors (motor coil) or capacitors on every AC cycle (usually 50 or 60 times per second).
• There are three terms need to be understand when dealing with alternating (AC) power.

1. First Term is kilowatt (kW) and it represents Real power. Real power can perform work. Utility meters on the side of House measure this quantity (Real Power) and Power Company charge for it.
2.  The second term is reactive power, measured in KVAR. Unlike kW, it cannot perform work. Residential customers do not pay for KVAR, and utility meters on houses do not record it too.
3.  The third term is apparent power, referred to as KVA. By use of multi meters we can measure current and voltage and then multiply the readings together we get apparent power in VA.

              

• Power Factor = Real Power (Watts) /Apparent Power (VA),
• Therefore, Real Power (Watts) = Apparent Power × PF = Voltage × Ampere × PF.
• Ideally a PF = 1, or unity, for an appliance defines a clean and a desired power consumption mostly Household Equipments (The dissipated output power becomes equal to the applied input power).
  In the above formula we can see that if PF is less than 1, the amperes (current consumption) of the appliances increase, and vice verse.
• With AC Resistive Load, the voltage is always in phase with the current and constitutes an ideal power factor equal to 1. However, with inductive or capacitive loads, the current waveform lags behind the voltage waveform and is not in tandem. This happens due to the inherent properties of these devices to store and release energy with the changing AC waveform, and this causes an overall distorted wave form, lowering the net PF of the appliance.
• Manufacture claim that the above problem may be solved by installing a well-calculated inductor/capacitor network and switching it automatically and appropriately to correct these fluctuations. A power saver unit is designed exactly for this purpose. This correction is able to bring the level of PF very close to unity, thus improving the apparent power to a great extent. An improved apparent power would mean less CURRENT consumption by all the domestic appliances. So far everything looks fine, but what’s the use of the above correction? The Utility Bill Which We pay is never based on Apparent Power (KVA) but it is based on Real Power (KW). The utility bill that we pay is never for the Apparent Power- it’s for the Real Power.
• By Reducing Current Consumption Does Not Reduce Power Bills of Household Consumer.

Study of Power Saver in Domestic Load

• Let us try to study Household’s Reactive-Resistive Electrical Load and Voltage Spike Characteristic by example.

(1)   Power Saver in Reactive Load of Home:

• Let’s take One Example for reactive Load: A refrigerator having a rated Real Power of 100 watts at 220 V AC has a PF = 0.6. So Power=Volt X Ampere X P.F becomes 100 = 220 × A × 0.6 Therefore, A = 0.75 Ampere
• Now suppose after Installing Power Saver if the PF is brought to about 0.9, the above result will now show as: 100 = 220 × A × 0.9 And A = 0.5 Ampere
• In the second expression we clearly show that a reduction in current consumption by the refrigerator, but interestingly in both the above cases, the Real Power remains the same, i.e. the refrigerator continues to consume 100 watts, and therefore the utility bill remains the same. This simply proves that although the PF correction done by an energy saver may decrease the Amperage of the appliances, it can never bring down their power consumption and the electric Bill amount.
• Reactive power is not a problem for a Reactive Load of Home appliances like A.C, Freeze, motor for its operation. It is a problem for the electric utility company when they charge for KW only. If two customers both use the same amount of real energy but one has a power factor of 0.5, then that customer also draws double the current. This increased current requires the Power Company to use larger transformers, wiring and related equipment. To recover these costs Power Company charged a Penalty to industrial customers for their Low power factors and give them benefits if they improve their Power Factor in. Residential customers (homes) are never charged extra for their reactive Power.

(2)   Power Saver in Resistive Load of Home:

• Since a resistive load does not carry a PF so there is not any issue regarding filtering of Voltage and Current, So Power = Voltage X Current.

(3)   In Voltage Spike/Fluctuation condition of Household Appliances:

• In above discussion simply proves that as long as the voltage and the current are constant, the consumed power will also be constant. However, if there’s any rise in the input voltage because of a fluctuation, then as explained above your appliances will be forced to consume a proportionate amount of power. This becomes more apparent because current, being a function of voltage, also rises proportionately. However, this rise in the power consumption will be negligibly small; the following simple math will prove this.
• Consider a bulb consuming 100 watts of power at 220 volts. This simply means at 240 volts it will use up about 109 watts of power. The rise is just of around 9% and since such fluctuations are pretty seldom, this value may be furthermore reduced to less than 1%, and that is negligible.
• Thus the above discussions convincingly prove that energy savers can never work and the concept is not practically feasible.

What happens when Power Saver is installed?

• The Fig shows the result of using Power Saver. The air conditioner (which has a large compressor motor) is still consuming reactive power but it is being supplied by a nearby capacitor (which is what is in those “KVAR” boxes). If you were to mount it at the air conditioner and switch it on with the air conditioner plus you sized the capacitor perfectly, then there would be no reactive power on the line going back to the fuse panel. If the wire between your fuse panels is very long and undersized, reducing the current would result in it running cooler and having a higher voltage at the air conditioner. These savings due to cooler wiring is minimal.

        

•  A further complication is that if you install the “KVAR” unit at the fuse panel, it does nothing for the heat losses except for the two feet of huge wire between the fuse panel and the utility meter. Many KVAR units are marketed as boxes that you install at a single location. If your power factor box is too large, then it will be providing reactive power for something else, perhaps your neighbor.

Conclusion:

• Power factor correction devices improve power quality but do not generally improve energy efficiency (meaning they would not reduce your energy bill). There are several reasons why their energy efficiency claims could be exaggerated.
• First, residential customers are not charged for KVA- hour usage, but by kilowatt-hour usage. This means that any savings in energy demand will not directly result in lowering a residential user’s utility bill.
• Second, the only potential for real power savings would occur if the product were only put near in the circuit while a reactive load (such as a motor) were running, and taken out of the circuit when the motor is not running. This is impractical, given that there are several motors in a typical home that can come on at any time (refrigerator, air conditioner, HVAC blower, vacuum cleaner, etc.), but the Power Saver itself is intended for permanent, unattended connection near the house breaker panel. And certainly not in the way the manufacturers recommend that they be installed, that is, permanently connecting them at the main panel. Doing that drags the power factor capacitive when the inductive motors are off and could create some real problems with ringing voltages.
• The KVAR needs to be sized perfectly to balance the inductive loads. Since our motors cycle on and off and we don’t use the air conditioner in the winter, there is no way to get it sized properly unless we have something to monitor the line and switch it on and off capacity (capacitors) as necessary.
• Adding a capacitor can increase the line voltage to dangerous levels because it interacts with the incoming power transmission lines.
• Adding a capacitor to a line that has harmonic frequencies (created by some electronic equipment) on it can result in unwanted resonance and high currents.
• For commercial facilities, power factor correction will rarely be cost-effective based on energy savings alone. The bulk of cost savings power factor correction can offer is in the form of avoided utility charges for low power factor. Energy savings are usually below 1% and always below 3% of load, the higher percentage occurring where motors are a large fraction of the overall load of a facility. Energy savings alone do not make an installation cost effective.

Types of Neutral Earthing in Power Distribution

Types of Neutral Earthing in Power Distribution:

 Introduction:

In the early power systems were mainly Neutral ungrounded due to the fact that the first ground fault did not require the tripping of the system. An unscheduled shutdown on the first ground fault was particularly undesirable for continuous process industries. These power systems required ground detection systems, but locating the fault often proved difficult. Although achieving the initial goal, the ungrounded system provided no control of transient over-voltages.

A capacitive coupling exists between the system conductors and ground in a typical distribution system. As a result, this series resonant L-C circuit can create over-voltages well in excess of line-to-line voltage when subjected to repetitive re-strikes of one phase to ground. This in turn, reduces insulation life resulting in possible equipment failure.

Neutral grounding systems are similar to fuses in that they do nothing until something in the system goes wrong. Then, like fuses, they protect personnel and equipment from damage. Damage comes from two factors, how long the fault lasts and how large the fault current is. Ground relays trip breakers and limit how long a fault lasts and Neutral grounding resistors limit how large the fault current is.

 Importance of Neutral Grounding:

There are many neutral grounding options available for both Low and Medium voltage power systems. The neutral points of transformers, generators and rotating machinery to the earth ground network provides a reference point of zero volts. This protective measure offers many advantages over an ungrounded system, like,

1. Reduced magnitude of transient over voltages
2. Simplified ground fault location
3. Improved system and equipment fault protection
4. Reduced maintenance time and expense
5. Greater safety for personnel
6. Improved lightning protection
7. Reduction in frequency of faults.

