# Buchholz relay

Canonical URL: https://eloragrid.ai/symbols/buchholz-relay
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: BR
Drawing use: Schematic, SLD
Reference families: IEC, Manufacturer

Gas-and-oil actuated relay in the pipe to the conservator, giving alarm on gas build-up and trip on oil surge.

## In plain English

### What does Buchholz relay mean?

A Buchholz relay is fitted in the pipe between an oil-filled transformer tank and its conservator. Slow gas accumulation from minor internal faults operates an alarm float, while a rapid surge of oil from a serious internal fault operates a trip element, so the schematic shows two separate contacts.

### A simple example

On single-line diagrams it may appear only as a label on the transformer, while the control schematic shows its alarm and trip contacts wired to alarm indication and the transformer trip relay. The symbol does not show the gas sampling procedure or the test arrangement used during maintenance.

### What the drawings help you do

Identify separate alarm and trip contacts. Follow the trip contact to the transformer lockout or trip relay. A Buchholz relay detects gas and oil movement; a sudden pressure relay responds to rapid tank pressure rise.

### This particular reference

Gas-and-oil actuated relay in the pipe to the conservator, giving alarm on gas build-up and trip on oil surge. A Buchholz relay detects gas and oil movement; a sudden pressure relay responds to rapid tank pressure rise.

### Terms

- Winding: A coil of conductor inside an electrical machine or transformer.
- Normally open / normally closed: The contact state in the normal condition defined for the drawing.
- Coil: A wound conductor that can produce a magnetic field when energised.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [ETR-4000 and ETR-5000 transformer protection function tables](https://www.eaton.com/us/en-us/catalog/electrical-circuit-protection/transformer-relays.html)
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/)


## What it represents

A Buchholz relay is fitted in the pipe between an oil-filled transformer tank and its conservator. Slow gas accumulation from minor internal faults operates an alarm float, while a rapid surge of oil from a serious internal fault operates a trip element, so the schematic shows two separate contacts.
On single-line diagrams it may appear only as a label on the transformer, while the control schematic shows its alarm and trip contacts wired to annunciation and the transformer trip relay. The symbol does not show the gas sampling procedure or the test arrangement used during maintenance.

## How to read it

- Identify separate alarm and trip contacts.
- Follow the trip contact to the transformer lockout or trip relay.

## Information to show or reference

- Show alarm contact
- Show trip contact
- Reference transformer trip relay

## Important distinction

A Buchholz relay detects gas and oil movement; a sudden pressure relay responds to rapid tank pressure rise.

## Facts

- Location: Pipe between tank and conservator
- Outputs: Gas alarm and oil-surge trip
- Preview: Functional illustration of the device role; not a product footprint or certified standard graphic

## Public references

- [ETR-4000 and ETR-5000 transformer protection function tables](https://www.eaton.com/us/en-us/catalog/electrical-circuit-protection/transformer-relays.html) — Eaton. Public transformer protection context in which mechanical transformer protections are wired.
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/) — IEC. IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.