Calculate Diesel Generator Protection Setting

Recommended Generator Protection are

Recommended Generator Protection
ANSI Code	Protection Function
27	Under Voltage
32	Reverse Power
37	Under Power
40	Loss of Excitation
46	Negative Phase Sequence /Un Balance Load
49T	Thermal Overload
50	Instantaneous Over Current
51	Time grade Over Current
51G	Earth Fault Time Overcurrent
50/51V	Voltage Restrained Overcurrent
59	Over voltage
60G	Fuse Failure Monitor
64S	Stator Earth Fault Protection
81	Under / Over Frequency
87	Three Phase Current Differential
87N	Neutral Current Differential
87G	Generator Differential Protection
24G	Over excitation (Volt/Hertz) Protection
21G	Impedance Protection
59N or 64G1	Stator EF protection (0-95%)
27TN or 64G2	Stator EF protection (100%)
50BF	Breaker Failure Protection
24G	Over excitation (Volt/Hertz) Protection
78G	Pole slip protection

Protection Setting Calculation:

(1) Under Voltage Relay (27):

• The Under Voltage Relay measure either phase-to-phase (Ph-Ph) or phase-to-neutral (Ph-N) fundamental RMS voltage depending on the input voltage setting. If the value of measured voltages deviates from the setting values, then these relays will give a trip indication.
• Reason:
• An under-voltage condition in a diesel generator can occur due to several reasons, overloading the generator beyond its capacity, faulty Automatic Voltage Regulator (AVR), issues with the stator windings, problems with the voltage sampling line, loose connections, low engine speed, fuel problems, and issues with the excitation system
• Setting:
• The Typical under-voltage setting is usually 80 % of the normal rated voltage. If the voltage falls below this level for the set amount of time, then the tripping command is issued by the relay and hence the system is isolated. The time setting is used to avoid tripping due to any transient disturbances. the exact setting can vary depending on the specific generator and system requirements.
• Usually, motors stall at below 80% of their rated voltage. An under-voltage element can be set to trip motor circuits once fall below 80% so that on the restoration of supply an overload is not caused by the simultaneous starting of all the motors.
• Normally Generators are designed to operate continuously at minimum voltage of 95% of its rated voltage.
• Two levels of tripping are provided depending on the severity of the condition, these under voltage elements are blocked from tripping when the generator breaker is open to allow for startup conditions.
• Calculation:
• For 415V Diesel Generator
• Level 1 (Slow)= 80% of Rated Voltage
• Level 1 (Slow)= 80% x 415V =332 V
• Time Delay = 5 sec.
• Level 2 (Fast): 70% of Rated Voltage
• Level 2 (Fast)= 70% x 415V =290 V
• Time Delay = 0 sec.

(2) Over Voltage Protection [59]:

• The Over Voltage Relay measure either phase-to-phase (Ph-Ph) or phase-to-neutral (Ph-N) fundamental RMS voltage depending on the input voltage setting. If the value of measured voltages deviates from the setting values, then these relays will give a trip indication.
• Reason:
• System over voltages can damage the insulation of components. Over voltages occur due to sudden loss of load, improper working of tap changer, Generator AVR malfunction, Reactive component malfunctions, etc.
• Setting:
• The Overvoltage setting is usually 110 to 130 % of the normal operating voltage depending on the system requirement.
• If the voltage rises above this level for the set amount of time then the tripping command issued by the relay and hence the system is isolated. The time setting is used to avoid tripping due to any transient disturbances.
• Calculation:
• For 415V Diesel Generator
• Level 1 (Slow)= 110% of Rated Voltage
• Level 1 (Slow)= 110% x 415V =456 V
• Time Delay = 5 sec.
• Level 2 (Fast): 130% of Rated Voltage
• Level 2 (Fast)= 130% x 415V =539 V
• Time Delay = 0 sec.