Method of Neutral Earthing:

• There are five methods for Neutral earthing.

1. Unearthed Neutral System
2. Solid Neutral Earthed System.
3. Resistance Neutral Earthing System.
   1. Low Resistance Earthing.
   2. High Resistance Earthing.
4. Resonant Neutral Earthing System.
5. Earthing Transformer Earthing.

 (1) Ungrounded Neutral Systems:

•  In ungrounded system there is no internal connection between the conductors and earth. However, as system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces. Consequently, the “ungrounded system” is, in reality, a “capacitive grounded system” by virtue of the distributed capacitance.
• Under normal operating conditions, this distributed capacitance causes no problems. In fact, it is beneficial because it establishes, in effect, a neutral point for the system; As a result, the phase conductors are stressed at only line-to-neutral voltage above ground.
• But problems can rise in ground fault conditions. A ground fault on one line results in full line-to-line voltage appearing throughout the system. Thus, a voltage 1.73 times the normal voltage is present on all insulation in the system. This situation can often cause failures in older motors and transformers, due to insulation breakdown.

•   Advantage:

1. After the first ground fault, assuming it remains as a single fault, the circuit may continue in operation, permitting continued production until a convenient shut down for maintenance can be scheduled.

• Disadvantages:

1. The interaction between the faulted system and its distributed capacitance may cause transient over-voltages (several times normal) to appear from line to ground during normal switching of a circuit having a line-to ground fault (short). These over voltages may cause insulation failures at points other than the original fault.
2. A second fault on another phase may occur before the first fault can be cleared. This can result in very high line-to-line fault currents, equipment damage and disruption of both circuits.
3. The cost of equipment damage.
4. Complicate for locating fault(s), involving a tedious process of trial and error: first isolating the correct feeder, then the branch, and finally, the equipment at fault. The result is unnecessarily lengthy and expensive down downtime.

 (2) Solidly Neutral Grounded Systems:

• Solidly grounded systems are usually used in low voltage applications at 600 volts or less.
• In solidly grounded system, the neutral point is connected to earth.
• Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of the system three phase fault current. However, if the reactance of the generator or transformer is too great, the problem of transient over voltages will not be solved.
• While solidly grounded systems are an improvement over ungrounded systems, and speed up the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides.
• To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must be extend from the source to the furthest point of the system within the same raceway or conduit. Its purpose is to maintain very low impedance to ground faults so that a relatively high fault current will flow thus insuring that circuit breakers or fuses will clear the fault quickly and therefore minimize damage. It also greatly reduces the shock hazard to personnel

• If the system is not solidly grounded, the neutral point of the system would “float” with respect to ground as a function of load subjecting the line-to-neutral loads to voltage unbalances and instability.
• The single-phase earth fault current in a solidly earthed system may exceed the three phase fault current. The magnitude of the current depends on the fault location and the fault resistance. One way to reduce the earth fault current is to leave some of the transformer neutrals unearthed.
• Advantage:

1. The main advantage of solidly earthed systems is low over voltages, which makes the earthing design common at high voltage levels (HV).

• Disadvantage:

1. This system involves all the drawbacks and hazards of high earth fault current: maximum damage and disturbances.
2. There is no service continuity on the faulty feeder.
3. The danger for personnel is high during the fault since the touch voltages created are high.

• Applications:

1. Distributed neutral conductor.
2. 3-phase + neutral distribution.
3. Use of the neutral conductor as a protective conductor with systematic earthing at each transmission pole.
4. Used when the short-circuit power of the source is low.

(3) Resistance earthed systems:

• Resistance grounding has been used in three-phase industrial applications for many years and it resolves many of the problems associated with solidly grounded and ungrounded systems.
• Resistance Grounding Systems limits the phase-to-ground fault currents. The reasons for limiting the Phase to ground Fault current by resistance grounding are:

1. To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers, cables, and rotating machines.
2. To reduce mechanical stresses in circuits/Equipments carrying fault currents.
3. To reduce electrical-shock hazards to personnel caused by stray ground fault.
4. To reduce the arc blast or flash hazard.
5. To reduce the momentary line-voltage dip.
6. To secure control of the transient over-voltages while at the same time.
7. To improve the detection of the earth fault in a power system.

• Grounding Resistors are generally connected between ground and neutral of transformers, generators and grounding transformers to limit maximum fault current as per Ohms Law to a value which will not damage the equipment in the power system and allow sufficient flow of fault current to detect and operate Earth protective relays to clear the fault. Although it is possible to limit fault currents with high resistance Neutral grounding Resistors, earth short circuit currents can be extremely reduced. As a result of this fact, protection devices may not sense the fault.
• Therefore, it is the most common application to limit single phase fault currents with low resistance Neutral Grounding Resistors to approximately rated current of transformer and / or generator.
• In addition, limiting fault currents to predetermined maximum values permits the designer to selectively coordinate the operation of protective devices, which minimizes system disruption and allows for quick location of the fault.
• There are two categories of resistance grounding:

(1)  Low resistance Grounding.

(2)  High resistance Grounding.

• Ground fault current flowing through either type of resistor when a single phase faults to ground will increase the phase-to-ground voltage of the remaining two phases. As a result, conductor insulation and surge arrestor ratings must be based on line-to-line voltage. This temporary increase in phase-to-ground voltage should also be considered when selecting two and three pole breakers installed on resistance grounded low voltage systems.
• The increase in phase-to-ground voltage associated with ground fault currents also precludes the connection of line-to-neutral loads directly to the system. If line-to neutral loads (such as 277V lighting) are present, they must be served by a solidly grounded system. This can be achieved with an isolation transformer that has a three-phase delta primary and a three-phase, four-wire, wye secondary

• Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated with phase-to-phase faults, but both systems significantly reduce or essentially eliminate the arc-flash hazards associated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses and reduce thermal damage to electrical equipment, circuits, and apparatus carrying faulted current.
• The difference between Low Resistance Grounding and High Resistance Grounding is a matter of perception and, therefore, is not well defined. Generally speaking high-resistance grounding refers to a system in which the NGR let-through current is less than 50 to 100 A. Low resistance grounding indicates that NGR current would be above 100 A.
• A better distinction between the two levels might be alarm only and tripping. An alarm-only system continues to operate with a single ground fault on the system for an unspecified amount of time. In a tripping system a ground fault is automatically removed by protective relaying and circuit interrupting devices. Alarm-only systems usually limit NGR current to 10 A or less.
• Rating of The Neutral grounding resistor:

1. 1.    Voltage: Line-to-neutral voltage of the system to which it is connected.
2. 2.    Initial Current: The initial current which will flow through the resistor with rated voltage applied.
3. 3.    Time: The “on time” for which the resistor can operate without exceeding the allowable temperature rise.

(A).Low Resistance Grounded:

• Low Resistance Grounding is used for large electrical systems where there is a high investment in capital equipment or prolonged loss of service of equipment has a significant economic impact and it is not commonly used in low voltage systems because the limited ground fault current is too low to reliably operate breaker trip units or fuses. This makes system selectivity hard to achieve. Moreover, low resistance grounded systems are not suitable for 4-wire loads and hence have not been used in commercial market applications
• A resistor is connected from the system neutral point to ground and generally sized to permit only 200A to 1200 amps of ground fault current to flow. Enough current must flow such that protective devices can detect the faulted circuit and trip it off-line but not so much current as to create major damage at the fault point.

• Since the grounding impedance is in the form of resistance, any transient over voltages are quickly damped out and the whole transient overvoltage phenomena is no longer applicable. Although theoretically possible to be applied in low voltage systems (e.g. 480V),significant amount of the system voltage dropped across the grounding resistor, there is not enough voltage across the arc forcing current to flow, for the fault to be reliably detected. For this reason, low resistance grounding is not used for low voltage systems (under 1000 volts line to-line).
• Advantages:

1. Limits phase-to-ground currents to 200-400A.
2. Reduces arcing current and, to some extent, limits arc-flash hazards associated with phase-to-ground arcing current conditions only.
3. May limit the mechanical damage and thermal damage to shorted transformer and rotating machinery windings.

• Disadvantages:

1. Does not prevent operation of over current devices.
2. Does not require a ground fault detection system.
3. May be utilized on medium or high voltage systems.
4. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer.

• Used: Up to 400 amps for 10 sec are commonly found on medium voltage systems.

