Buchholz relay: The relay is located in the path of the oil from transformer tank to conservator. As seen from diagram, the upper mercury switch operates the alarm circuit due to tilting of the float by accumulation of gas evolved slowly in the transformer tank due to minor faults which may develop into major ones if the alarm is not investigated. Further lower mercury switch operates the trip circuit to switch off the circuit breaker related to the transformer when there is a sudden flow of oil from the transformer tank to conservator. Such flow occurs when there is serious fault in the transformer tank. Here the float (lower) is placed in such a manner that it senses the sudden violent movement of oil from transformer tank to conservator.