---
title: "Gas operated Buchholz relay"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:buchholz-relay-working"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/buchholz-relay-working
site: BCT Engineering
---

# Gas operated Buchholz relay

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2078 Chaitra, 6 marks
- Video: [buchholz relay | buchholz relay working animation | buchholz relay in transformer | in hindi | relay](https://www.youtube.com/watch?v=TdUwHXjCjnQ) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/buchholz-relay-working
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

Explain the working of gas operated Buchholz relay used for the protection of a transformer. State its limitations and advantages.

## Short answer

It is mounted in the pipe to the conservator. Slow gas formation from incipient faults lowers the oil level and closes an alarm; a heavy oil surge from severe faults trips the transformer. It is simple, needs no CTs, detects early faults and gas analysis is possible, but only for oil-immersed conservator-type transformers, slow (0.2 s), and cannot protect external faults.

## Answer

A **Buchholz relay** is a gas-operated relay mounted in the pipe connecting the main tank of an oil-immersed transformer to the conservator. It protects against all internal faults that produce gas or an oil surge.

#### Construction
- A domed, oil-filled chamber in the inclined connecting pipe.
- **Upper float (hinged)** with a mercury switch connected to an **alarm** circuit.
- **Lower float or baffle flap** in the path of oil flow, with a mercury switch connected to the **trip** circuit of the transformer breakers.
- A gas release cock at the top and an inspection window.

```text
                     gas cock
                        │
  Tank  ════════╗  ┌────┴───────┐  ╔════════ Conservator
   oil  ───────►╠══│ (o)─upper  │══╣ (pipe rises gently
                ║  │   float→A  │  ║  towards conservator)
                ║  │ [flap]─lower│  ║
                ║  │   float→T  │  ║
                   └────────────┘
          A = alarm switch, T = trip switch
```

#### Working
1. **Normal condition:** the relay is full of oil; both floats stay up and their contacts are open.
2. **Incipient faults** (core insulation failure, inter-turn short, hot spot, bad joint): heat slowly decomposes the oil; gas bubbles rise to the relay chamber and collect at the top. The oil level falls, and the upper float tilts and closes the **alarm** circuit.
3. **Severe faults** (earth fault, phase-to-phase winding fault): gas forms rapidly, and the pressure drives a surge of oil through the pipe towards the conservator. This surge tilts the lower float/flap and closes the **trip** circuit, disconnecting the transformer from both sides.
4. **Loss of oil** (leak): the oil level falls first below the upper float (alarm) and then below the lower float (trip).
5. The collected gas is sampled; its colour and analysis show the nature of the fault.

#### Advantages
- Simplest form of transformer protection; no CTs or external supply needed.
- Detects **incipient faults** at an early stage, before they become serious, and gives an alarm.
- Detects inter-turn and core faults that differential protection may not see.
- Gas analysis helps to identify the type of fault.
- Also protects against loss of oil.

#### Limitations
- Usable only for **oil-immersed transformers with a conservator**.
- Protects only against faults **inside the tank**; it does not cover bushings, cables or external connections.
- **Slow**: the minimum operating time is about 0.1 s (average 0.2 s), so differential protection is still needed for fast clearance.
- Can maloperate on oil surges caused by heavy through faults, earthquakes or mechanical shocks, or on air entering after oil filling.
- Economical only for transformers above about 500 kVA.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Where is a Buchholz relay fitted and what do its two floats do?
   A. Inside the winding; both trip
   B. In the pipe between main tank and conservator; the upper float gives alarm, the lower float/flap gives trip
   C. On the conservator vent; both give alarm
   D. On the bushing; upper float trips
   Answer: B. It sits in the inclined connecting pipe; the upper float closes the alarm circuit, the lower float or flap closes the trip circuit. (Key point: Location and types of float)

2. If transformer oil slowly leaks away, the relay:
   A. Does not respond
   B. Trips first, then alarms
   C. Alarms first (upper float) and then trips (lower float)
   D. Only alarms
   Answer: C. The falling oil level uncovers the upper float first, then the lower. (Key point: Loss of oil)

3. Which is a limitation of the Buchholz relay?
   A. It needs CTs and a DC supply
   B. It covers external faults on cables
   C. It works only on oil-immersed transformers with a conservator and covers only faults inside the tank
   D. It cannot detect inter-turn faults
   Answer: C. It is restricted to oil-filled conservator-type transformers, protects only inside the tank, and is relatively slow (about 0.1-0.2 s). (Key point: Limitations)

## Related in Switchgear and Protection

- [Buchholz relay for internal faults](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abuchholz-internal-faults) — Switchgear and Protection, chapter 7
- [Percentage differential relay: 125 MVA transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apercentage-differential-125mva) — Switchgear and Protection, chapter 7
- [Percentage differential relay: 150 MVA transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apercentage-differential-150mva) — Switchgear and Protection, chapter 7
- [Static differential relay for two-winding transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-differential-two-winding) — Switchgear and Protection, chapter 8
- [Distance protection relay and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adistance-protection-relay) — Switchgear and Protection, chapter 7
- [Differential protection of power transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Atransformer-differential-protection) — Switchgear and Protection, chapter 7

## Related in other subjects

- [Working of combined gas-steam plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-gas-steam-working) — Power Plant Equipment, chapter 8
- [Protection for alternator and step-up transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Amega-project-protection) — Power Plant Design, chapter 4
- [Transformer protection, selection and vector group](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Atransformer-protection-vector-group) — Power Plant Design, chapter 4