(B).High Resistance Grounded:

• High resistance grounding is almost identical to low resistance grounding except that the ground fault current magnitude is typically limited to 10 amperes or less. High resistance grounding accomplishes two things.
• The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable damage is done at the fault point. This means that the faulted circuit need not be tripped off-line when the fault first occurs. Means that once a fault does occur, we do not know where the fault is located. In this respect, it performs just like an ungrounded system.
• The second point is it can control the transient overvoltage phenomenon present on ungrounded systems if engineered properly.
• Under earth fault conditions, the resistance must dominate over the system charging capacitance but not to the point of permitting excessive current to flow and thereby excluding continuous operation

• High Resistance Grounding (HRG) systems limit the fault current when one phase of the system shorts or arcs to ground, but at lower levels than low resistance systems.
• In the event that a ground fault condition exists, the HRG typically limits the current to 5-10A.
• HRG’s are continuous current rated, so the description of a particular unit does not include a time rating. Unlike NGR’s, ground fault current flowing through a HRG is usually not of significant magnitude to result in the operation of an over current device. Since the ground fault current is not interrupted, a ground fault detection system must be installed.
• These systems include a bypass contactor tapped across a portion of the resistor that pulses (periodically opens and closes). When the contactor is open, ground fault current flows through the entire resistor. When the contactor is closed a portion of the resistor is bypassed resulting in slightly lower resistance and slightly higher ground fault current.
• To avoid transient over-voltages, an HRG resistor must be sized so that the amount of ground fault current the unit will allow to flow exceeds the electrical system’s charging current. As a rule of thumb, charging current is estimated at 1A per 2000KVA of system capacity for low voltage systems and 2A per 2000KVA of system capacity at 4.16kV.
• These estimated charging currents increase if surge suppressors are present. Each set of suppressors installed on a low voltage system results in approximately 0.5A of additional charging current and each set of suppressors installed on a 4.16kV system adds 1.5A of additional charging current.
•  A system with 3000KVA of capacity at 480 volts would have an estimated charging current of 1.5A.Add one set of surge suppressors and the total charging current increases by 0.5A to 2.0A. A standard 5A resistor could be used on this system. Most resistor manufacturers publish detailed estimation tables that can be used to more closely estimate an electrical system’s charging current.
• Advantages:

1. Enables high impedance fault detection in systems with weak capacitive connection to earth
2. Some phase-to-earth faults are self-cleared.
3. The neutral point resistance can be chosen to limit the possible over voltage transients to 2.5 times the fundamental frequency maximum voltage.
4. Limits phase-to-ground currents to 5-10A.
5. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase-to-ground arcing current conditions only.
6. Will eliminate the mechanical damage and may limit thermal damage to shorted transformer and rotating machinery windings.
7. Prevents operation of over current devices until the fault can be located (when only one phase faults to ground).
8. May be utilized on low voltage systems or medium voltage systems up to 5kV. IEEE Standard 141-1993 states that “high resistance grounding should be restricted to 5kV class or lower systems with charging currents of about 5.5A or less and should not be attempted on 15kV systems, unless proper grounding relaying is employed”.
9. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer.

• Disadvantages:

1. Generates extensive earth fault currents when combined with strong or moderate capacitive connection to earth Cost involved.
2. Requires a ground fault detection system to notify the facility engineer that a ground fault condition has occurred.

(4) Resonant earthed system:

• Adding inductive reactance from the system neutral point to ground is an easy method of limiting the available ground fault from something near the maximum 3 phase short circuit capacity (thousands of amperes) to a relatively low value (200 to 800 amperes).
• To limit the reactive part of the earth fault current in a power system a neutral point reactor can be connected between the transformer neutral and the station earthing system.
• A system in which at least one of the neutrals is connected to earth through an

1. Inductive reactance.
2. Petersen coil / Arc Suppression Coil / Earth Fault Neutralizer.

• The current generated by the reactance during an earth fault approximately compensates the capacitive component of the single phase earth fault current, is called a resonant earthed system.
• The system is hardly ever exactly tuned, i.e. the reactive current does not exactly equal the capacitive earth fault current of the system.
• A system in which the inductive current is slightly larger than the capacitive earth fault current is over compensated. A system in which the induced earth fault current is slightly smaller than the capacitive earth fault current is under compensated

• However, experience indicated that this inductive reactance to ground resonates with the system shunt capacitance to ground under arcing ground fault conditions and creates very high transient over voltages on the system.
• To control the transient over voltages, the design must permit at least 60% of the 3 phase short circuit current to flow underground fault conditions.
• Example. A 6000 amp grounding reactor for a system having 10,000 amps 3 phase short circuit capacity available. Due to the high magnitude of ground fault current required to control transient over voltages, inductance grounding is rarely used within industry.

• Petersen Coils:

• A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that the capacitive current in the earth fault is compensated by an inductive current passed by the Petersen Coil. A small residual current will remain, but this is so small that any arc between the faulted phase and earth will not be maintained and the fault will extinguish. Minor earth faults such as a broken pin insulator, could be held on the system without the supply being interrupted. Transient faults would not result in supply interruptions.
• Although the standard ‘Peterson coil’ does not compensate the entire earth fault current in a network due to the presence of resistive losses in the lines and coil, it is now possible to apply ‘residual current compensation’ by injecting an additional 180° out of phase current into the neutral via the Peterson coil. The fault current is thereby reduced to practically zero. Such systems are known as ‘Resonant earthing with residual compensation’, and can be considered as a special case of reactive earthing.
• Resonant earthing can reduce EPR to a safe level. This is because the Petersen coil can often effectively act as a high impedance NER, which will substantially reduce any earth fault currents, and hence also any corresponding EPR hazards (e.g. touch voltages, step voltages and transferred voltages, including any EPR hazards impressed onto nearby telecommunication networks).
• Advantages:

1. Small reactive earth fault current independent of the phase to earth capacitance of the system.
2. Enables high impedance fault detection.

• Disadvantages:

1. Risk of extensive active earth fault losses.
2. High costs associated.

(5) Earthing Transformers:

• For cases where there is no neutral point available for Neutral Earthing (e.g. for a delta winding), an earthing transformer may be used to provide a return path for single phase fault currents

• In such cases the impedance of the earthing transformer may be sufficient to act as effective earthing impedance. Additional impedance can be added in series if required. A special ‘zig-zag’ transformer is sometimes used for earthing delta windings to provide a low zero-sequence impedance and high positive and negative sequence impedance to fault currents.

Conclusion:

• Resistance Grounding Systems have many advantages over solidly grounded systems including arc-flash hazard reduction, limiting mechanical and thermal damage associated with faults, and controlling transient over voltages.
• High resistance grounding systems may also be employed to maintain service continuity and assist with locating the source of a fault.
• When designing a system with resistors, the design/consulting engineer must consider the specific requirements for conductor insulation ratings, surge arrestor ratings, breaker single-pole duty ratings, and method of serving phase-to-neutral loads.

Comparison of Neutral Earthing System:

Condition	Un grounded	Solid Grounded	Low Resistance Grounded	High Resistance Grounded	Reactance Grounding
Immunity to Transient Over voltages	Worse	Good	Good	Best	Best
73% Increase in Voltage Stress Under Line-to-Ground Fault Condition	Poor	Best	Good	Poor
Equipment Protected	Worse	Poor	Better	Best	Best
Safety to Personnel	Worse	Better	Good	Best	Best
Service Reliability	Worse	Good	Better	Best	Best
Maintenance Cost	Worse	Good	Better	Best	Best
Ease of Locating First Ground Fault	Worse	Good	Better	Best	Best
Permits Designer to CoordinateProtective Devices	Not Possible	Good	Better	Best	Best
Reduction in Frequency of Faults	Worse	Better	Good	Best	Best
Lighting Arrestor	Ungroundedneutral type	Grounded-neutraltype	Ungroundedneutral type	Ungroundedneutral type	Ungroundedneutral type
Current for phase-to ground fault in percent ofthree-phase fault current	Less than 1%	Varies, may be 100% or greater	5 to 20%	Less than 1%	5 to 25%

Reference:

• By Michael D. Seal, P.E., GE Senior Specification Engineer.
• IEEE Standard 141-1993, “Recommended Practice for Electrical Power Distribution for Industrial Plants”
• Don Selkirk, P.Eng, Saskatoon, Saskatchewan Canada

EHV/HV Cable Sheath Earthing

EHV/HV Cable Sheath Earthing:

 Introduction:

