---
title: "Classes of fire and fire extinguishers"
subject: "Workshop Technology"
chapter: 1
key: "semester_2/workshop_technology:fire-classes-extinguishers"
source: https://www.bctengineer.com/q/semester_2/workshop_technology/fire-classes-extinguishers
site: BCT Engineering
---

# Classes of fire and fire extinguishers

Workshop Technology · Chapter 1: Workshop, its Applications and Safety Considerations

- Asked in: year not given, 3+3 marks
- Video: [Fire Extinguishers & Classification of Fire](https://www.youtube.com/watch?v=o5QdAEJH43c) — Engineering Motive
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/workshop_technology/fire-classes-extinguishers
- Whole chapter: https://www.bctengineer.com/md/semester_2/workshop_technology/chapter-1

## Question

Classify fires according to the material burning. Name the extinguisher suitable for each class and explain the working of a carbon dioxide extinguisher.

## Short answer

Class A fires (wood, paper) use water or foam; B (liquids) use foam or CO₂; C (gases) dry powder; D (metals) special powder; E (electrical) CO₂ or dry powder. A CO₂ extinguisher contains liquid CO₂; upon release it expands to a gas at -78°C, smothering by displacing oxygen and cooling. Suitable for B and E fires.

## Answer

Fire needs fuel, heat and oxygen (the fire triangle). Removing any one of them puts the fire out.

#### Classes of fire and suitable extinguishers
| Class | Fuel | Suitable extinguisher |
|---|---|---|
| A | Wood, paper, cloth, rubber | Water, foam, dry powder (ABC) |
| B | Oils, petrol, paint, grease, solvents | Foam, CO2, dry chemical powder |
| C | Gases (LPG, acetylene) | Dry powder; first shut off the gas supply |
| D | Metals (Mg, Al, Na) | Special dry powder (sand, graphite) |
| E | Live electrical equipment | CO2, dry powder; never water |

Water must not be used on oil fires (it spreads the burning oil) or on live electrical fires (shock hazard).

#### Carbon dioxide (CO2) extinguisher
It is a steel cylinder holding liquefied CO2 at high pressure (about 50 to 60 bar) with a valve, a discharge hose and a horn. The horn is made of a non-conducting material, because a metal horn could conduct current on an electrical fire.

```text
   [Valve + pin]
        |
   +----+----+
   |  CO2    |-----> hose --> horn
   | (liquid)|
   | dip tube|
   +---------+
```

- Working: the safety pin is pulled and the valve is opened. Liquid CO2 rises up the dip tube and leaves through the horn, where it expands suddenly into gas and cools to about -78 °C, forming dry-ice snow.
- Action: CO2 is heavier than air, so it forms a blanket that displaces oxygen (smothering) and also cools the fuel.
- Use: Class B and electrical fires, since it leaves no residue and does not conduct electricity. It is poor for deep-seated Class A fires.
- Precaution: aim at the base of the fire; in a closed room CO2 can cause suffocation.

## Check yourself

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

1. According to the fire triangle, a fire is extinguished by
   A. Adding more fuel
   B. Raising the heat
   C. Removing any one of fuel, heat or oxygen
   D. Removing only the oxygen
   Answer: C. Fire needs fuel, heat and oxygen together; removing any one puts it out. (Key point: Fire triangle)

2. Which extinguisher agent must never be used on a live electrical fire?
   A. Water
   B. CO2
   C. Dry powder
   D. None of these
   Answer: A. Water conducts electricity and gives a shock hazard; CO2 and dry powder are suitable. (Key point: Class of fire for live electrical)

3. What happens when liquid CO2 leaves the horn of an extinguisher?
   A. It stays liquid and floods the fire
   B. It expands to gas, cools to about -78 °C and forms dry-ice snow
   C. It burns with the fuel
   D. It reacts chemically with the fuel
   Answer: B. Sudden expansion cools the CO2 to about -78 °C, giving gas and snow that smother and cool the fire. (Key point: Working of CO2 extinguisher)

4. Why does CO2 smother a fire, and what is a precaution in using it?
   A. It is lighter than air and rises; avoid it near floors
   B. It is heavier than air, displaces oxygen; in a closed room it can cause suffocation
   C. It removes fuel; it leaves a harmful residue
   D. It conducts heat away; it must be sprayed at the top of flames
   Answer: B. CO2 is heavier than air and blankets the fire by displacing oxygen, but it can suffocate people in closed rooms. Aim at the base of the fire. (Key point: Action and precaution of CO2)

## Related in Workshop Technology

- [Cutting tool materials and carbon steels](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Acutting-tool-materials-carbon-steels) — Workshop Technology, chapter 7

## Related in other subjects

- [Carbon dioxide and atmospheric pollution](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aco2-pollution) — Engineering Chemistry, chapter 3
- [Short note: Fire extinguisher](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ashort-fire-extinguisher) — Building Technology, chapter 12
- [Create classes from design class diagrams](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aclasses-from-design-class-diagram) — Object Oriented Analysis and Design, chapter 4, asked 2×
- [Classes of insulation by thermal stability](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulation-thermal-classes) — Electric Machine Design, chapter 1, asked 4×