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

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

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).

Voltage-ELCBs were first introduced about sixty years ago and CurrentELCB
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.

Working Principle of Voltage ELCB:

Voltage ELCB is a voltage operated device. It has a coil and if the voltage across the coil exceeds a predetermined value such as 50 V, the current through the coil will be sufficient enough to trip the circuit.

Voltage ELCB is connected in between the metallic part of equipment and the Earth. If we take an example of insulation failure, then the voltage across the coil of Voltage ELCB will drive enough current to cut the power supply till the manually reset.

Earth Leakage Circuit Breaker ELCB
Earth Leakage Circuit Breaker ELCB

The way to identify an ELCB is by looking for green or green and yellow earth wires entering the device. They rely on voltage returning to the trip via the earth wire during a fault and afford only limited protection to the installation and no personal protection at all.
You should use plug in 30mA RCD’s for any appliances and extension leads that may be used outside as a minimum.

Working Principle of Current ELCB:

The working of Current ELCB is quite interesting but easy. Current operated ELCB is also known as Residual Current Device, RCD. A Residual Current Device (RCD) has a toroidal iron core over which phase and neutral windings are wound. A search coil is also wound on the same iron core which in turn is connected to the trip coil. Figure below shows the constructional detail of RCD or Current ELCB.

 Residual Current Device, RCD
Residual Current Device, RCD

Under normal operating condition, the current through the phase winding and neutral winding are same but both the windings are wound in such a manner to oppose the mmfs of each other, therefore net mmf in the toroidal iron core will be zero.

Let us consider a condition where earth leakage current exists in the load side. In this case the current through the phase and neutral will no longer be equal rather phase current will be more than the neutral current.

Thus mmf produced by phase winding will be more than the mmf produced by neutral winding because of which a net mmf will exist in the toroidal iron core.

Net mmf in Core = mmf by phase winding  – mmf by neutral winding

This net mmf in the core will link with the Search Coil and as the mmf is changing in nature (current is AC), an emf will be induced across the terminals of the Search Coil. This emf will in turn drive a current through the Trip Coil which will pull (because of current flow through the Trip Coil, it will behave as an electromagnet and hence will pull the lever to open contact) the supply contacts to isolate the power supply. Notice that Current ELCB works on Residual Current that is the reason it is also called Residual Current Device.

A RCD / Current ELCB is also provided with test button to check the healthiness of the safety device. If you carefully observe the figure, you will notice that, when we press the Test Button, Load and phase winding are bypassed due to which only mmf because of neutral winding will exist in the core (as there is no opposing mmf as was the case with both the windings in service) which will cause RCD to trip to isolate the supply.

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