Buchholz Relay


Buchholz Relay

Buchholz relay is a safety device which is generally used in large oil immersed transformers. It is a type of oil and gas actuated protection relay. It is used for the protection of a transformer from the faults occurring inside the transformer, such as impulse breakdown of the insulating oil, insulation failure of turns etc.
Buchholz relay is a type of oil and gas actuated protection relay universally used on all oil immersed transformers having rating more than 500 kVA. Buchholz relay is not provided in relays having rating below 500 kVA from the point of view of economic considerations.

Working principle

The Buchholz relay comprises hinged float and mercury switch assembly for both the alarm and trip circuits. The entire assembly is in an oil proof case which has two glass windows. When the oil level is reduced from the desired level, the float switch moves down that will touch the contact.

In case of major faults, the gases generated in transformer tank due to decomposing of oil rush towards conservator tank through Buchholz relay. These gases pressurize the oil and reduce the oil level in buchholz relay and the float switch go down to close the trip circuit as shown in the below figure.

While reducing the oil level, the alarm will get activated. If the pressure is higher in the transformer tank the trip circuit will be activated to close the mercury switch and trip the power to transformer.

Construction of Buchholz Relay

Buchholz relay consists of an oil filled chamber. There are two hinged floats, one at the top and other at the bottom in the chamber. Each float is accompanied by a mercury switch. The mercury switch on the upper float is connected to an alarm circuit and that on the lower float is connected to an external trip breaker. The alignment of mercury switch hence depends upon the position of the float. The construction of a  buchholz relay is shown in the figure.

Operation of Buchholz Relay

Operation of the Buchholz relay is very simple. Whenever any minor fault occurs inside the transformer heat is produced by the fault currents. The transformer oil gets decomposed and gas bubbles are produced. These gas bubbles moves towards the conservator through the pipe line.

These gas bubbles get collected in the relay chamber and displaces oil equivalent to the volume of gas collected. The displacements of oil tilts the hinged float at the top of the chamber thereby the mercury switch closes the contacts of the alarm circuit.

The amount of gas collected can be viewed through the window provided on the walls of the chamber. The samples of gas are taken and analyzed. The amount of gas indicates the severity of and its color indicates the nature of fault occurred. In case of minor faults the float at the bottom of the chamber remains unaffected because the gases produced will not be sufficient to operate it.

During the occurrence of severe faults such as phase to earth faults and faults in tap changing gear, the amount of volume of gas evolves will be large and the float at the bottom of the chamber is tilted and the trip circuit is closed. This trip circuit will operate the circuit breaker and isolates the transformer.


  1. fault can be determined without even dismantling the transformer.
  2. indicates the internal faults due to heating and it helps in avoiding the major faults.


If any query or suggestion about Buchholz Relay please comment below or Email on sohal@electricalidea.com


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