(3) Reverse Power Protection [32R]:

• Reverse power relay is an electronic, microprocessors-based protection device which is used for monitoring and stopping the power supply flowing grid side to the DG side or generator running in parallel with another generator. If accidentally leakage current is received by generator, then it can start to running as motor. This situation may be very dangerous for generator set.
• The function of the reverse power relay is to prevent a reverse power condition in which power flows from the bus bar into the generator.
• This condition can occur when there is a failure in the prime mover such as an engine or a turbine which drives the generator.
• Relay detects the reverse flow of power from the load back to the generator, which can occur during system faults or abnormal operating conditions. By sensing this reverse power flow, the relay triggers a protective action, typically disconnecting the generator to prevent further issues.
• The generator are classified by their Prime Mover which determine the amount of Reverse power they can motor.

Sr. No	Prime Mover	Motorizing Power in % of Unit Rating
1	Gas Turbine (single shaft)	100%
2	Gas Turbine (Double Shaft)	10-15%
3	4 Cycle Diesel	15%
4	2 Cycle Diesel	25%
5	Hydraulic Turbine	2-100
6	Steam Turbine (Conventional)	1-4%
7	Steam Turbine (Cond Cooled)	0.5 to 1.0%

• Reason:
• When Two or more unit running in parallel
• In LT panel if the DG supply is running then grid supply should be switched off and if the grid supply is running then DG supply should be switched off. When one source is on then second source accidentals starts to leakage current resultant a large fault may be occurred and system can be failed. So, for prevention of other source leakage the RPR relay is used.
• Failure of Speed controller or another breakdown. When the prime mover of a generator running in a synchronized condition fails. There is a condition known as motoring, where the generator draws power from the bus bar, runs as a motor and drives the prime mover. This happens as in a synchronized condition all the generators will have the same frequency. Any drop in frequency in one generator will cause the other power sources to pump power into the generator. The flow of power in the reverse direction is known as the reverse power relay.
• If the frequency of the machine to be synchronized is slightly lesser than the bus bar frequency and the breaker is closed, power will flow from the bus bar to the machine. Hence, during synchronization(forward), frequency of the incoming machine is kept slight higher than that of the bus bar i.e. the synchroscope is made to rotate in the “Too fast” direction. This ensures that the machine takes on load as soon as the breaker is closed.
• Loss of excitation:
• Failure of AVR
• Setting:
• A generator reverse power relay setting is typically set between 2% to 8% of the generator’s rated power, depending on the type of prime mover (like a diesel engine or steam turbine), with diesel engines generally requiring a higher setting (around 8%) compared to turbines (around 2 to 5%) to prevent unnecessary tripping during transient conditions; this setting essentially determines the threshold at which the relay will activate to protect the generator from reverse power flow, which can damage the machine if it becomes too significant. 
• Calculation:
• Generator capacity :500KVA ,415V,0.9 Power factor
• Full Load Current =500×1000/(1.732*415)
• Full Load Current =695A
• Setting at 5%
• Reverse Power = -5%*500*0.9 = -22.5KW
• Relay Setting= Reverse Power / Real Power =-22.5 / 500 =-4.50%
• Relay Setting =-4.50%
• Time delay proposed=5 sec

(4) Negative Phase Sequence (Unbalance Phase) Relay (46):