• In urban areas, high voltage underground cables are commonly used for the transmission and distribution of electricity. Such high voltage cables have metallic sheaths or screens surrounding the conductors, and/or armour and metallic wires surrounding the cables. During earth faults applied to directly earthed systems, these metallic paths are expected to carry a substantial proportion of the total fault current, which would otherwise flow through the general mass of earth, while returning to system neutrals. These alternative return paths must be considered when determining the extent of the grid potential rise at an electrical plant due to earth faults.
• For safety and reliable operation, the shields and metallic sheaths of power cables must be grounded. Without grounding, shields would operate at a potential considerably above ground. Thus, they would be hazardous to touch and would cause rapid degradation of the jacket or other material intervening between shield and ground. This is caused by the capacitive charging current of the cable insulation that is on the order of 1 mA/ft of conductor length.
• This current normally flows, at power frequency, between the conductor and the earth electrode of the cable, normally the shield. In addition, the shield or metallic sheath provides a fault return path in the event of insulation failure, permitting rapid operation of the protection devices.
• In order to reduce Circulating current and electric potential difference between the sheathings of single core three-phase cables, the sheathing is grounded and bonded at one or both ends of the cables. If the cable is long, double bonding has to be carried out which leads to circulating currents and increased total power loss. Raising the sheath’s resistance, by decreasing its cross section and increasing its resistivity, can reduce this almost to the level of the core losses.
• However, in case of an earth fault, a considerable portion of the fault current flows through the increased sheath resistance, creating much higher power in the sheaths than in the faulty core. A simple solution, a conductor rod buried into the soil above or under the cable can divert this power from the sheaths.

Cable Screen:

 (1) Purpose of cable screen:

•                    
• Cable screen controls the electric field stress in the cable insulation.
• Cable Screen Provides return path for Cable neutral and fault current.
• If the screen is earthed at two ends than it provides Shielding for electromagnetic radiation.
• Enclosing dangerous high voltage with earth potential for safety.

 (2) Purpose of bonding cable screens at both ends:

• The electric power losses in a cable circuit are dependent on the currents flowing in the metallic sheaths of the cables so by reducing the current flows in metallic sheath by different methods of bonding we can increases the load current carrying capacity (ampacity) of the cable.
• It provides low impedance fault current return path and provides neutral point for the circuit.
• It provides shielding of electromagnetic field.

(3) Induced voltage & circulating circulating current in cable screen:

•  Electromagnetic coupling between the core and screen Electromagnetic screen.
• If the cable screen is single point bonded, no electrical continuity and mmf generates a voltage.
• If the cable screen is bonded at both ends, the mmf will cause circulating current to flow if there is electrical continuity.
• The circulating current produces an opposing magnetic field.
• Suitable bonding method should be employed to meet the standing voltage limit and keep Circulating current to an acceptable level.

Laying Method of Cable:

•  The three Single core cables in a 3-phase circuit can be placed in different formations. Typical formations include trefoil (triangular) and flat formations.

(1) Trefoil Formation:

• To minimize the electromechanical forces between the cables under short-circuit conditions, and to avoid eddy-current heating in nearby steelwork due to magnetic fields set up by load currents, the three single-core cables comprising the three phases of a 3-phase circuit are always run clamped in ‘Trefoil’ formation.
• Advantage:

1. This type of Formation minimizes the sheath circulating currents induced by the magnetic flux linking the cable conductors and metallic sheath or copper wire screens.
2. This configuration is generally used for cables of lower voltages (33 to 132kV) and of smaller conductor sizes.

• Disadvantages:

1.  The trefoil formation is not appropriate for heat dissipation because there is an appreciable mutual heating effect of the three cables.
2. The cumulated heat in cables and cable trench has the effect of reducing the cable rating and accelerating the cable ageing.

(2) Flat Formation:

• This is a most common method for Laying LT Cable.
• This formation is appropriate for heat dissipation and to increase cable rating.
• The Formation choice is totally deepened on several factors like screen bonding method, conductor area and available space for installation.

 

Type of Core and Induced Voltage:

 (1)  Three Core Cable:

• For LT application, typically for below 11 kV.
• Well balanced magnetic field from Three Phase.
• Induced voltages from three phases sum to zero along the entire length of the cable.
• Cable screen should be earthed at both ends
• Virtually zero induced voltage or circulating current under steady state operation.

(2)  Single Core Cable:

• For HV application, typically for 11 kV and above.
• Single–core cables neglects the use of ferromagnetic material for screen, sheath and armoring.
• Induced voltage is mainly contributed by the core currents in its own phase and other two phases.If cables are laid in a compact and symmetrical formation, induced in the screen can be minimized.
• A suitable screen bonding method should be used for single–core cables to prevent Excessive circulating current, high induced standing voltage.igh voltage.

Accessories for HT Cable Sheath Bonding:

 (1)  Function of Link Box?

• Link Box is electrically and mechanically one of the integral accessories of HV underground above ground cable bonding system, associated with HV XLPE power cable systems.
• Link boxes are used with cable joints and terminations to provide easy access to shield breaks for test purposes and to limit voltage build-up on the sheath

• Lightning, fault currents and switching operations can cause over voltages on the cable sheath. The link box optimizes loss management in the cable shield on cables grounded both sides.
• In HT Cable the bonding system is so designed that the cable sheaths are bonded and earthed or with SVL in such way as to eliminate or reduce the circulating sheath currents.
• Link Boxes are used with cable joints and terminations to provide easy access to shield breaks for test purposes and to limit voltage build-up on the sheath. The link box is part of bonding system, which is essential of improving current carrying capacity and human protection.

(2)  Sheath Voltage Limiters (SVL) (Surge Arrestors):

• SVL is protective device to limit induce voltages appearing on the bonded cable system due to short circuit.
• It is necessary to fit SVL’s between the metallic screen and ground inside the link box. The screen separation of power cable joint (insulated joint) will be protected against possible damages as a result of induced voltages caused by short circuit/break down.

Type of Sheath Bonding for HT Cable:

 There is normally Three Type of Bonding for LT/HT Cable Screen.

(1)  Single Point Bonded.

1. One Side Single Point Bonded System.
2. Split Single Point Bonded System.

(2)  Both End Bonded System

(3)  Cross Bonded System

(1) Single point bonded system:

 (A) One Side Single Bonded System:

• A system is single point bonded if the arrangements are such that the cable sheaths provide no path for the flow of circulating currents or external fault currents.
• This is the simplest form of special bonding. The sheaths of the three cable sections are connected and grounded at one point only along their length. At all other points there will be a voltage between sheath and ground and between screens of adjacent phases of the cable circuit that will be at its maximum at the farthest point from the ground bond.
• This induced voltage is proportional to the cable length and current. Single-point bonding can only be used for limited route lengths, but in general the accepted screen voltage potential limits the length

• The sheaths must therefore be adequately insulated from ground. Since there is no closed sheath circuit, except through the sheath voltage limiter, current does not normally flow longitudinally along the sheaths and no sheath circulation current loss occurs.
• Open circuit in cable screen, no circulating current.
• Zero volt at the earthed end, standing voltage at the unearthed end.
• Optional PVC insulated earth continuity conductor required to provide path for fault current, if returning from earth is undesirable, such as in a coal mine.
• SVL installed at the unearthed end to protect the cable insulation during fault conditions.
• Induced voltage proportional to the length of the cable and the current carried in the cable .
• Zero volt with respect to the earth grid voltage at the earthed end, standing voltage at the unearthed end.
• Circulating current in the earth–continuity conductor is not significant, as magnetic fields from phases are partially balanced.
• The magnitude of the standing voltage is depended on the magnitude of the current flows in the core, much higher if there is an earth fault.
• High voltage appears on the unearthed end can cause arcing and damage outer PVC sheath.
• The voltage on the screen during a fault also depends on the earthing condition.

Standing voltage at the unearthed end during earth fault condition.

• During a ground fault on the power system the zero sequence current carried by the cable conductors could return by whatever external paths are available. A ground fault in the immediate vicinity of the cable can cause a large difference in ground potential rise between the two ends of the cable system, posing hazards to personnel and equipment.
•  For this reason, single-point bonded cable installations need a parallel ground conductor, grounded at both ends of the cable route and installed very close to the cable conductors, to carry the fault current during ground faults and to limit the voltage rise of the sheath during ground faults to an acceptable level.
• The parallel ground continuity conductor is usually insulated to avoid corrosion and transposed, if the cables are not transposed, to avoid circulating currents and losses during normal operating conditions.
  • Voltage at the unearthed end during an earth fault consists of two voltage components. Induced voltage due to fault current in the core.

Advantage:

• No circulating current.
• No heating in the cable screen.
• Economical.

