Buchholz relay

In electric power distribution and transmission, a Buchholz relay is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a "conservator".

The Buchholz relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment.

[2] Buchholz relays have also been used in electric trains that operate on AC Electrified lines, including locomotives and multiple units.

These trains take a high voltage supply from overhead wires, at up to 25,000 volts AC, which is then fed to an oil cooled transformer that 'steps down' the supply to voltage suitable for use in the trains traction motors - usually between 750 and 1000 volts.

This switch normally will operate a circuit breaker to isolate the apparatus before the fault causes additional damage.

Two ball-shaped floats and two glass-enclosed reed switches are visible inside this cutaway view of a Buchholz relay
Schematic diagram of a large oil-filled power transformer. The conservator tank, green, at right, is marked 3 and the Buchholz relay is marked 5