• The Negative Sequence Overcurrent function provides protection against possible rotor overheating and damage due to unbalanced faults or other system conditions which can cause unbalanced three phase currents in the generator.
• Negative Phase Sequence detects imbalances in the network that does not cause energy loss out of the system.
• Reason:
• Generator or Motor are design to operate in balance three phase loading.
• Generator negative phase sequence currents can result from any unbalance condition on the system including un transposed lines, single phase loads, unbalanced type line faults and open conductors. the unbalance condition leads negative sequence currents having opposite rotation that of power system in generator leads. This reversed rotating current produce double frequency current in rotor structure. This resulting over heating of rotor.
• Setting:
• A generator Negative Phase Sequence (NPS) relay setting is typically set between 2 to 10% of the full load current depending on the specific generator design and manufacturer’s recommendations, aiming to detect significant unbalances in the power system while avoiding unnecessary tripping due to normal load variations; this setting should be based on the generator’s maximum allowable negative sequence I² (current squared) value to prevent excessive rotor heating. 
• Generator withstand limit against negative sequence overcurrent (K) = 10 (As per IEC-60034-1)
• Normally Generator continuous withstand limit: 8 %
• Calculation:
• Generator capacity :500KVA ,415V , CT is 800/1
• Full Load Current =500×1000/(1.732*415)
• Full Load Current =695A
• Setting at 10% .
• Desired pickup current = 10% of rated current
• Relay setting = (0.1 x Rated Current) / CT ratio 
• Relay setting =(0.1×695) / 800
• Relay setting =0.0868A

(5) Thermal Overload Relay (49T):

• In general, generators can operate successfully at rated kVA, frequency, and power factor for a voltage variation of 5% above or below rated voltage. Under emergency condition, it is possible to exceed the continuous output capability for a short time.
• The stator overload function provides protection against possible damage during overload conditions.
• Reason:
• A generator becomes overloaded when too many appliances or devices are plugged in and drawing power simultaneously, exceeding the generator’s rated capacity, often happening when attempting to power heavy appliances like air conditioners, heaters, or electric stoves at the same time; essentially, drawing more power than the generator can supply. 
• Peak usage times: Running multiple high-power appliances simultaneously. 
• Damaged components: Faulty electrical components within the generator can contribute to overload issues. 
• Improper load management: Not prioritizing which appliances to run on the generator. 
• Adding new equipment: Plugging in additional appliances without considering the generator’s capacity. 
• Setting:
• A generator thermal overload relay setting is typically based on a percentage of the motor’s full load current.
• Common settings are:
• For motors with Service Factor (SF) ≥ 1.15, Set to 125% of FLA.
• For motors with Service Factor (SF) < 1.15, Set to 115% of FLA
• As per IEEE Generator short time thermal capability for balanced three-phase loading diagram (Short time capability curve) the wining will withstand 117% rated current for 120 second.
• Calculation:
• Generator capacity :500KVA ,415V , CT is 800/1
• Full Load Current =500×1000/(1.732*415)
• Full Load Current =695A
• Setting at 117% .
• Desired pickup current = 117% of rated current
• Relay setting = (1.17 x Rated Current) / CT ratio 
• Relay setting =(1.17×695) / 800
• Relay setting =1.016A

(6) Generator Under Frequency Protection (81 G):

• Prevents the steam turbine and generator from exceeding the permissible operating time at reduced frequencies.
• Ensures that the generating unit is separated from the network at a preset value of frequency.
• Prevent over fluxing (v/f) of the generator (large over fluxing for short times).
• The stator under frequency relay measures the frequency of the stator terminal voltage.
• Setting Recommendations:
• within 0.2 to 0.5Hz below the nominal frequency
• For Alarm: 48.0 Hz, 2.0 Sec. time delay. 
• For Trip: 47.5 Hz, 1.0 Sec. or as recommended by Generator Manufacturers.

(7) Instantaneous Over Current Relay (50):

• Instantaneous overcurrent protection is where a protective relay initiates a breaker trip based on current exceeding a pre-programmed “pickup” value for any length of time. 
• Setting:
• A generator phase instantaneous overcurrent relay setting is typically set between 2 to 1.5 times the full load current (FLA) of the generator, ensuring quick tripping in case of a severe fault while avoiding unnecessary trips due to momentary current surges during starting or load fluctuations; this setting is usually referred to as the “pickup current” of the relay. 
• This is back up protection for Generator. To avoid unnecessary trip of the generator we recommend making OFF this function in generator protection.
• Calculation:
• Generator full Load Current = 130A & CT is 300/5 =60
• Setting =1.5 times of Full Load Current
• Setting= 1.5×130 =195A
• 51 Current Setting = Setting / CT Ratio = 195/60 =3.25A.
• Time setting =5 Second.
• The proposed above setting is coordinated with other O/C protection setting.

