Earth Leakage Relay (ELR): Working Principle, Fundamentals

An undetected fault current may lead to cables overheating which could start a fire. If high fault currents are involved, hazardous voltages may also appear on earthed equipment, putting lives at risk. That’s why a special protection device is needed to disconnect the energy and the load. The earth leakage relay (Earth fault relay) is used for these purposes. Let me give you more details including its full definition and operating principle.

What is an Earth Leakage Relay?

The earth leakage relay is an electronic device that detects earth failures. The relay uses a toroid to measure the earth leakage current.

A definite time operating characteristic of the earth leakage relay is provided with adjustable trip sensitivity and time delay. When a fault occurs, the relay’s trip function is activated. Thus, switching the trip contacts connected in the control circuit.

The earth leakage relay needs a toroidal transformer to detect the leakage current. The toroidal transformer is designed around a high-quality magnetic core that detects fault currents, even in very low values. The different diameters are used to adapt to a wide range of wire sections and leakage currents.

The core balance transformer allows the direct measurement of residual currents (earth leakage current). The presence of a homopolar component is significant to an earth default. This component is measured via a toroidal transformer placed around the 3 phases + neutral or phase + neutral or on the connexion to the earth.

The vector sum of these currents is null when there is no fault. This balance is broken when an earth leakage current exists. At this time, the vector sum of currents is no longer null and the toroid measures this difference.

All phases should pass through the inner part of the toroid, even the neutral if the same is distributed. The earthing wire shouldn’t pass through the toroid.

The choice of a toroidal transformer depends on the conductor or on the bar to be used. It is suggested to use the open versions in case of revamping or upgrading an existing installation.

How Does an Earth Leakage Relay Work?

Thanks to the earth leakage relay it is possible to measure the leakage current to the earth. This relay works in conjunction with a separate external toroid. The active conductors that pass through the toroid create a magnetic field proportional to the current flow.

Under normal conditions and in the absence of leakage current, the vector sum of the current is zero. Any fault condition causes an unbalance in the vector sum proportional to the value of the leakage current. The value of the fault current is constantly detected by the toroid. When the residual current relay receives a signal from the toroid, it switches its output contacts. Then the shunt-trip opens the circuit breaker.

Summary:

1. A residual current causes an unbalance detected by the toroid.
2. The earth leakage relay receives the signal from the toroid and switches its output contacts.
3. The shunt-trip relay of the circuit breaker opens the circuit breaker.

Why an Earth Leakage Relay is Needed?

We need earth leakage relays because compact residual current devices are manufactured up to 125A and their sensitivity is up to 1000mA.

Earth leakage relays can be used in circuits carrying higher currents. We can measure the leakage of a 400A…6300A busbar with an earth leakage relay and toroid but we cannot measure it with a compact residual current device.