Construction of Buchholz Relay: It takes the form of a domed vessel placed in the connecting pipe between the main tank and the conservator. The device has two elements. The upper element consists of a mercury switch attached to a float. The lower elements contain a mercury switch mounted on a hinged type flap located in the direct path of the flow of oil from the transformer to the conservator. The upper element closes an alarm circuit during incipient faults whereas the lower element is arranged to trip the circuit breaker in case of severe internal faults.
Operation: 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/gas from transformer tank to conservator.