(8) Time grade Over Current Relay (51):

• Time overcurrent protection is where a protective relay initiates a breaker trip based on the combination of overcurrent magnitude and overcurrent duration, the relay tripping sooner with greater current magnitude. This is a more sophisticated form of overcurrent protection than instantaneous.
• Setting:
• This is back-up protection of the generator, for better time gradings the overcurrent setting should be co-ordinate with load connected feeder overcurrent setting.
• A generator Phase Overcurrent (51) setting is typically set between 125% and 150% of the generator’s full load current, however, the exact setting depends on the specific application and should be coordinated with other system protections.
• Calculation:
• Generator full Load Current = 130A & CT is 300/5 =60
• Setting =150% of Full Load Current
• Setting= 1.5×130 =195A
• 51 Current Setting = Setting / CT Ratio = 195/60 =3.25A.
• Time setting =5 Second.
• The proposed above setting is coordinated with other O/C protection setting.

(9) Earth Fault Time Overcurrent (51G)

• This is back-up protection in Earth Fault of generator, for better time gradings the overcurrent setting should be co-ordinate with load connected feeder setting.
• Setting:
• Earth Fault Relay setting shall be 10 to 20 % Full Load Current
• Calculation:
• Generator full Load Current = 130A & CT is 300/5 =60
• Setting =20% of Full Load Current
• Setting= 0.2×130 =26A
• 51G Current Setting = Setting / CT Ratio = 26/60 =0.43A.
• Time setting =5 Second.
• The proposed above setting is coordinated with other O/C protection setting.

(9) Ground Differential (87 N)

• The ground differential element (87N) that operates based on the difference between the measured neutral current and the sum of the three-phase current inputs.
• The 87N element provides sensitive ground fault detection on resistance-grounded particularly where multiple generators are connected directly to a load bus.
• The relay provides two definite-time delayed ground current differential elements designed to detect ground faults on resistance grounded generator.
• The relay uses the neutral CT connected to the relay input to measure the generator neutral current. It then calculates the residual current, which is the sum of the three phase current inputs (from CTs located at generator terminals).
• The relay adjusts the residual current by the ratio of the CTR and CTRN settings to scale the residual current in terms of the secondary neutral current. It then calculates the difference. Normally, under balanced load or external ground fault conditions, the difference current should be zero. In the event of an internal ground fault, the difference current is nonzero. If the difference current magnitude is greater than the element pickup setting, the element picks up and begins to operate the definite time-delay.
• Setting:
• Earth Fault Relay setting shall be 10 to 20 % Maximum Ground Fault Current
• Calculation:
• Generator grounded through 39.8 Ohms Resistance.
• Generator rated Voltage=13800V, Current 130A
• Maximum Earth Fault Current =(138000 / 1.732) / 39.8
• Maximum Earth Fault Current =7967.4 / 39.8
• Maximum Earth Fault Current = 200 A
• 87N pickup current setting = 10% x 200 / CT Ratio
• 87N pickup current setting = 20 / 60
• 87N pickup current setting = 0.3
• 87N Time delay =0.2s

MCB/MCCB/ELCB/RCCB

MCB/MCCB/ ELCB /RCBO/ RCCB:

MCB (Miniature Circuit Breaker)

• Rated current not more than 100 A.
• Trip characteristics normally not adjustable.
• Thermal or thermal-magnetic operation.

MCCB (Moulded Case Circuit Breaker):

• Rated current up to 1000 A.
• Trip current may be adjustable.
• Thermal or thermal-magnetic operation.

