- Practice · 3+3 marks
Classes of fire and fire extinguishers
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.
Classify fires according to the material burning. Name the extinguisher suitable for each class and explain the working of a carbon dioxide extinguisher.
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.
[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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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