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What Is a Buchholz Relay and How Does It Work?

I am an electrical and automation engineer specialized in LV switchgear design and process automation.

Buchholz relay

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 that have a rating of more than 500 kVA. Buchholz relay is not provided in relays that have a rating below 500 kVA from the point of view of economic considerations.

Why Is Buchholz Relay 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.

Diagram 2

Diagram 2


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?

When does a Buchholz relay operate?


The 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 move towards the conservator through the pipeline. These gas bubbles get collected in the relay chamber and displace 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 and its color indicates the nature of the fault that 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.

Operation of Buchholz Relay

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.
  • The nature and severity of the 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.

Diagram 3

Diagram 3

Additional Reference


Salim Kironde on July 09, 2020:

Am exhaustively answered, Thanks

Medline Chinyowa on June 25, 2020:

Useful information

Kishore on May 27, 2020:

Super explained

Elias on January 09, 2020:

Good doc

Jaya on December 02, 2019:

Is mercury switch operating because of difference in upthrust of air and liquid - transformer oil?

Jagapathi Babu on November 04, 2019:

Thanks for information brother

mabor Adut on August 20, 2019:

very good information


Henry Kafumbe on August 06, 2019:

Very useful and helpful information

Tejas Shinde on July 05, 2019:

Very informative and useful.

ImRan Hussain on June 18, 2019:

Very nice information

Nduka on May 11, 2019:

Is it the same as light gas alarm

daniel on May 02, 2019:

Switching the transformer on the primary side using HV fuses connecting not simultaneously won`t cause any harm to the transformer?

Emovon on April 18, 2019:

Since the buchholz relay is only used to detect the internal faults in transformers with ratings above 500KVA. How can the faults in transformers with below ratings be detected?

Emmanuel Adu on April 02, 2019:

Thank you for this very important information and education on this very important component on the transformer.

muzi on March 08, 2019:

what operates the relay. is it the accumulated gas on the rising oil due to the gas

P.SRINIVASULU on January 16, 2019:

thank u very much this topic

papa on December 06, 2018:

Nice one

sriraj on November 10, 2018:

if bucholz relay acts, what action to be taken on trnsformer

Chris Lombaard on November 02, 2018:

Hi i am from South Africa can you please send me BS25 ,BS50 &BS80 Magnetic buchholz relay specifications complete with the test report with 1 trip 1 alarm and the low gas volume (trip ) specs

Srinivasan C on October 09, 2018:

Hi sir,

Easy to understand with proper pictures. Thanks & Regards

Srinivasan C


Ashraf on October 07, 2018:

Thanks to explain very simply

Rishi on August 24, 2018:

Hi . How closure of contacts take place with help of mercury

Sunil Kolekar on August 10, 2018:


what is operating voltage of buchholz relay

nelson on July 30, 2018:

simply explained

krishna on June 28, 2018:

very simply explained the working principle .so thank you so much

Godfrey on June 17, 2018:

Thank you so much for the clear notes on the Buchholz relay. Notes are well structured with good diagrams to illustrate. Thx . Once again. Keep up . I wish you add something over setting & operation of the Arcing horns together with the Faults current suppressors.

OSBERT JOEL C (author) from CHENNAI on May 11, 2018:

Thank you Mr. Emovon

EMOVON ALFRED on May 10, 2018:

Straight to the point and easy to understand.

EMOVON ALFRED on May 10, 2018:

Straight to the point and easy to understand.

P.nanda on May 08, 2018:


Very very easy to understand

irfan munawar on March 08, 2018:

very nice

Divy Patel on February 25, 2018:

nicely explained with easy terms.

Aitizaz Malik on February 09, 2018:

very nice and easy to understand.

viresh on January 21, 2018:


N.Krishnaswamy on January 16, 2018:

Good information about the operation of Buchholz relay and how it protects the transformer.Good for final year Engineering students.

Shaiju on December 26, 2017:

Thank you...l was learned lot of things from this article

Anil yadav on August 31, 2017:

Can I test the performance of Buchholz relay in normal condition?

I mean consentancy of relay

monday eliab on August 16, 2017:

Thanks so much for this info. So sounding!!!

ashok Gupta Asssitnat Engineer9 Distribution) MSEDCL Maharashtra on August 15, 2017:

description is very clear and very easy to understand the working principal

Christian Lumembang, lecturer of Polytechnic State Of Ujung Pandang, Makassar, Indonesia on July 27, 2017:

Description is so clear and easy to understand. thanks.

Shubham Rohilla on July 19, 2017:


Prashnt on July 07, 2017:

Is bucholz relay used in air cooled transformer??

Bhushan on June 28, 2017:

Can we relate buchholz relay operation with operating current?

i.e. Can it be set to operate for 20% overload or short circuit external to transformer?

Kapila A on June 27, 2017:

Thank you very much, it saved my time. Anyone can understand easily.

Sandeep singh on June 17, 2017:

Buchholz relay is a voltage sensitive device ??


Rabbani shaik on May 31, 2017:

Thank you so much.......

I got what I want so easily

Nazeem on May 24, 2017:

What is the effect if you use a bigger buchholz relay on a smaller transformer. Say using a 50mm instead of a 25mm on a 1000kVA transformer with ±800 litres of oil.

Mehman on April 06, 2017:

Thanks. Very useful.

kumaraguru on March 20, 2017:


Engr. Shola on March 13, 2017:

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

K on October 11, 2016:

Why use oil inmmersed transformer in buchloz relay

OSBERT JOEL C (author) from CHENNAI on September 04, 2016:

Dear Satya,

Thank you..

Yadvir Singh on January 15, 2016:

Method is very effective to understand everybody.

Yadvir Singh

Avinah Kulkarni on December 09, 2015:

Usefull info

OSBERT JOEL C (author) from CHENNAI on March 25, 2015:

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.

peter on March 23, 2015:

what are the functional tests carried on the Buchholz relay?

OSBERT JOEL C (author) from CHENNAI on February 10, 2015:

Thank you Olleny Kalima..

Olleny Kalima on February 10, 2015:

Thanks it really helps me as a young engeneer

vijayakumar.d on August 24, 2014:

very good and know easy operation

Vikash Mj on August 17, 2014:

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

Contents are in vary common language.

Once again thanks.

OSBERT JOEL C (author) from CHENNAI on May 05, 2014:

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

Rose on May 03, 2014:

Are Buchholz relay present in all transformers..