Air Circuit Breaker:

• Rated current up to 10,000 A.
• Trip characteristics often fully adjustable including configurable trip thresholds and delays.
• Usually electronically controlled—some models are microprocessor controlled.
• Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

Vacuum Circuit Breaker:

• With rated current up to 3000 A,
• These breakers interrupt the arc in a vacuum bottle.
• These can also be applied at up to 35,000 V. Vacuum breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

RCD (Residual Current Device) / RCCB( Residual Current Circuit Breaker) :

• Phase (line) and Neutral both wires connected through RCD.
• It trips the circuit when there is earth fault current.
• The amount of current flows through the phase (line) should return through neutral .
• It detects by RCD. any mismatch between two currents flowing through phase and neutral detect by RCD and trip the circuit within 30Miliseconed.
• If a house has an earth system connected to an earth rod and not the main incoming cable, then it must have all circuits protected by an RCD (because u mite not be able to get enough fault current to trip a MCB)
• The most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection (hundred of amps)
• A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, where the risk of electric shock is small
• RCDs are an extremely effective form of shock protection

Limitation of RCCB:

• Standard electromechanical RCCBs are designed to operate on normal supply waveforms and cannot be guaranteed to operate where none standard waveforms are generated by loads. The most common is the half wave rectified waveform sometimes called pulsating dc generated by speed control devices, semi conductors, computers and even dimmers.
• Specially modified RCCBs are available which will operate on normal ac and pulsating dc.
• RCDs don’t offer protection against current overloads: RCDs detect an imbalance in the live and neutral currents. A current overload, however large, cannot be detected. It is a frequent cause of problems with novices to replace an MCB in a fuse box with an RCD. This may be done in an attempt to increase shock protection. If a live-neutral fault occurs (a short circuit, or an overload), the RCD won’t trip, and may be damaged. In practice, the main MCB for the premises will probably trip, or the service fuse, so the situation is unlikely to lead to catastrophe; but it may be inconvenient.
• It is now possible to get an MCB and and RCD in a single unit, called an RCBO (see below). Replacing an MCB with an RCBO of the same rating is generally safe.
• Nuisance tripping of RCCB: Sudden changes in electrical load can cause a small, brief current flow to earth, especially in old appliances. RCDs are very sensitive and operate very quickly; they may well trip when the motor of an old freezer switches off. Some equipment is notoriously `leaky’, that is, generate a small, constant current flow to earth. Some types of computer equipment, and large television sets, are widely reported to cause problems.
• RCD will not protect against a socket outlet being wired with its live and neutral terminals the wrong way round.
• RCD will not protect against the overheating that results when conductors are not properly screwed into their terminals.
• RCD will not protect against live-neutral shocks, because the current in the live and neutral is balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals of a light fitting), you may still get a nasty shock.

ELCB (Earth Leakage Circuit Breaker):

• Phase (line), Neutral and Earth wire connected through ELCB.
• ELCB is working based on Earth leakage current.
• Operating Time of ELCB:
• The safest limit of Current which Human Body can withstand is 30ma sec.
• Suppose Human Body Resistance is 500Ω and Voltage to ground is 230 Volt.
• The Body current will be 500/230=460mA.
• Hence ELCB must be operated in  30maSec/460mA = 0.65msec

RCBO (Residual Circuit Breaker with OverLoad):

• It is possible to get a combined MCB and RCCB in one device (Residual Current Breaker with Overload RCBO), the principals are the same, but more styles of disconnection are fitted into one package

Difference between ELCB and RCCB.

