Figure 1.19 shows Buchholz relay having two elements mounted in a small chamber. It is located in the pipe connection between the conservator and the transformer tank. Heat is produced due to leakage current for any minor fault and some of the oil in the transformer tank evaporates. Some vapour comes to the top of the chamber while passing through to the conservator. The oil level falls due to accumulation of vapour, and the mercury tape attached to the float is tilted closing the alarm circuit to ring the bell.
A release cock is attached to the top of the chamber to release the pressure of the chamber after operation and gas is emitted. It allows refilling of oil in the chamber once again. During severe fault, a large volume of gas is evolved, which tilts the lower element containing a mercury switch mounted on a hinged-type flap and the trip coil is energized. A test cock is provided at the bottom of the chamber, which allows air to be pumped into the chamber for test purposes.