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

An Earth Leakage Circuit Breaker (ELCB) is a safety device used to directly detect the leakage current to the Earth from an installation and cut the power supply. Basically ELCB is used where the earth impedance is high. Because of high earth impedance, the voltage difference between the Metallic part of the Installation and Earth will be quite high and dangerous from human safety point of view.

It may strike in your smart mind that �What is the difference between Earth Fault current and Earth Leakage Current?� This is very important to know as we are going to discuss about safety device used to sense earth leakage current.

Well, according to IEC 60947-2, Earth fault current is the current flowing to earth due to insulation fault and Earth leakage current is the current flowing from the live parts of the installation to earth in the absence of an insulation fault.

In case of degradation of electrical insulation, the live conductor may get in touch with the metallic part of the equipment and because of high earth impedance; the potential difference between the body of equipment to the Earth will be high enough to result in shock to the working personnel. Earth Leakage Circuit Breaker (ELCB) detects the leakage current to the earth and trips the associated breaker to isolate the supply.

There are two types of Earth Leakage Circuit Breaker (ELCB). One is Voltage Earth Leakage Circuit Breaker, also called Voltage ELCB and another is Current Earth Leakage Circuit Breaker, also known as Current ELCB.

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.

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.

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.