• ELCB is the old name and often refers to voltage operated devices that are no longer available and it is advised you replace them if you find one.
• RCCB or RCD is the new name that specifies current operated (hence the new name to distinguish from voltage operated).
• The new RCCB is best because it will detect any earth fault. The voltage type only detects earth faults that flow back through the main earth wire so this is why they stopped being used.
• The easy way to tell an old voltage operated trip is to look for the main earth wire connected through it.
• RCCB will only have the line and neutral connections.
• ELCB is working based on Earth leakage current. But RCCB is not having sensing or connectivity of Earth, because fundamentally Phase current is equal to the neutral current in single phase. That’s why RCCB can trip when the both currents are deferent and it withstand up to both the currents are same. Both the neutral and phase currents are different that means current is flowing through the Earth.
• Finally both are working for same, but the thing is connectivity is difference.
• RCD does not necessarily require an earth connection itself (it monitors only the live and neutral).In addition it detects current flows to earth even in equipment without an earth of its own.
• This means that an RCD will continue to give shock protection in equipment that has a faulty earth. It is these properties that have made the RCD more popular than its rivals. For example, earth-leakage circuit breakers (ELCBs) were widely used about ten years ago. These devices measured the voltage on the earth conductor; if this voltage was not zero this indicated a current leakage to earth. The problem is that ELCBs need a sound earth connection, as does the equipment it protects. As a result, the use of ELCBs is no longer recommended.

MCB Selection:

• The first characteristic is the overload which is intended to prevent the accidental overloading of the cable in a no fault situation. The speed of the MCB tripping will vary with the degree of the overload. This is usually achieved by the use of a thermal device in the MCB.
• The second characteristic is the magnetic fault protection, which is intended to operate when the fault reaches a predetermined level and to trip the MCB within one tenth of a second. The level of this magnetic trip gives the MCB its type characteristic as follows: – ·
• Type               Tripping Current                                      Operating Time
• Type B            3 To 5 time full load current                    0.04 To 13 Sec
• Type C             5 To 10 times full load current               0.04 To 5 Sec
• Type D            10 To 20 times full load current              0.04 To 3 Sec
• The third characteristic is the short circuit protection, which is intended to protect against heavy faults maybe in thousands of amps caused by short circuit faults.
• The capability of the MCB to operate under these conditions gives its short circuit rating in Kilo amps (KA). In general for consumer units a 6KA fault level is adequate whereas for industrial boards 10KA fault capabilities or above may be required.

Fuse and MCB characteristics

• Fuses and MCBs are rated in amps. The amp rating given on the fuse or MCB body is the amount of current it will pass continuously. This is normally called the rated current or nominal current.
• Many people think that if the current exceeds the nominal current, the device will trip, instantly. So if the rating is 30 amps, a current of 30.00001 amps will trip it, right? This is not true.
• The fuse and the MCB, even though their nominal currents are similar, have very different  properties.
• For example, For 32Amp MCB and 30 Amp Fuse, to be sure of tripping in 0.1 seconds, the MCB requires a current of 128 amps, while the fuse requires 300 amps.
• The fuse clearly requires more current to blow it in that time, but notice how much bigger both these currents are than the `30 amps’ marked current rating.
• There is a small likelihood that in the course of, say, a month, a 30-amp fuse will trip when carrying 30 amps. If the fuse has had a couple of overloads before (which may not even have been noticed) this is much more likely. This explains why fuses can sometimes `blow’ for no obvious reason
• If the fuse is marked `30 amps’, but it will actually stand 40 amps for over an hour, how can we justify calling it a `30 amp’ fuse? The answer is that the overload characteristics of fuses are designed to match the properties of modern cables. For example, a modern PVC-insulated cable will stand a 50% overload for an hour, so it seems reasonable that the fuse should as well.

Typical methods of provision of the main earthing terminal:

Supply type code : TN-S

• Supplier provides a separate earth connection, usually direct from the distribution station and via the metal sheath of the supply cable.

Supply type code : TN-C-S

• Supplier provides a combined earth/neutral connection; your main earth terminal is connected to their neutral

Supply type code : TT

• Supplier provides no earth; you have an earth spike near your premises.

Difference between MCB/MCCB/ELCB/RCCB/RCBO

MCB/MCCB/ ELCB /RCBO/ RCCB:

MCB (Miniature Circuit Breaker)

• Rated current not more than 100 A.
• Trip characteristics normally not adjustable.
• Thermal or thermal-magnetic operation.

