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Home / Technical Articles / Protecting Oil Type Transformer with Buchholz Relay

Introduction to Buchholz Relay

Buchholz relay is a gas-actuated relay installed in oil immersed transformers for protection against all kinds of faults. Named afteri ts inventor mr. Max Buchholz (1875–1956) in 1921, relay is used to produce an alarm in case of incipient (i.e.slow-developing) faults in the transformer and to disconnect the transformer from the supply in the event of severe internal faults.

Protecting Oil Type Transformer with Buchholz Relay
Protecting Oil Type Transformer with Buchholz Relay

It is usually installed in the pipe connecting the conservator to the main tank. It is a universal practice to use Buchholz relays on all such oil immersed transformers having ratings in excess of 750 kVA. The Buchholz relay is a protective rely for equipment immersed in oil for insulating and cooling purpose.

It is intended mainly for transformers or choke coils having a conservator vessel.
The relay responds to the accumulation of gas or air inside the apparatus when the oil level is too low or the flow of oil unusually strong. The relay neither gives a warning signal or disconnects the endangered equipment.

The Buchholz relay operates even on very slight faults which are just in process of developing, so that greater damage may be prevented.

Protection Range

The relay is particularly effective in case of:

  1. Falling oil level owing to leaks
  2. Short circuited core laminations
  3. Short – circuits between phases
  4. Broken-down core bolt insulation
  5. Earth faults
  6. Bad contacts
  7. Puncture of bushing insulators inside tank
  8. Overheating of some part of the windings

In the event of a fault, oil or insulations decomposes by heat, producing gas or developing an impulse oil flow. To detect these phenomena, a Buchholz relay is installed.

The Buchholz relay is installed at the middle of the connection pipe between the transformer oil tank and the conservator, so that, upon a fault development inside a oil transformer, an alarm is set off or the transformer is disconnected from the circuit.

Construction and technical characteristics

Buchholz relay contruction
Buchholz relay contruction

1-2Release terminals
3-4Alarm terminals
5Earth terminal
6Breather cock
7Valve of pneumatic test
8Mechanical test of the alarm/release circuit
9Cable gland
10Drain plug

Casing Non porous weatherproof compact casting of light aluminium alloy painted.

Cover Non porous weatherproof compact casting of light aluminium alloy painted. On the cover are located : the terminal box, the valve of pneumatic test, the breather cock, the button for mechanical test of alarm and trip circuits.

Inspection windows Special tempered glass with graduated scale in cm3.

Contacts – They can be mercury switches or magnetic actuated switches (reed contacts). On request it’s possible to supply change-over switches.

Switches characteristics
Rated voltage:
24 ÷ 250V AC or DC
Rated current: 0,5 A (10000 tests)
Breaking capacity: 2 A AC (cos = 0,4 ÷ 25% – 50Hz) 2A DC (T = L / R = 40 msec).

Mechanical protection degree IP 54

Insulation 2000V 50Hz between terminals and earth for a 60 seconds time.

Working temperature Oil temperature range: – 25 / +115°C.

Vibration test (in normal operative conditions)
Oscillation amplitude: 2mm
Time diagram:
0Hz ÷ 100Hz 30 sec.
100Hz (200 vibrations/sec.) 60 sec.
100Hz ÷ 0Hz. 30 sec.

Contacts capacity to withstand vibrations
Mercury sw: 150 horizontal vibrations/sec. (75Hz) first signals of closing contacts 120 vertical vibrations/sec. (75Hz) first signals of closing contacts.
Magnetic sw: 200 horizontal vibrations/sec. (100Hz) no one signal of closing contacts 200 vertical vibrations/sec. (100Hz) no one signal of closing contacts.

Function of Buchholz relay

In the following the operation of a Buchholz relay is explained using the example of a double-float  Buchholz relay. The relay is built in the connecting pipe between the transformer tank and the  conservator. During normal operation it is filled completely with insulating liquid.

Due to buoyancy the  floats are at their top position. If a fault occurs inside the transformer, the Buchholz relay responds as  follows:

Gas accumulation

Gas accumulation in Buchholz relay
Gas accumulation in Buchholz relay

Fault: Free gas is available in the insulating  liquid.

Response: The gas in the liquid moves upwards, accumulates in the Buchholz relay and displaces the insulating liquid level. The moving float actuates a switch contact (magnet contact tube). An alarm signal is tripped. The lower float is not affected as from a certain gas volume the gas flows through a piping to the conservator.

Insulating liquid loss

Insulating liquid loss in Buchholz relay
Insulating liquid loss in Buchholz relay

Fault: Insulating liquid loss due to leakage.

Response: As the liquid level falls the top float moves downward. An alarm is tripped. If the liquid loss continues, conservator and piping as well as the Buchholz relay will be emptied. As the liquid level falls, the lower float moves downward. The moving float actuates a switch contact so that the transformer is disconnected.

Insulating liquid flow

Fault: A spontaneous incident generates a pressure wave moving in the direction of the conservator.

Response: The liquid flow reaches a damper arranged in the liquid flow. If the flow rate exceeds the operating threshold of the damper, the latter moves in flow direction. Due to this movement a switch contact is actuated so that the transformer is disconnected.

Insulating liquid flow - Buchholz relay
Insulating liquid flow – Buchholz relay

The upper and lower switching system form a functional unit in the single-float Buchholz relay. In case of a fault, the single-float Buchholz relay normally isolates the transformer immediately from the mains system.

Wiring Diagrams

Standard Wiring Diagram

Buchholz relay - Standard wiring diagram
Buchholz relay – Standard wiring diagram

Mounting Sketch

Buchholz relay - Mounting sketch
Buchholz relay – Mounting sketch


1. It is the simplest form of transformer protection.
2.It detects the incipient faults at a stage much earlier than is possible withother forms of protection.


1. It can only be used with oil immersed transformers equipped withconservator tanks.
2. The device can detect only faults below oil level in the transformer. Therefore, separate protection is needed for connecting cables.

  1. Terman – Buchholz relay broshure
  2. EMB Elektromotoren und Gerätebau Barleben GmbH – Transformer Protection Relays (Buchholz Principle)

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


    Nov 30, 2021

    how on puncher of insulation core ,this bucholz will work?

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