The sensitive earth fault relay is a protective device that works by measuring the residual current across the three phases in a system. This is done using a Core Balance Current Transformer (CBCT). In the ideal condition, the residual current will be zero as all the currents flow through the three wires and their magnetic fields cancel each other out.
In the event of a fault, the residual current over the three phases will not be equal to zero as the current from the faulted phase flows through the earth.
The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding. High resistance grounding restricts the earth fault current to less than 10A. High resistance grounding enables electrical systems to continue running when one of the phases is faulted. This prevents interruptions to the power supply. This kind of earthing system provides time to identify and isolate the fault.
Once an earth fault occurs in the high resistance grounding system, an alarm needs to be generated and the fault needs to be traced. For this a reliable protection which detects earth faults even when the fault current is very low is necessary. Undetected earth faults in this system are dangerous as a second earth fault in another phase may result in a short-circuit. Conventional earth fault relays may not be accurate in detecting an earth fault at such low current values.
The sensitive earth fault protection, as the name suggests, is a highly sensitive relay. It can sense currents as low as 0.2% of the CT secondary current.
The sensitive earth fault relay may be configured to either generate an alarm or a trip signal.