MCCB (Moulded Case Circuit Breaker):

• Rated current up to 1000 A.
• Trip current may be adjustable.
• Thermal or thermal-magnetic operation.

Air Circuit Breaker:

• Rated current up to 10,000 A.
• Trip characteristics often fully adjustable including configurable trip thresholds and delays.
• Usually electronically controlled—some models are microprocessor controlled.
• Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

Vacuum Circuit Breaker:

• With rated current up to 3000 A,
• These breakers interrupt the arc in a vacuum bottle.
• These can also be applied at up to 35,000 V. Vacuum breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

RCD (Residual Current Device) / RCCB( Residual Current Circuit Breaker) :

• Phase (line) and Neutral both wires connected through RCD.
• It trips the circuit when there is earth fault current.
• The amount of current flows through the phase (line) should return through neutral .
• It detects by RCD. any mismatch between two currents flowing through phase and neutral detect by RCD and trip the circuit within 30Miliseconed.
• If a house has an earth system connected to an earth rod and not the main incoming cable, then it must have all circuits protected by an RCD (because u mite not be able to get enough fault current to trip a MCB)
• The most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection (hundred of amps)
• A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, where the risk of electric shock is small
• RCDs are an extremely effective form of shock protection

Limitation of RCCB:

• Standard electromechanical RCCBs are designed to operate on normal supply waveforms and cannot be guaranteed to operate where none standard waveforms are generated by loads. The most common is the half wave rectified waveform sometimes called pulsating dc generated by speed control devices, semi conductors, computers and even dimmers.
• Specially modified RCCBs are available which will operate on normal ac and pulsating dc.
• RCDs don’t offer protection against current overloads: RCDs detect an imbalance in the live and neutral currents. A current overload, however large, cannot be detected. It is a frequent cause of problems with novices to replace an MCB in a fuse box with an RCD. This may be done in an attempt to increase shock protection. If a live-neutral fault occurs (a short circuit, or an overload), the RCD won’t trip, and may be damaged. In practice, the main MCB for the premises will probably trip, or the service fuse, so the situation is unlikely to lead to catastrophe; but it may be inconvenient.
• It is now possible to get an MCB and and RCD in a single unit, called an RCBO (see below). Replacing an MCB with an RCBO of the same rating is generally safe.
• Nuisance tripping of RCCB: Sudden changes in electrical load can cause a small, brief current flow to earth, especially in old appliances. RCDs are very sensitive and operate very quickly; they may well trip when the motor of an old freezer switches off. Some equipment is notoriously `leaky’, that is, generate a small, constant current flow to earth. Some types of computer equipment, and large television sets, are widely reported to cause problems.
• RCD will not protect against a socket outlet being wired with its live and neutral terminals the wrong way round.
• RCD will not protect against the overheating that results when conductors are not properly screwed into their terminals.
• RCD will not protect against live-neutral shocks, because the current in the live and neutral is balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals of a light fitting), you may still get a nasty shock.

ELCB (Earth Leakage Circuit Breaker):

• Phase (line), Neutral and Earth wire connected through ELCB.
• ELCB is working based on Earth leakage current.
• Operating Time of ELCB:
• The safest limit of Current which Human Body can withstand is 30ma sec.
• Suppose Human Body Resistance is 500Ω and Voltage to ground is 230 Volt.
• The Body current will be 500/230=460mA.
• Hence ELCB must be operated in  30maSec/460mA = 0.65msec

RCBO (Residual Circuit Breaker with OverLoad):

• It is possible to get a combined MCB and RCCB in one device (Residual Current Breaker with Overload RCBO), the principals are the same, but more styles of disconnection are fitted into one package

Difference between ELCB and RCCB.

