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 transposefor 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.
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