Working Principle of Earth Leakage Circuit Breaker (ELCB) and Residual Current Device (RCD)

Earth Leakage Circuit Breaker (ELCB)

The operation of an earth leakage circuit breaker (or shorten ELCB) is based on the fact that the algebraic sum of the currents in any healthy single phase, three phase and three phase and neutral system is zero. This simply means that in a single phase system the current flowing in the line conductor is equal and opposite to the current in the neutral conductor, and in a three phase system the vectorial current in one line is equal and opposite to the vectorial sum of the currents in the other two lines.

The same applies in three phase and neutral systems. If these conductors are passed through a core balance transformer, and the circuit is healthy, no excitation of the transformer results.

However, if an earth fault occurs in the system, the earth current returns to the earth connection on the supply transformer via, an earth conductor or general earth. Since these conductors are external to the core balance transformer, the currents passing through the core balance transformer are no longer balanced, and an EMF is induced into the secondary winding of the core balance transformer.

If the secondary winding is connected to a relay, the voltage induced is applied across the terminals of the relay and if of sufficient value will cause the relay to operate. This is the principle on which current balance earth leakage circuit breakers operate.

Requirements for Earth Leakage Circuit Breaker

Requirement #1

On systems where conductivity of the earth connections are poor, as is sometimes the case in rural areas (but in urban areas too!), there is always a risk of fire damage resulting from an earth fault. This is so because, if the high impedance of the earth conductor limits the fault current to a value below the fusing value of the main fuse, no means of isolating the fault exists. The longer this fault current continues to flow the higher is the risk of a fire.

In situations such as this the use of an earth leakage circuit breaker is necessary to isolate the supply immediately an earth fault occurs.

Of course, it is always possible to use an earth leakage circuit breaker with the lowest tripping value available irrespective of earth loop impedance. It would certainly trip on lower values of earth fault current and could reduce the possible framework-to-earth voltage to less than 40 volts.

However, consideration should be given to the extent of the network protected and the natural leakage to earth of the system or nuisance tripping may occur.

Figure 1 – Schematics of an Earth Leakage Circuit Breaker (ELCB)

Requirement #2

There are installations where conditions are such that the possibility of a person receiving an electric shock are higher than normal. For example, any portable equipment has a risk higher than fixed installations for two reasons.

Firstly, the fact that the equipment, be it a drill or an electric iron, is constantly subjected to movement there is a danger that the earth connection could become loose or the earth conductor in the cable, which is also subjected to movement, could be broken.

Secondly, because the cable is being constantly moved about there is a danger that damage could result which would expose a live conductor. A perfect example of this is the flex on an electric iron. It is quite common to see one that is frayed, caused by the movement of the iron.

In the case of a portable piece of equipment, that for one reason or another has become disconnected from earth, should an earth fault occur on that equipment the enclosure could be raised to line voltage. Obviously if it is touched by a person who is in contact with earth they will receive an electric shock. The same applies to a person who comes into contact with a live conductor as could happen where the cable is damaged.

The objective in cases as described above must be to have an earth leakage circuit breaker protecting the system which will operate at a level of trip current and have a cut-off time that are considered safe.

How Earth Leakage Circuit Breaker works?

Earth Leakage Circuit Breaker (ELCB) is a device used to directly detect currents leaking to earth from an installation and cut the power and mainly used in TT earthing systems.

There are two types of ELCBs:

1. Voltage Earth Leakage Circuit Breaker (voltage-ELCB)
2. Current Earth Leakage Current Earth Leakage Circuit Breaker (Current-ELCB).

Figure 2 – 4-pole earth leakage circuit breaker, 63A

An Voltage-ELCBs were first introduced about sixty years ago and Current-ELCB was first introduced about forty years ago. For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCBs because it was a simpler name to remember. But the use of a common name for two different devices gave rise to considerable confusion in the electrical industry.

If the wrong type was used on an installation, the level of protection given could be substantially less than that intended.

To ignore this confusion, IEC decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs. Residual current refers to any current over and above the load current.