Disadvantage:

• Standing voltage at the un–earthed end.
• Requires SVL if standing voltage during fault is excessive.
• Requires additional earth continuity conductor for fault current if earth returned current is undesirable. Higher magnetic fields around the cable compared to solidly bonded system.
• Standing voltage on the cable screen is proportional to the length of the cable and the magnitude of current in the core.
• Typically suitable for cable sections less than 500 m, or one drum length.

(B) Split Single Point-bonded System:

• It is also known as double length single point bonding System.
• Cable screen continuity is interrupted at the midpoint and SVLs need to be fitted at each side of the isolation joint.
• Other requirements are identical to single–point–bonding system like SVL, Earth continuity Conductor, Transposition of earth continuity conductor.
• Effectively two sections of single–point–bonding.
• No circulating current and Zero volt at the earthed ends, standing voltage at the sectionalizing joint.

 

Advantages:

• No circulating current in the screen.
• No heating effect in the cable screen.
• Suitable for longer cable section compared to single–point–bonding system and solidly bonded single-core system.
• Economical.

Disadvantages:

• Standing voltage exists at the screen and sectionalizing insulation joint.
• Requires SVL to protect the un–earthed end.
• Requires separate earth continuity conductor for zero sequence current.
• Not suitable for cable sections over 1000 m.
• Suitable for 300~1000 m long cable sections, double the length of single–point–bonding system.

(2) Both End Solidly Bonded (Single-core cable) systems.

• Most Simple and Common method.
• Cable screen is bonded to earth grids at both ends (via link box).
• To eliminate the induced voltages in Cable Screen is to bond (Earth) the sheath at both ends of the cable circuit.
•  This eliminates the need for the parallel continuity conductor used in single bonding systems. It also eliminates the need to provide SVL, such as that used at the free end of single-point bonding cable circuits
• Significant circulating current in the screen Proportional to the core current and cable length and de rates cable.
• Could lay cable in compact trefoil formation if permissible.
• Suitable for route length of more than 500 Meter.
• Very small standing voltage in the order of several volts.

                                 

 Advantages:

• Minimum material required.
• Most economical if heating is not a main issue.
• Provides path for fault current, minimizing earth return current and EGVR at cable destination.
• Does not require screen voltage limiter (SVL).
• Less electromagnetic radiation.

Disadvantages:

• Provides path for circulating current.
• Heating effects in cable screen, greater losses .Cable therefore might need to be de–rated or larger cable required.
• Transfers voltages between sites when there is an EGVR at one site.
• Can lay cables in trefoil formation to reduce screen losses .
• Normally applies to short cable section of tens of meters long. Circulating current is proportional to the length of the cable and the magnitude of the load current.

(3) Cross-bonded cable system.

• A system is cross-bonded if the arrangements are such that the circuit provides electrically continuous sheath runs from earthed termination to earthed termination but with the sheaths so sectionalized and cross-connected in order to reduce the sheath circulating currents.
• In This Type voltage will be induced between screen and earth, but no significant current will flow.
• The maximum induced voltage will appear at the link boxes for cross-bonding. This method permits a cable current-carrying capacity as high as with single-point bonding but longer route lengths than the latter. It requires screen separation and additional link boxes.
• For cross bonding, the cable length is divided into three approximately equal sections. Each of the three alternating magnetic fields induces a voltage with a phase shift of 120° in the cable shields.
• The cross bonding takes place in the link boxes. Ideally, the vectorial addition of the induced voltages results in U (Rise) = 0. In practice, the cable length and the laying conditions will vary, resulting in a small residual voltage and a negligible current. Since there is no current flow, there are practically no losses in the screen.
• The total of the three voltages is zero, thus the ends of the three sections can be grounded.
• Summing up induced voltage in sectionalized screen from each phase resulting in neutralization of induced voltages in three consecutive minor sections.
• Normally one drum length (500 m approx) per minor section.
• Sectionalizing position and cable jointing position should be coincident.
• Solidly earthed at major section joints.
• Transpose cable core to balance the magnitude of induced voltages to be summed up.
• Link box should be used at every sectionalizing joint and balanced impedance in all phases.
• Induced voltage magnitude profile along the screen of a major section in the cross–bonding cable system.
• Virtually zero circulating current and Voltage to the remote earth at the solidly earthed ends.
• In order to obtain optimal result, two ‘‘crosses’’ exist. One is Transposition of cable core crossing cable core at each section and second is Cross bond the cable screens effectively no transposition of screen.
  
• Cross bonding of cable screen: It is cancelled induced voltage in the screen at every major Section joint.
  
• Transposition of cables: It is ensure voltages to be summed up have similar magnitude .Greater standing voltage at the screen of the outer cable.
• Standing voltages exist at screen and majority of section joints cable and joints must be installed as an insulated screen system.

Requirement of transpose for cables core.

• If core not transposed, not well neutralized resulting in some circulating currents.
• Cable should be transposed and the screen needs to be cross bonded at each sectionalizing joint position for optimal neutralization

                 

 Advantage:

• Not required any earth continuity conductor.
• Virtually zero circulating current in the screen.
• Standing voltage in the screen is controlled.
• Technically superior than other methods.
• Suitable for long distance cable network.

Disadvantage:

• Technically complicated.
• More expensive.

Bonding Method Comparison:

Earthing Method	Standing Voltage at Cable End	Sheath Voltage Limiter Required	Application
Single End Bonding	Yes	Yes	Up to 500 Meter
Double End Bonding	No	No	Up to 1 Km and Substations  short  connections, hardly applied for HV cables, rather for MV and LV cables
Cross Bonding	Only at cross bonding points	Yes	Long distance connectionswhere joints are required

Sheath Losses according to type of Bonding:

• Sheath losses are current-dependent losses and are generated by the induced currents when load current flows in cable conductors.
• The sheath currents in single-core cables are induced by “transformer” effect; i.e.by the magnetic field of alternating current flowing in cable conductor which induces voltages in cable sheath or other parallel conductors.
• The sheath induced electromotive forces (EMF) generate two types of losses: circulating current losses (Y1) and eddy current losses (Y2), so the total losses in cable metallic sheath are: Y= Y1+Y2
• The eddy currents circulating radially and longitudinally of cable sheaths are generated on similar principles of skin and proximity effects i.e. they are induced by the conductor currents, sheath circulating currents and by currents circulating in close proximity current carrying conductors.
• They are generated in cable sheath irrespective of bonding system of single core cables or of three-core cables
• The eddy currents are generally of smaller magnitude when comparing with circuit (circulating) currents of solidly bonded cable sheaths and may be neglects except in the case of large segmental conductors and are calculated in accordance with formulae given in the IEC60287.
• Circulating currents are generated in cable sheath if the sheaths form a closed loop when bonded together at the remote ends or intermediate points along the cable route.
• These losses are named sheath circulating current losses and they are determined by the magnitude of current in cable conductor, frequency, mean diameter, the resistance of cable sheath and the distance between single-core cables.

Conclusion:

• There is much disagreement as to whether the cable shield should be grounded at both ends or at only one end. If grounded at only one end, any possible fault current must traverse the length from the fault to the grounded end, imposing high current on the usually very light shield conductor. Such a current could readily damage or destroy the shield and require replacement of the entire cable rather than only the faulted section.
• With both ends grounded, the fault current would divide and flow to both ends, reducing the duty on the shield, with consequently less chance of damage.
• Multiple grounding, rather than just grounding at both ends, is simply the grounding of the cable shield or sheath at all access points, such as manholes or pull boxes. This also limits possible shield damage to only the faulted section.

References:

1. Mitton Consulting.
2. EMElectricals

Abstract of NEC for Size of Cable for Single or Group of Motors

Abstract of National Electrical Code for Size of Cable for Motors:

NEC Code 430.22 (Size of Cable for Single Motor):

• Size of Cable for Branch circuit which has Single Motor connection is 125% of Motor Full Load Current Capacity.
• Example:what is the minimum rating in amperes for Cables supplying 1 No of 5 hp, 415-volt, 3-phase motor at 0.8 Power Factor.
  • Full-load currents for 5 hp = 7Amp.
  • Min Capacity of Cable= (7X125%) =8.75 Amp.

NEC Code 430.6(A) (Size of Cable for Group of Motors or Elect.Load).

