How Does Earth Leakage Relay Work in Electrical Network

How Does Earth Leakage Relay Work
Figure: Earth Leagage Relay

Earth Leakage relay in short ELR or also known as ground fault relays are residual current monitor in short  RCM that are used in electrical wiring to detect potentially dangerous ground fault electrical-hazard that may causes mechanical damage of cables and devices.

If fault currents becomes in a significant value that is involved, hazardous voltages may also appear on earthed equipment, putting lives at risk.

Operational Principle of Earth Leakage Relay

An earth leakage relay checks that there is no significant leakage between the electrical supply and earth, means it check the flowing current through phase conductor and neutral conductor of the elctrical circuit.

Electrical circuits and devices are in a domestic wiring, single phase, there are equal windings on a solenoid, from both phase and neutral connections.

Earth leakage relay just check the phase and neutral current, if the two currents are equal and balanced, there is no net magnetic force generated in the solenoid. Relay no need operation.


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But, if there is a leak to earth in any point of the circuit, means the currents is passing through anywhere. Current in the phase and neutral will not be balanced during this situation. A net magnetic force will be generated which is used to operate the relay and trip the breaker.

The advantage of earth leakage relay is that it no need any external DC or AC supply to operate the relay coil. The whole system of earth leakage realay is self-contained to enable the trip.

You may find the operation principle of earth lekage relay in above figure where single phage working procedure is shown. At electrical substations and other large electrical installations where three phase system is used, Three phase relay also works in same principle. The lines would be connected via current transformers, and the tripping would be by external high voltage circuit breakers. However, the earth leakage detection relies on checking for balanced currents in the supply lines and tripping the system if the balance is disturbed by a leakage to earth.

Different Between Earth leakage relay and Earth Fault Relay


Earth Leakage relay: We found from above that the earth leakage relays are used to sense small magnitude of currents electrical wiring system in domestic or industrial circuits. These relays are designed in such a way that whenever they sense a preset value like 10mA and 3A,  it trips the circuit breaker and interrupts the flow of current. 

Earth leakage relays can be found in domestic distribution boards and are used to sense insulation failures and to prevent electric shocks.

Earth fault relays: Earth fault relays are used in high and medium voltage systems and in transformers. The working principle is allmost same just this relay is capable of sensing greater fault currents. The contacts of the earth fault relay are connected to the trip circuit of circuit breakers at the output side of the transformer. Hence on the detection of a fault, earth fault relay trips the circuit breaker.

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