Voltage-Based ELCB

Voltage-based earth leakage circuit breaker is a voltage operated circuit breaker. The device will function when the Current passes through the ELCB. Voltage-ELCB contains relay Coil which it being connected to the metallic load body at one end and it is connected to ground wire at the other end.

If the voltage of the Equipment body is rise (by touching phase to metal part or failure of insulation of equipment) which could cause the difference between earth and load body voltage, the danger of electric shock will occur. This voltage difference will produce an electric current from the load metallic body passes the relay loop and to earth.

The ELCB detects fault currents from live to the earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCB’s sense coil, it will switch off the power, and remain off until manually reset. A voltage-sensing ELCB does not sense fault currents from live to any other earthed body.

Figure 3 – Voltage Base ELCB

These ELCBs monitored the voltage on the earth wire, and disconnected the supply if the earth wire voltage was over 50 volts.

These devices are no longer used due to its drawbacks like if the fault is between live and a circuit earth, they will disconnect the supply. However, if the fault is between live and some other earth (such as a person or a metal water pipe), they will NOT disconnect, as the voltage on the circuit earth will not change. Even if the fault is between live and a circuit earth, parallel earth paths created via gas or water pipes can result in the ELCB being bypassed.

You should use plug in 30mA RCD’s for any appliances and extension leads that may be used outside as a minimum.

 Advantages of ELCB

• ELCBs have one advantage over RCDs: they are less sensitive to fault conditions, and therefore have fewer nuisance trips.
• While voltage and current on the earth line is usually fault current from a live wire, this is not always the case, thus there are situations in which an ELCB can nuisance trip.
• When an installation has two connections to earth, a nearby high current lightning strike will cause a voltage gradient in the soil, presenting the ELCB sense coil with enough voltage to cause it to trip.
• If the installation’s earth rod is placed close to the earth rod of a neighboring building, a high earth leakage current in the other building can raise the local ground potential and cause a voltage difference across the two earths, again tripping the ELCB.
• If there is an accumulated or burden of currents caused by items with lowered insulation resistance due to older equipment, or with heating elements, or rain conditions can cause the insulation resistance to lower due to moisture tracking. If there is a some mA which is equal to ELCB rating than ELCB may give nuisance Tripping.
• If either of the earth wires become disconnected from the ELCB, it will no longer trip or the installation will often no longer be properly earthed.
• Some ELCBs do not respond to rectified fault current.
  This issue is common for ELCBs and RCDs, but ELCBs are on average much older than RCB, so an old ELCB is more likely to have some uncommon fault current waveform that it will not respond to.
• Voltage-operated ELCB are the requirement for a second connection, and the possibility that any additional connection to earth on the protected system can disable the detector.
• Nuisance tripping especially during thunderstorms.

Watch Video – Earth Leakage Circuit Breaker (ELCB) -Definition, Working, Uses, Diagram

Disadvantages of ELCB

• They do not detect faults that don’t pass current through the CPC to the earth rod.
• They do not allow a single building system to be easily split into multiple sections with independent fault protection, because earthing systems are usually use common earth Rod.
• They may be tripped by external voltages from something connected to the earthing system such as metal pipes, a TN-S earth or a TN-C-S combined neutral and earth.
• As electrically leaky appliances such as some water heaters, washing machines and cookers may cause the ELCB to trip.
• ELCBs introduce additional resistance and an additional point of failure into the earthing system.

Checking the health of the ELCB is simple and you can do it easily by pressing TEST Push Button Switch of ELCB. The test push-button will test whether the ELCB unit is working properly or not. Can we assume that If ELCB is Trip after Pressing TEST Switch of ELCB than your system is protected against earth protection? Then you are wrong.

The test facility provided on the home ELCB will only confirm the health of the ELCB unit, but that test does not confirm that the ELCB will trip when an electric shock hazard does occur. It is a really sad fact that all the while this misunderstanding has left many homes totally unprotected from the risk of electric shocks.

