DC earth fault relay –Working principle

DC earth fault relay is commonly used to detect earth fault in DC systems. An ungrounded DC system is used to control the supply of protection systems, control panels, communication, and other networking devices.

When one terminal of the DC system is grounded, it is not creating any problem. However, when both the DC terminal are grounded due to cable insulation failure or wrong wiring, it creates a short circuit and the connected DC breaker will open.

To prevent the earth fault in both terminals, the DC earth fault relay is used in the DC system. When one of the DC terminals is grounded, it will detect by the DC earth fault locator. The earth fault in the DC terminal needs to attend immediately.

Working Principle of DC earth fault Relay

As the DC is an ungrounded and floating system, there is no connection between the ground. So in this case to identify the earth fault of the DC terminal, the DC system is connected to the ground from the center point between the 02 nos. equal value resistances (Rx) which are connected to the DC (+Ve) and DC(-Ve) as shown in the diagram. The value of the resistance is decided in a way that there is a negligible current flow between the DC terminals. The resistance value also depends on the voltage level of the DC system.

DC earth fault relay
DC earth fault relay

Under the healthy condition, if there is no earth fault in the system, the voltage between the positive terminal to ground and negative terminal to ground are the same. For example, if the system voltage is 220V DC, the voltage between (+Ve) to ground should be (+ )110V and (-Ve) to ground should be (-) 110V. under the balance condition, there is no current flow through the CT of the earth fault relay.

Suppose, the positive terminal of any feeder is grounded, under this condition, it will create the parallel path of the Resistance (Rx) which is connected to the DC positive terminal. Due to earth fault in the positive terminal, the effective resistance connected to the positive is very negligible as compared to the Resistance(Rx) of the negative terminal.

In this condition, the voltage between (+Ve) to the ground is 0 V, and (-Ve) to the ground is (-) 220V. Under the fault condition, current will flow through the CT of the earth fault relay, and the earth fault relay will trigger based on the setting’s value.

DC earth fault relay can identify the earth fault in the system, however, it cannot detect the location of the earth fault. DC earth fault locator is used to detect the location of the fault.

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