What Is a Buchholz Relay and What Does It Protect?

Published 27 Aug 2026Updated 27 Aug 20265 min readIntermediateComponents

Quick answer — A Buchholz relay is a gas-actuated protection device fitted in the pipe between the transformer conservator and tank. It collects gas released by an internal fault and trips the unit on a major fault, or gives an alarm on a slow accumulation (e.g. minor overheating). The gas collected can be analysed to diagnose the fault.

Contents

How it works
What it senses
Buchholz gas analysis

What is a Buchholz relay and how does it protect a transformer?

How it works

The relay is a chamber with float switches (or a baffle/reed) that sense oil level and gas accumulation. An internal fault generates gas that rises into the relay; a large fault creates a surge of oil that trips a second element.

What it senses

Gas collection indicates a slow fault (e.g. a hot spot, partial discharge or cellulose overheating); an oil surge indicates a more energetic arcing fault. The amount and type of gas, and the pressure, guide the diagnosis.

Buchholz gas analysis

Collecting the gas and testing it (combustible, CO2, colour) helps identify the fault — for example a combustible, odourless gas typically points to a thermal or dielectric event.

References & standards

IEC 60076-1
IEEE C57.12.90

Book series Academy

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.

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