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

# Buchholz relay for internal faults

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2070 Bhadra, 4 marks
- Video: [SGP402 Buchholz Relay](https://www.youtube.com/watch?v=PUKGoU7q_Qk) — Lectures in Electrical Engineering
- 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-internal-faults
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

How does Buchholz relay protect the internal faults of transformer?

## Short answer

Internal faults produce gas; minor faults cause slow accumulation, lowering oil level to tilt the upper float and close an alarm. Major faults create an oil surge that deflects the lower float/flap, tripping the transformer breakers.

## Answer

A **Buchholz relay** is a gas-actuated relay fitted in the pipe between the main tank and the conservator of oil-immersed transformers (above about 500 kVA). It detects internal faults by the gas and oil surge they produce.

#### How it protects
Any fault inside the tank (winding or core) produces heat; the heat decomposes the oil and insulation into gas (H₂, CO, C₂H₂, etc.). The gas rises towards the conservator and passes through the relay.

```text
  Main tank ──pipe──► [ Buchholz relay ] ──► Conservator
                       ┌──────────────┐
                       │ float1 → alarm│ (upper)
                       │ flap/float2 → │
                       │   trip        │ (lower)
                       └──────────────┘
```

1. **Incipient (minor) faults** such as insulation breakdown between core laminations, core-bolt insulation failure, poor joints, inter-turn faults or loss of oil: gas forms slowly and collects at the top of the relay. The oil level in the relay falls, the **upper float** tilts and closes the **alarm** circuit.
2. **Severe faults** such as phase-to-earth or phase-to-phase winding faults: a large volume of gas is produced suddenly and forces a **surge of oil** towards the conservator. This surge deflects the **lower float/flap**, which closes the **trip** circuit and opens the transformer circuit breakers.
3. **Oil leakage:** if oil level falls, first the alarm and then the trip float operate.
4. The gas collected can be drawn off through a cock and analysed: its colour and composition indicate the type of fault (e.g. white: paper insulation; yellow: wood; dark grey: oil).

The relay therefore gives early warning of slowly developing faults that differential protection cannot see, and trips quickly on major internal faults.

## Check yourself

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

1. How does a Buchholz relay detect an internal fault?
   A. By measuring winding current
   B. By sensing the gas and oil surge produced when heat decomposes oil and insulation
   C. By measuring tank voltage
   D. By sensing bushing temperature only
   Answer: B. Fault heat decomposes oil and insulation into gas, which rises to the relay; strong faults also cause an oil surge. (Key point: Fault detection mechanism)

2. Which response is correct?
   A. Incipient faults trip instantly; severe faults only alarm
   B. Incipient faults: slow gas collects, upper float tilts and gives alarm; severe faults: oil surge deflects lower flap and trips
   C. Both only alarm
   D. Both trip via the upper float
   Answer: B. Slow gas operates the upper (alarm) float; a sudden surge operates the lower flap and closes the trip circuit. (Key point: Incipient vs severe fault response)

3. Why is a Buchholz relay valuable in addition to differential protection?
   A. It is faster than differential protection
   B. It gives early warning of slowly developing faults the differential cannot see
   C. It protects the bushings
   D. It works on dry-type transformers
   Answer: B. Incipient faults such as core insulation failure produce gas but little current imbalance, so only the Buchholz relay sees them. (Key point: Advantage over differential)

## Related in Switchgear and Protection

- [Gas operated Buchholz relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abuchholz-relay-working) — 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
- [Voltage-monitored overcurrent relay timing](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avoltage-monitored-overcurrent) — Switchgear and Protection, chapter 7