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What Is a Buchholz Relay and How Does It Work?

Updated on June 13, 2016

Buchholz relay

What is a Buchholz relay?

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.

Why Buchholz relay is used in transformers?

Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer. Short circuit faults such as inter turn faults, incipient winding faults, and core faults may occur due to the impulse breakdown of the insulating oil or simply the transformer oil. Buchholz relay will sense such faults and closes the alarm circuit.

Working principle

Buchholz relay relies on the fact that an electrical fault inside the transformer tank is accompanied by the generation of gas and if the fault is high enough it will be accompanied by a surge of oil from the tank to the conservator

Whenever a fault occurs inside the transformer, the oil in the transformer tank gets overheated and gases are generated. The generation of the gases depends mainly on the intensity of fault produced. The heat generated during the fault will be high enough to decompose the transformer oil and the gases produced can be used to detect the winding faults. This is the basic principle behind the working of the Buchholz relay.


Buchholz relay can be used in the transformers having the conservators only. It is placed in the pipe connecting the conservator and the transformer tank. It consists of an oil filled chamber. Two hinged floats, one at the top of the chamber and the other at the bottom of the chamber which accompanies a mercury switch each is present in the oil filled chamber. The mercury switch on the upper float is connected to an external alarm circuit and the mercury switch on the lower is connected to an external trip circuit.

When does a buchholz relay operate?


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.

When does a buchholz relay operate?

Buchholz relay operates during three conditions:

1. Whenever gas bubbles are formed inside the transformer due to severe fault.

2. Whenever the level of transformer oil falls.

3. Whenever transformer oil flows rapidly from the conservation tank to the main or from the main tank to the conservation tank.

Advantages of Buchholz relay

  • Buchholz relay indicates inter turn faults and faults due to heating of core and helps in the avoidance of severe faults.
  • Nature and severity of fault can be determined without dismantling the transformer by testing the air samples.

Limitation of Buchholz relay

It can sense the faults occurring below the oil level only. The relay is slow and has a minimum operating range of 0.1second and an average operating range of 0.2 seconds.

Operation of Buchholz Relay


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    • profile image

      Rose 2 years ago

      Are Buchholz relay present in all transformers..

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      OSBERT JOEL C 2 years ago from CHENNAI

      No. It used only in oil immersed transformers having ratings above 500kVA

    • profile image

      Vikash Mj 2 years ago

      Thanks a lot it helps to me lot and saves my time .

      Contents are in vary common language.

      Once again thanks.

    • profile image

      vijayakumar.d 2 years ago

      very good and know easy operation

    • profile image

      Olleny Kalima 2 years ago

      Thanks it really helps me as a young engeneer

    • goodnews11 profile image

      OSBERT JOEL C 2 years ago from CHENNAI

      Thank you Olleny Kalima..

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      peter 2 years ago

      what are the functional tests carried on the Buchholz relay?

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      OSBERT JOEL C 2 years ago from CHENNAI

      Buchholz relay is subjected to vibration tests and stability test. It is very sensitive device. It must withstand a minimum of 150 horizontal vibrations/sec for mercury switches and 200 horizontal vibrations/sec for magnetic actuated switches. It is also subjected to temperature tests.

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      Avinah Kulkarni 15 months ago

      Usefull info

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      Yadvir Singh 14 months ago

      Method is very effective to understand everybody.

      Yadvir Singh

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      satya 6 months ago

      It is usefull to every one and understand everyone,

    • goodnews11 profile image

      OSBERT JOEL C 6 months ago from CHENNAI

      Dear Satya,

      Thank you..

    • profile image

      5 months ago

      Why use oil inmmersed transformer in buchloz relay

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      Engr. Shola 2 weeks ago

      What is the different between Buchoiz relay and TIM 3 relay?

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      kumaraguru 7 days ago


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