• Cables or Feeder which is supplying more than one motors other load(s), shall have an ampacity not less than 125 % of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A).
• For Calculating minimum Ampere Capacity of Main feeder and Cable is 125% of Highest Full Load Current + Sum of Full Load Current of remaining Motors.
• Example:what is the minimum rating in amperes for Cables supplying 1 No of 5 hp, 415-volt, 3-phase motor at 0.8 Power Factor , 1 No of 10 hp, 415-volt, 3-phase motor at 0.8 Power Factor, 1 No of 15 hp, 415-volt, 3-phase motor at 0.8 Power Factor and 1 No of 5hp, 230-volt, single-phase motor at 0.8 Power Factor?
  • Full-load currents for 5 hp = 7Amp
  • Full-load currents for 10 hp = 13Amp
  • Full-load currents for 15 hp = 19Amp
  • Full-load currents for 10 hp (1 Ph) = 21Amp
  • Here Capacity wise Large Motor is 15 Hp but Highest Full Load current is 21Amp of 5hp Single Phase Motor so 125% of Highest Full Load current is 21X125%=26.25Amp
  • Min Capacity of Cable= (26.25+7+13+19) =65.25 Amp.

NEC Code 430.24 (Size of Cable for Group of Motors or Electrical Load).

• As specified in 430.24, conductors supplying two or more motors must have an ampacity not less than 125 % of the full-load current rating of the highest rated motor +  the sum of the full-load current ratings of all the other motors in the group or on the same phase.
•  It may not be necessary to include all the motors into the calculation. It is permissible to balance the motors as evenly as possible between phases before performing motor-load calculations.
• Example:what is the minimum rating in amperes for conductors supplying 1No of 10 hp, 415-volt, 3-phase motor at 0.8 P.F and 3 No of 3 hp, 230-volt, single-phase motors at 0.8 P.F.
  • The full-load current for a 10 hp, 415-volt, 3-phase motor is 13 amperes.
  • The Full-load current for single-phase 3 hp motors is 12 amperes.
  • Here for Load Balancing one Single Phase Motor is connected on R Phase Second in B Phase and third is in Y Phase.
  • Because the motors are balanced between phases, the full-load current on each phase is 25 amperes (13 + 12 = 25).
  • Here multiply 13 amperes by 125 %=(13 × 125% = 16.25 Amp). Add to this value the full-load currents of the other motor on the same phase (16.25 + 12 = 28.25 Amp).
  •  The minimum rating in amperes for conductors supplying these motors is 28 amperes.

NEC 430/32 Size of Overload Protection for Motor:

• Overload protection (Heater or Thermal cut out protection) would be a device that thermally protects a given motor from damage due to heat when loaded too heavy with work.
• All continuous duty motors rated more than 1HP must have some type of an approved overload device.
• An overload shall be installed on each conductor that controls the running of the motor rated more than one horsepower. NEC 430/37 plus the grounded leg of a three phase grounded system must contain an overload also. This Grounded leg of a three phase system is the only time you may install an overload or over – current device on a grounded conductor that is supplying a motor.
• To Find the motor running overload protection size that is required, you must multiply the F.L.C. (full load current) with the minimum or the maximum percentage ratings as follows;

Maximum Overload

• Maximum overload = F.L.C. (full load current of a motor) X allowable % of the maximum setting of an overload,
• 130% for motors, found in NEC Article 430/34.
• Increase of 5% allowed if the marked temperature rise is not over 40 degrees or the marked service factor is not less than 1.15.

Minimum Overload

• Minimum Overload = F.L.C. (full load current of a motor) X allowable % of the minimum setting of an overload,
• 115% for motors found in NEC Article 430/32/B/1.
• Increase of 10% allowed to 125% if the marked temperature rise is not over 40 degrees or the marked service factor is not less than 1.15.

HIPOT Testing

What is HIPOT Testing (Dielectric Strength Test):

• Hipot Test is short name of high potential (high voltage) Teat and It also known as Dielectric Withstand Test. A hipot test checks for “good isolation.” Hipot test makes surety of no current will flow from one point to another point. Hipot test is the opposite of a continuity test.
• Continuity Test checks surety of current flows easily from one point to another point while Hipot Test checks surety of current would not flow from one point to another point (and turn up the voltage really high just to make sure no current will flow).

Importance of HIPOT Testing:

• The hipot test is a nondestructive test that determines the adequacy of electrical insulation for the normally occurring over voltage transient. This is a high-voltage test that is applied to all devices for a specific time in order to ensure that the insulation is not marginal.
• Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive or corrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in cables. Inadequate creepage and clearance distances introduced during the manufacturing process.
• HIPOT test is applied after tests such as fault condition, humidity, and vibration to determine whether any degradation has taken place.
• The production-line hipot test, however, is a test of the manufacturing process to determine whether the construction of a production unit is about the same as the construction of the unit that was subjected to type testing. Some of the process failures that can be detected by a production-line hipot test include, for example, a transformer wound in such a way that creepage and clearance have been reduced. Such a failure could result from a new operator in the winding department. Other examples include identifying a pinhole defect in insulation or finding an enlarged solder footprint.
• As per IEC 60950, The Basic test Voltage for  Hipot test is the 2X (Operating Voltage) + 1000 V
• The reason for using 1000 V as part of the basic formula is that the insulation in any product can be subjected to normal day-to-day transient over voltages. Experiments and research have shown that these over voltages can be as high as 1000 V.

Test method for HIPOT Test:

• Hipot testers usually connect one side of the supply to safety ground (Earth ground). The other side of the supply is connected to the conductor being tested. With the supply connected like this there are two places a given conductor can be connected: high voltage or ground.
• When you have more than two contacts to be hipot tested you connect one contact to high voltage and connect all other contacts to ground. Testing a contact in this fashion makes sure it is isolated from all other contacts.
• If the insulation between the two is adequate, then the application of a large voltage difference between the two conductors separated by the insulator would result in the flow of a very small current. Although this small current is acceptable, no breakdown of either the air insulation or the solid insulation should take place.
• Therefore, the current of interest is the current that is the result of a partial discharge or breakdown, rather than the current due to capacitive coupling.

Time Duration for HIPOT Test:

• The test duration must be in accordance with the safety standard being used.
• The test time for most standards, including products covered under IEC 60950, is 1 minute.
• A typical rule of thumb is 110 to 120% of 2U + 1000 V for 1–2 seconds.

 Current Setting for HIPOT Test:

• Most modern hipot testers allow the user to set the current limit. However, if the actual leakage current of the product is known, then the hipot test current can be predicted.
• The best way to identify the trip level is to test some product samples and establish an average hipot current. Once this has been achieved, then the leakage current trip level should be set to a slightly higher value than the average figure.
• Another method of establishing the current trip level would be to use the following mathematical formula:  E(Hipot) / E(Leakage) = I(Hipot) / 2XI(Leakage)
• The hipot tester current trip level should be set high enough to avoid nuisance failure related to leakage current and, at the same time, low enough not to overlook a true breakdown in insulation.

Test Voltage for HIPOT Test:

• The majority of safety standards allow the use of either ac or dc voltage for a hipot test.
• When using ac test voltage, the insulation in question is being stressed most when the voltage is at its peak, i.e., either at the positive or negative peak of the sine wave.
• Therefore, if we use dc test voltage, we ensure that the dc test voltage is under root 2 (or 1.414) times the ac test voltage, so the value of the dc voltage is equal to the ac voltage peaks.
•  For example, for a 1500-V-ac voltage, the equivalent dc voltage to produce the same amount of stress on the insulation would be 1500 x 1.414 or 2121 V dc.

Advantage / Disadvantage of use DC Voltage for Hipot Test:

• One of the advantages of using a dc test voltage is that the leakage current trip can be set to a much lower value than that of an ac test voltage. This would allow a manufacturer to filter those products that have marginal insulation, which would have been passed by an ac tester.
• when using a dc hipot tester, the capacitors in the circuit could be highly charged and, therefore, a safe-discharge device or setup is needed. However, it is a good practice to always ensure that a product is discharged, regardless of the test voltage or its nature, before it is handled.
• It applies the voltage gradually. By monitoring the current flow as voltages increase, an operator can detect a potential insulation breakdown before it occurs. A minor disadvantage of the dc hipot tester is that because dc test voltages are more difficult to generate, the cost of a dc tester may be slightly higher than that of an ac tester.
• The main advantage of the dc test is DC Voltage does not produce harmful discharge as readily occur in AC.
  It can be applied at higher levels without risk or injuring good insulation. This higher potential can literally “sweep-out” far more local defects.
• The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thus producing a “snow balling” effect which leads to the small visible dielectric puncture usually observed. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance.

