It’s a protection relay used in oil immersed transformer to protect the transformer from insulation failure, core heating or any other type of internal faults, which may cause the heating of winding beyond the specified temperature. This relay is placed in between the pipe connecting the conservator and the tank. Generally used in power transformer of above 500 kVA.
It consists of two operating floats and is operated by two mercury switches separately provided for the alarm and trip. Due to an internal fault (collection of gases) or leakage of oil, if the oil level comes down the alarm relay first, operates and then the trip relay operates to isolate the transformer from the circuit.
