Buchholz Relay of Transformer and Working of Buchholz Relay

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Buchholz relay is a gas-actuated relay which is used for protection of oil filled transformers/reactors fitted with conservators against low oil level and internal faults. The Buchholz relay is provided with two hinged floats/buckets which on tilting operate mercury switches inside the oil tight enclosure. Mercury switches in turn actuated alarm and trip circuits depending upon nature of fault.


a)The Buchholz relay provided with counter balance aluminium buckets operate as given below:-

With minor faults gas formation is slow. As the gas accumulates the oil level in the buchholz relay falls, leaving the upper buckets full of the oil which predominates over the balance weight and the assembly tilts closing the mercury switch to operate alarm circuit. If the fault persists the oil level falls still further and the lower mercury switch operates the trip circuit which totally disconnects the equipment from the supply.

b)The Buchholz relay provided with two hinged floats operate as given below:-

Gas produced by abnormal conditions in the transformer, collects in the Buchholz relay which lowers the oil level so that the top float gradually comes down and ultimately operates the mercury switch when it reaches a certain level. This switch is usually connected to external alarm circuit.

Abnormal condition associated with major fault cause a surge in the transformer oil which moves towards the expansion vessel. This surge displaces the lower float operation the mercury switch associated with it. This switch usually is connected to the external trip circuit. This switch would also operate if the oil level falls below that of the float due to any reason.

The relay Setting:-

Relay size Trip contact oil Velocity Alarm contact Gas Volume Transformer ratting
25 mm 70-130cm/sec 110-160 c.c Upto 1000 KVA
50 mm 75-140 cm/Sec 180-220 c.c 1000 to 10000 KVA
80 mm 90-160 cm/sec 220-260 c.c Above 10,000 KVA


In the Buchholz relay,two pairs of electrically separate self reset ‘make’ contact one for alarm and one for trip, provided by mercury switch. They are wired to terminal box.
General construction of the Buchholz relay:

  1. The working parts are housed in a chamber with oil sight window.
  2. There is petcock provided at the top of the relay. These petcocks are used for releasing all the air trapped in the transformer or the relay. It can also be used taking out gas sample.

Installation :

  1. The Buchholz relay is connected in the pipe connection between the transformer and conservator so that normally it is completely filled with oil, and the “ ARROW” marked on the relay is pointing towards the conservator.
  2. The operation of the surge bucket /float ( lower bucket/ float ) is caused by the flow of oil to relay. Therefore, it is given as upward slope varying between three and seven degrees from the horizontal.
  3. A flat surface is provided on the top of the Buchholz relay case for the purpose of testing and inclination of the relay in pipe connection.

Testing on site:-

Air may be introduced slowly through the petcock provided either at the top or bottom. Oil level will gradually fall till the upper switch operates the alarm circuit.

Fault analysis after nature of gas collected from Buchholz relay:

By studying the nature off gas collected , fault analysis can be made as shown in Table below:-

Nature of Gas Probable fault
1. Colorless & neutral Air trapped in oil or in insulation
2. Grayish white with pungent smell non-inflammable. Overheating of insulation, press board etc.
3. Yellowish inflammable Decomposing of wood insulation.
4. Dark grey inflammable Flash over in all . Excessive overheating due to fault of major nature

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