• ELCB is the old name and often refers to voltage operated devices that are no longer available and it is advised you replace them if you find one.
• RCCB or RCD is the new name that specifies current operated (hence the new name to distinguish from voltage operated).
• The new RCCB is best because it will detect any earth fault. The voltage type only detects earth faults that flow back through the main earth wire so this is why they stopped being used.
• The easy way to tell an old voltage operated trip is to look for the main earth wire connected through it.
• RCCB will only have the line and neutral connections.
• ELCB is working based on Earth leakage current. But RCCB is not having sensing or connectivity of Earth, because fundamentally Phase current is equal to the neutral current in single phase. That’s why RCCB can trip when the both currents are deferent and it withstand up to both the currents are same. Both the neutral and phase currents are different that means current is flowing through the Earth.
• Finally both are working for same, but the thing is connectivity is difference.
• RCD does not necessarily require an earth connection itself (it monitors only the live and neutral).In addition it detects current flows to earth even in equipment without an earth of its own.
• This means that an RCD will continue to give shock protection in equipment that has a faulty earth. It is these properties that have made the RCD more popular than its rivals. For example, earth-leakage circuit breakers (ELCBs) were widely used about ten years ago. These devices measured the voltage on the earth conductor; if this voltage was not zero this indicated a current leakage to earth. The problem is that ELCBs need a sound earth connection, as does the equipment it protects. As a result, the use of ELCBs is no longer recommended.

MCB Selection:

• The first characteristic is the overload which is intended to prevent the accidental overloading of the cable in a no fault situation. The speed of the MCB tripping will vary with the degree of the overload. This is usually achieved by the use of a thermal device in the MCB.
• The second characteristic is the magnetic fault protection, which is intended to operate when the fault reaches a predetermined level and to trip the MCB within one tenth of a second. The level of this magnetic trip gives the MCB its type characteristic as follows: – ·
• Type               Tripping Current                                      Operating Time
• Type B            3 To 5 time full load current                    0.04 To 13 Sec
• Type C             5 To 10 times full load current               0.04 To 5 Sec
• Type D            10 To 20 times full load current              0.04 To 3 Sec
• The third characteristic is the short circuit protection, which is intended to protect against heavy faults maybe in thousands of amps caused by short circuit faults.
• The capability of the MCB to operate under these conditions gives its short circuit rating in Kilo amps (KA). In general for consumer units a 6KA fault level is adequate whereas for industrial boards 10KA fault capabilities or above may be required.

Fuse and MCB characteristics

• Fuses and MCBs are rated in amps. The amp rating given on the fuse or MCB body is the amount of current it will pass continuously. This is normally called the rated current or nominal current.
• Many people think that if the current exceeds the nominal current, the device will trip, instantly. So if the rating is 30 amps, a current of 30.00001 amps will trip it, right? This is not true.
• The fuse and the MCB, even though their nominal currents are similar, have very different  properties.
• For example, For 32Amp MCB and 30 Amp Fuse, to be sure of tripping in 0.1 seconds, the MCB requires a current of 128 amps, while the fuse requires 300 amps.
• The fuse clearly requires more current to blow it in that time, but notice how much bigger both these currents are than the `30 amps’ marked current rating.
• There is a small likelihood that in the course of, say, a month, a 30-amp fuse will trip when carrying 30 amps. If the fuse has had a couple of overloads before (which may not even have been noticed) this is much more likely. This explains why fuses can sometimes `blow’ for no obvious reason
• If the fuse is marked `30 amps’, but it will actually stand 40 amps for over an hour, how can we justify calling it a `30 amp’ fuse? The answer is that the overload characteristics of fuses are designed to match the properties of modern cables. For example, a modern PVC-insulated cable will stand a 50% overload for an hour, so it seems reasonable that the fuse should as well.

Typical methods of provision of the main earthing terminal:

Supply type code : TN-S

• Supplier provides a separate earth connection, usually direct from the distribution station and via the metal sheath of the supply cable.

Supply type code : TN-C-S

• Supplier provides a combined earth/neutral connection; your main earth terminal is connected to their neutral

Supply type code : TT

• Supplier provides no earth; you have an earth spike near your premises.