Advantage / Disadvantage of use AC Voltage for Hipot Test:

• One of the advantages of an ac hipot test is that it can check both voltage polarities, whereas a dc test charges the insulation in only one polarity. This may become a concern for products that actually use ac voltage for their normal operation. The test setup and procedures are identical for both ac and dc hipot tests.
• A minor disadvantage of the ac hipot tester is that if the circuit under test has large values of Y capacitors, then, depending on the current trip setting of the hipot tester, the ac tester could indicate a failure. Most safety standards allow the user to disconnect the Y capacitors prior to testing or, alternatively, to use a dc hipot tester. The dc hipot tester would not indicate the failure of a unit even with high Y capacitors because the Y capacitors see the voltage but don’t pass any current.

 Step for HIPOT Testing:

• Only electrically qualified workers may perform this testing.
• Open circuit breakers or switches to isolate the circuit or Cable that will be hi-pot tested.
• Confirm that all equipment or Cable that is not to be tested is isolated from the circuit under test.
• The limited approach boundary for this hi-pot procedure at 1000 volts is 5 ft. (1.53m) so place barriers around the terminations of cables and equipment under test to prevent unqualified persons from crossing this boundary.
• Connect the ground lead of the HIPOT Tester to a suitable building ground or grounding electrode conductor. Attach the high voltage lead to one of the isolated circuit phase conductors.
• Switch on the HIPOT Tester. Set the meter to 1000 Volts or pre decide DC Voltage. Push the “Test” button on the meter and after one minute observe the resistance reading.  Record the reading for reference.
• At the end of the one minute test, switch the HIPOT Tester from the high potential test mode to the voltage measuring mode to confirm that the circuit phase conductor and voltage of HIPOT Tester are now reading zero volts.
• Repeat this test procedure for all circuit phase conductors testing each phase to ground and each phase to each phase.
• When testing is completed disconnect the HIPOT Tester from the circuits under test and confirm that the circuits are clear to be re-connected and re-energized.
• To PASS the unit or Cable under Test must be exposed to a minimum Stress of pre decide Voltage for 1 minute without any Indication of Breakdown. For Equipments with total area less than 0.1 m2, the insulation resistance shall not be less than 400 MΩ. For Equipment with total area larger than 0.1 m2 the measured insulation resistance times the area of the module shall not be less than 40 MΩ⋅m2.

Safety precautions during HIPOT Test:

• During a HIPOT Test, There may be at some risk so to minimize risk of injury from electrical shock make sure HIPOT equipment follows these guidelines:

1. The total charge you can receive in a shock should not exceed 45 uC.
2. The total hipot energy should not exceed 350 mJ.
3. The total current should not exceed 5 mA peak (3.5 mA rms)
4. The fault current should not stay on longer than 10 mS.
5. If the tester doesn’t meet these requirements then make sure it has a safety interlock system that guarantees you cannot contact the cable while it is being hipot tested.

• For Cable:

1. Verify the correct operation of the safety circuits in the equipment every time you calibrate it.
2. Don’t touch the cable during hipot testing.
3. Allow the hipot testing to complete before removing the cable.
4. Wear insulating gloves.
5. Don’t allow children to use the equipment.
6. If you have any electronic implants then don’t use the equipment.

What is Earthing

The main reason for doing earthing in electrical network is for the safety. When all metallic parts in electrical equipments are grounded then if the insulation inside the equipments fails there are no dangerous voltages present in the equipment case. If the live wire touches the grounded case then the circuit is effectively shorted and fuse will immediately blow. When the fuse is blown then the dangerous voltages are away.

Purpose of Earthing:

(1)  Safety for Human life/ Building/Equipments:

• To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user
• To protect buildings, machinery & appliances under fault conditions.
• To ensure that all exposed conductive parts do not reach a dangerous potential.
• To provide safe path to dissipate lightning and short circuit currents.
• To provide stable platform for operation of sensitive  electronic equipments   i.e. To maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment .

(2)  Over voltage protection:

• Lightning, line surges or unintentional contact with higher voltage lines can cause dangerously high voltages to the electrical distribution system. Earthing provides an alternative path around the electrical system to minimize damages in the System.

(3)  Voltage stabilization:

• There are many sources of electricity. Every transformer can be considered a separate source. If there were not a common reference point for all these voltage sources it would be extremely difficult to calculate their relationships to each other. The earth is the most omnipresent conductive surface, and so it was adopted in the very beginnings of electrical distribution systems as a nearly universal standard for all electric systems.

Conventional methods of earthing:

(1)  Plate type Earthing:

• Generally for plate type earthing normal Practice is to use
• Cast iron plate of size 600 mm x600 mm x12 mm. OR
• Galvanized iron plate of size 600 mm x600 mm x6 mm. OR
• Copper plate of size 600 mm * 600 mm * 3.15 mm
• Plate  burred at the depth of 8 feet in the vertical position and GI strip of size 50 mmx6 mm bolted with the plate is brought up to the ground level.
• These types of earth pit are generally filled with alternate layer of charcoal & salt up to 4 feet from the bottom of the pit.

(2)  Pipe type Earthing:

• For Pipe type earthing normal practice is to use
• GI pipe [C-class] of 75 mm diameter, 10 feet long welded with 75 mm diameter GI flange having 6 numbers of holes for the connection of earth wires and inserted in ground by auger method.
• These types of earth pit are generally filled with alternate layer of charcoal & salt or earth reactivation compound.

Method for Construction of Earthing Pit (Indian Electricity Board):

• Excavation on earth for a normal earth Pit size is 1.5M X 1.5M X 3.0 M.
• Use 500 mm X 500 mm X 10 mm GI Plate or Bigger Size for more Contact of Earth and reduce Earth Resistance.
•  Make a mixture of Wood Coal Powder Salt & Sand all in equal part
•  Wood Coal Powder use as good conductor of electricity, anti corrosive, rust proves for GI Plate for long life.
• The purpose of coal and salt is to keep wet the soil permanently.
• The salt percolates and coal absorbs water keeping the soil wet.
• Care should always be taken by watering the earth pits in summer so that the pit soil will be wet.
• Coal is made of carbon which is good conductor minimizing the earth resistant.
• Salt use as electrolyte to form conductivity between GI Plate Coal and Earth with humidity.
• Sand has used to form porosity to cycle water & humidity around the mixture.
• Put GI Plate (EARTH PLATE) of size 500 mm X 500 mm X 10 mm in the mid of mixture.
• Use Double GI Strip size 30 mm X 10 mm to connect GI Plate to System Earthling.
•  It will be better to use GI Pipe of size 2.5″ diameter with a Flange on the top of GI Pipe to cover GI Strip from EARTH PLATE to Top Flange.
• Cover Top of GI pipe with a T joint to avoid jamming of pipe with dust & mud and also use water time to time through this pipe to bottom of earth plate.
• Maintain less than one Ohm Resistance from EARTH PIT conductor to a distance of 15 Meters around the EARTH PIT with another conductor dip on the Earth at least 500 mm deep.
• Check Voltage between Earth Pit conductors to Neutral of Mains Supply 220V AC 50 Hz it should be less than 2.0 Volts.

Factors affecting on Earth resistivity:

(1)  Soil Resistivity:    

• It is the resistance of soil to the passage of electric current. The earth resistance value (ohmic value) of an earth pit depends on soil resistivity. It is the resistance of the soil to the passage of electric current.
• It varies from soil to soil. It depends on the physical composition of the soil, moisture, dissolved salts, grain size and distribution, seasonal variation, current magnitude etc.

• In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its distribution, seasonal variation, current magnitude.

(2)  Soil Condition:

• Different soil conditions give different soil resistivity. Most of the soils are very poor conductors of electricity when they are completely dry. Soil resistivity is measured in ohm-meters or ohm-cm.
• Soil plays a significant role in determining the performance of Electrode.
• Soil with low resistivity is highly corrosive. If soil is dry then soil resistivity value will be very high.
• If soil resistivity is high, earth resistance of electrode will also be high.

(3)  Moisture:  

• Moisture has a great influence on resistivity value of soil. The resistivity of a soil can be determined by the quantity of water held by the soil and resistivity of the water itself. Conduction of electricity in soil is through water.
• The resistance drops quickly to a more or less steady minimum value of about 15% moisture. And further increase of moisture level in soil will have little effect on soil resistivity. In many locations water table goes down in dry weather conditions. Therefore, it is essential to pour water in and around the earth pit to maintain moisture in dry weather conditions. Moisture significantly influences soil resistivity

(4)  Dissolved salts:

• Pure water is poor conductor of electricity.
• Resistivity of soil depends on resistivity of water which in turn depends on the amount and nature of salts dissolved in it.

