The Residual Current based earth Fault relay works by measuring the vector sum of the three phase currents.
Under healthy conditions, the vector sum of the three currents is zero. In the event of an earth fault, however, the fault current flows through the ground and hence, the vector sum of the currents is not equal to zero. This is known as the residual current. This current can be used to operate the earth fault relay.
The connection of the earth fault relay consists of three current transformers connected in parallel to each other. This kind of earth fault protection is also known as unrestricted earth fault protection.
The residual current protection is usually set to operate at around 10% of the nominal current. For fault currents lower than this value, as may be the case in high resistance grounded generators and transformers, the sensitive earth fault relay is used. This is because, the three current transformers used in the residual current protection may not be exactly identical in response, even if they are from the same manufacturer. Thus for very low setting, there is the risk of false operation of the relay due to errors in the current transformers. Since the sensitive earth fault relay uses one Core Balance Current Transformer instead of three individual current transformers, it can be set to lower values of earth fault current.