This brings us or alarming us to think over second basic requirement for earth protection. The second requirement for the proper operation of a home shock protection system is electrical grounding.

In addition to routine inspections that should be done by the qualified electrician, this grounding should preferably be inspected regularly at shorter intervals by the homeowner and need to pour Water in Earthing Pit at Regular interval of Time to minimize Earth Resistance.

Current-operated ELCB (RCD)

Current-operated ELCBs are generally known as Residual-current devices (RCD). These also protect against earth leakage. Both circuit conductors (supply and return) are run through a sensing coil; any imbalance of the currents means the magnetic field does not perfectly cancel. The device detects the imbalance and trips the contact.

When the term ELCB is used it usually means a voltage-operated device. Similar devices that are current operated are called residual-current devices. However, some companies use the term ELCB to distinguish high sensitivity current operated 3 phase devices that trip in the milliamp range from traditional 3 phase ground fault devices that operate at much higher currents.

Figure 4 – Residual current device (RCD)

Typical RCD circuit:

The supply coil, the neutral coil and the search coil all wound on a common transformer core. On a healthy circuit the same current passes through the phase coil, the load and return back through the neutral coil. Both the phase and the neutral coils are wound in such a way that they will produce an opposing magnetic flux. With the same current passing through both coils, their magnetic effect will cancel out under a healthy circuit condition.

In a situation when there is fault or a leakage to earth in the load circuit, or anywhere between the load circuit and the output connection of the RCB circuit, the current returning through the neutral coil has been reduced. Then the magnetic flux inside the transformer core is not balanced anymore. The total sum of the opposing magnetic flux is no longer zero. This net remaining flux is what we call a residual flux.

Since the trip current is driven by the residual magnetic flux (the resulting flux, the net effect between both fluxes) between the phase and the neutral coils, it is called the residual current device, or shorten RCD.

Figure 5 – Typical RCD circuit

With a circuit breaker incorporated as part of the circuit, the assembled system is called residual current circuit breaker (RCCB) or residual current devise (RCD). The incoming current has to pass through the circuit breaker first before going to the phase coil. The return neutral path passes through the second circuit breaker pole. During tripping when a fault is detected, both the phase and neutral connection is isolated.

• RCD sensitivity is expressed as the rated residual operating current, noted IΔn. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value.
• High sensitivity (HS): 6- 10- 30 mA (for direct-contact / life injury protection)
• Standard IEC 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending on the characteristics of the fault current.
• Type AC: RCD for which tripping is ensured for residual sinusoidal alternating currents

Sensitivity of RCB:

• Medium sensitivity (MS): 100- 300- 500- 1000 mA (for fire protection)
• Low sensitivity (LS): 3- 10- 30 A (typically for protection of machine)

Types of RCB:

Type A: RCD for which tripping is ensured

• for residual sinusoidal alternating currents
• for residual pulsating direct currents
• For residual pulsating direct currents superimposed by a smooth direct current of 0.006 A, with or without phase-angle control, independent of the polarity.

Type B: RCD for which tripping is ensured

• same as for type A
• for residual sinusoidal currents up to 1000 Hz
• for residual sinusoidal currents superposed by a pure direct current
• for pulsating direct currents superposed by a pure direct current
• for residual currents which may result from rectifying circuits
  • three pulse star connection or six pulse bridge connection
  • two pulse bridge connection line-to-line with or without phase-angle monitoring, independently of the polarity

There are two groups of devices:

Break time of RCB:

1. G (general use) for instantaneous RCDs (i.e. without a time delay)

• Minimum break time: immediate
• Maximum break time: 200 ms for 1x IΔn, 150 ms for 2x IΔn, and 40 ms for 5x IΔn

2. S (selective) or T (time delayed) for RCDs with a short time delay (typically used in circuits containing surge suppressors)

• Minimum break time: 130 ms for 1x IΔn, 60 ms for 2x IΔn, and 50 ms for 5x IΔn
• Maximum break time: 500 ms for 1x IΔn, 200 ms for 2x IΔn, and 150 ms for 5x IΔn

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