• Small quantity of salts in water reduces soil resistivity by 80%. common salt is most effective in improving conductivity of soil. But it corrodes metal and hence discouraged.

(5)  Climate Condition:

• Increase or decrease of moisture content determines the increase or decrease of soil resistivity.
• Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low.

 (6)  Physical Composition:

• Different soil composition gives different average resistivity. Based on the type of soil, the resistivity of clay soil may be in the range of 4 – 150 ohm-meter, whereas for rocky or gravel soils, the same may be well above 1000 ohm-meter.

 (7)  Location of Earth Pit :

• The location also contributes to resistivity to a great extent. In a sloping landscape, or in a land with made up of soil, or areas which are hilly, rocky or sandy, water runs off and in dry weather conditions water table goes down very fast. In such situation Back fill Compound will not be able to attract moisture, as the soil around the pit would be dry. The earth pits located in such areas must be watered at frequent intervals, particularly during dry weather conditions.
• Though back fill compound retains moisture under normal conditions, it gives off moisture during dry weather to the dry soil around the electrode, and in the process loses moisture over a period of time. Therefore, choose a site that is naturally not well drained.

 (8)  Effect of grain size and its distribution:

• Grain size, its distribution and closeness of packing are also contributory factors, since they control the manner in which the moisture is held in the soil.
• Effect of seasonal variation on soil resistivity: Increase or decrease of moisture content in soil determines decrease or increase of soil resistivity. Thus in dry weather resistivity will be very high and during rainy season the resistivity will be low.

(9)  Effect of current magnitude:

• Soil resistivity in the vicinity of ground electrode may be affected by current flowing from the electrode into the surrounding soil.
• The thermal characteristics and the moisture content of the soil will determine if a current of a given magnitude and duration will cause significant drying and thus increase the effect of soil resistivity

(10) Area Available:

• Single electrode rod or strip or plate will not achieve the desired resistance alone.
•  If a number of electrodes could be installed and interconnected the desired resistance could be achieved. The distance between the electrodes must be equal to the driven depth to avoid overlapping of area of influence. Each electrode, therefore, must be outside the resistance area of the other.

(11)  Obstructions:

• The soil may look good on the surface but there may be obstructions below a few feet like virgin rock. In that event resistivity will be affected. Obstructions like concrete structure near about the pits will affect resistivity. If the earth pits are close by, the resistance value will be high.

(12)    Current Magnitude:

• A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity.

Measurement of Earth Resistance by use of Earth Tester:

• For measuring soil resistivity Earth Tester is used. It is also called the “MEGGER”.
• It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connect terminal to Earth Electrode and Spikes.
• It is measured by using Four Terminal Earth Tester Instrument. The terminals are connected by wires as in illustration.
• P=Potential Spike and C=Current Spike. The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, and 50M.
• All spikes are equidistant and in straight line to maintain electrical continuity.  Take measurement in different directions.
• Soil resistivity =2πLR.
• R= Value of Earth resistance in ohm.
• Distance between the spikes in cm.
• π  =  3.14
• P = Earth resistivity ohm-cm.
• Earth resistance value is directly proportional to Soil resistivity value

Measurement of Earth Resistance (Three point method):

 

• In this method earth tester terminal C1 & P1 are shorted to each other and connected to the earth electrode (pipe) under test.
• Terminals P2 & C2 are connected to the two separate spikes driven in earth.  These two spikes are kept in same line at the distance of 25 meters and 50 meters due to which there will not be mutual interference in the field of individual spikes.
• If we rotate generator handle with specific speed we get directly earth resistance on scale.
• Spike length in the earth should not be more than 1/20th distance between two spikes.
• Resistance must be verified by increasing or decreasing the distance between the tester electrode and the spikes by 5 meter. Normally, the length of wires should be 10 and 15 Meter or in proportion of 62% of ‘D’.
• Suppose, the distance of Current Spike from Earth Electrode D = 60 ft, Then, distance of Potential Spike would be 62 % of D = 0.62D i.e.  0.62 x 60 ft = 37 ft.

Four Point Method:

• In this method 4 spikes are driven in earth in same line at the equal distance.  Outer two spikes are connected to C1 & C2 terminals of earth tester.  Similarly inner two spikes are connected to P1 & P2 terminals.  Now if we rotate generator handle with specific speed, we get earth resistance value of that place.
• In this method error due to polarization effect is eliminated and earth tester can be operated directly on A.C.

 GI Earthing Vs Copper Earthing:

• As per IS 3043, the resistance of Plate electrode to earth (R) = (r/A) X under root(P/A).
• Where r = Resistivity of Soil Ohm-meter.
• A=Area of Earthing Plate m3.
• The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d).
• Where L= Length of Pipe/Rod in cm
• d=Diameter of Pipe/Rod in cm.
• The resistivity of the soil and the physical dimensions of the electrode play important role of resistance of Rod with earth.
• The material resistivity is not considered important role in earth resistivity.
• Any material of given dimensions would offer the same resistance to earth. Except the sizing and number of the earthing conductor or the protective conductor.

 Pipe Earthing Vs Plate Earthing:

• Suppose Copper Plate having of size 1.2m x 1.2m x 3.15mm thick. soil resistivity of 100 ohm-m,
• The resistance of Plate electrode to earth (R)=( r/A)X under root(π/A) = (100/2.88)X(3.14/2.88)=36.27 ohm
• Now, consider a GI Pipe Electrode of 50 mm Diameter and 3 m Long. soil resistivity of 100 Ohm-m,
• The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d) = (100X100/2X3.14X300) X loge (4X300/5) =29.09 Ohm.
• From the above calculation the GI Pipe electrode offers a much lesser resistance than even a copper plate electrode.
• As per IS 3043 Pipe, rod or strip has a much lower resistance than a plate of equal surface area.

 Length of Pipe Electrode and Earthing Pit:

•  The resistance to earth of a pipe or plate electrode reduces rapidly within the first few feet from ground (mostly 2 to 3 meter) but after that soil resistivity is mostly uniform.

• After about 4 meter depth, there is no appreciable change in resistance to earth of the electrode. Except a number of rods in parallel are to be preferred to a single long rod.

 Amount of Salt and Charcoal (more than 8Kg) :

•  To reduce soil resistivity, it is necessary to dissolve in the moisture particle in the Soil.
• Some substance like Salt/Charcoal is highly conductive in water solution but the additive substance would reduce the resistivity of the soil, only when it is dissolved in the moisture in the soil after that additional quantity does not serve the Purpose.
• 5% moisture in Salt reduces earth resistivity rapidly and further increase in salt content will give a very little decrease in soil resistivity.
• The salt content is expressed in percent by weight of the moisture content in the soil. Considering 1M3 of Soil, the moisture content at 10 percent will be about 144 kg. (10 percent of 1440 kg). The salt content shall be 5% of this (i.e.) 5% of 144kg, that is, about 7.2kg.

 Amount of  Water Purring:

•  Moisture content is one of the controlling factors of earth resistivity.
• Above 20 % of moisture content, the resistivity is very little affected. But below 20% the resistivity increases rapidly with the decrease in moisture content.
• If the moisture content is already above 20% there is no point in adding quantity of water into the earth pit, except perhaps wasting an important and scarce national resource like water.

 Length Vs Diameter of Earth Electrode:

• Apart from considerations of mechanical strength, there is little advantage to be gained from increasing the earth electrode diameter with the object in mind of increasing surface area in contact with the soil.
• The usual practice is to select a diameter of earth electrode, which will have enough strength to enable it to be driven into the particular soil conditions without bending or splitting. Large diameter electrode may be more difficult to drive than smaller diameter electrode.
• The depth to which an earth electrode is driven has much more influence on its electrical resistance characteristics than has its diameter.

Maximum allowable Earth resistance:

• Major power station= 0.5 Ohm.
• Major Sub-stations= 1.0 Ohm
• Minor Sub-station = 2 Ohm
• Neutral Bushing. =2 Ohm
• Service connection = 4 Ohm
• Medium Voltage Network =2 Ohm
• L.T.Lightening Arrestor= 4 Ohm
• L.T.Pole= 5 Ohm
• H.T.Pole =10 Ohm
• Tower =20-30 Ohm

 Treatments to for minimizing Earth resistance:

• Remove Oxidation on joints and joints should be tightened.
• Poured sufficient water in earth electrode.
• Used bigger size of Earth Electrode.
• Electrodes should be connected in parallel.
• Earth pit of more depth & width- breadth should be made.