Transformer Buchholz Relay


A Buchholz relay is an electrical transformer protection device. For conservator type electrical transformers, a gas actuated relay or Buchholz relay is installed between the conservator tank and the main tank. Gas actuated relays have two functions, whereas a Buchholz Relay has three. The Buchholz relay is named after its inventor, Max Buchholz.

Location of Buchholz Relay on Transformer

Location of Buchholz Relay on Transformer

Buchholz Relay Components

Buchholz relays are usually installed with two floats and one baffle plate. The floats are installed in high and low positions within the relay housing. The baffle plate is attached to a spindle running vertically through the relay housing.

Buchholz Relay Components

Buchholz Relay Components (3D model based upon design at

How Buchholz Relays Work

The below video is an extract from our Introduction to Electrical Transformers Online Video Course.


Fault 1: Small Electrical Faults

Operator: Upper Float

Effect: Alarm

Small electrical faults within the transformer tank heat the insulating liquid and create small gas bubbles. These bubbles rise to the top of the tank and exit to the Buchholz relay. It is not possible for the gas to pass through the relay so it will slowly accumulate within the relay housing and displace the insulating liquid. If the fault continues, the volume of the gas will become so large that the upper float will fall (it is floating on the liquid) triggering an alarm.

Fault 2: Low Insulating Liquid Level

Operator: Lower Float

Effect: Alarm and Shutdown A large reduction in insulating liquid within the transformer could lead to a catastrophic failure of the transformer. In order to avoid this, the Buchholz relay will shut down the transformer should a low liquid level be detected. A decreasing liquid level will first be detected by the upper float which will activate an alarm as the liquid level within the relay drops. If the liquid level continues to drop, the lower float will fall and the transformer will trip/shutdown.

Fault 3: Large Electrical Faults

Operator: Baffle Plate

Effect: Shutdown

A large electrical fault will create a large volume of gas and this will displace a large volume of liquid within the transformer tank. As there is no room for this displaced liquid within the transformer tank, the insulating liquid moves in the direction of the conservator tank at high velocity (pressure/shock wave). The sudden movement of liquid through the relay causes the baffle plate to rotate, which activates a switch and/or pushes the lower float down, and trips the transformer.

Fault and Operator Summary

Small Electrical Faults = Upper float drops. Lower float no movement. Baffle plate no movement.

Low Insulating Liquid Level = Upper float drops. Lower float drops. Baffle plate no movement.

Large Electrical Faults = Upper float drops maybe. Lower float drops. Baffle plate pushes lower float down.

How Buchholz Relays Work

The below video is an extract from our Introduction to Electrical Transformers Online Video Course.


What is the difference between a gas actuated relay and a Buchholz relay?

Gas actuated relays operate based upon the amount of gas generated by small and/or large electrical faults, they do not measure the transformer liquid level. Buchholz relays operate based upon the amount of gas generated by small electrical faults, large electrical faults and the transformer insulating liquid level.

General Faults

General faults within the transformer may also lead to one or more of the operators within the relay causing alarms or trips. Whatever the reason for the fault, all alarms and trips should be fully investigated. If the transformer has tripped, the reason for this trip must be ascertained prior to the transformer being returned to service.

Reasons for Faults

Small electrical faults could be caused by eddy currents, local overheating, partial discharges, or, poor insulating liquid conditions.

Large electrical faults are more likely due to winding-winding faults, phase-phase faults, or ground faults.

A reduction in insulating liquid level is most likely due to leakage from the transformer tank or associated piping and appendages. Gaskets, seals, joints and welds are the usual leakage locations.

Testing and Maintenance

It is normally possible to test the float operators by pumping air into the float chamber to displace the liquid, which causes the floats to drop. The baffle plate can be tested by rotating a rod, or handheld lever, which is mounted on the casing of the relay. The condition of the electrical switches themselves can be checked by measuring the resistance when using a standard multimeter.

Note: Some people disapprove of pumping air into the float chamber to test the relay, although its the author's experience that there are as many people for the procedure as against. There are pros and cons, but these are discussed in the saVRee Transformer Maintenance course and are beyond the scope of this article.

Buchholz Relay Cross Sectional View

Buchholz Relay Cross Sectional View


Additional Resources

Encyclopedia - saVRee