How Does a CO2 Extinguisher Work?
Carbon dioxide is stored as a liquid under pressure inside the extinguisher cylinder. When the operating lever is depressed, CO2 is released through a discharge horn, where it immediately expands and converts to gas. This rapid expansion also creates a significant cooling effect (the horn becomes extremely cold — never hold the horn directly).
The CO2 gas displaces oxygen around the fire, breaking the combustion triangle. Because it leaves no residue — unlike powder or foam extinguishers — it is ideal for protecting electronic equipment, server rooms, commercial kitchens (for electrical fires only), and any environment where contamination would cause secondary damage.
Which Fires Can CO2 Extinguishers Be Used On?
- Class B fires — flammable liquids such as petrol, oils (excluding cooking oils), paint, and solvents
- Electrical fires — CO2 is non-conductive and safe for live electrical equipment (up to 1,000 V according to BS EN 3)
CO2 extinguishers are not suitable for Class A fires (ordinary combustibles like wood and paper — CO2 does not cool the material sufficiently to prevent re-ignition), Class F fires (cooking oils — risk of hot oil splatter), or use in confined spaces (risk of oxygen depletion for the user).
Where Are CO2 Extinguishers Used?
CO2 extinguishers are the standard choice for:
- Server rooms and data centres
- Electrical switchgear rooms and transformer areas
- Office environments where computer equipment is present
- Workshops and laboratories where flammable solvents are used
- Locations where a dry powder extinguisher would cause unacceptable damage to equipment
Under BS 5306-8, CO2 extinguishers should generally be provided alongside a water or foam extinguisher in offices and commercial premises, since CO2 alone is not effective on Class A materials which make up the majority of office combustibles.
Colour Coding and Identification
Under BS EN 3 and the UK's colour coding scheme (BS 7863:1996), CO2 extinguishers are identified by a black label panel on a red body. This replaced the older all-black body that was used before 1997. Never use a fire extinguisher unless you have confirmed it is the correct type for the fire — check the label and colour coding.
Maintenance and Servicing Requirements
Under BS 5306-3:2017, CO2 extinguishers require:
- Annual basic service — visual inspection, weight check, label check, and seal inspection
- Extended service at 5 years — full internal inspection of the valve and body
- Hydraulic pressure test (discharge test) at 10 years — the cylinder is tested to 1.5× working pressure
- Replacement or retest as required after any use or significant damage
Servicing must be carried out by a competent person as defined in BS 5306-3. The BFC EAL Level 3 Fire Extinguisher qualification is the recognised standard for competence in extinguisher maintenance across all extinguisher types, including CO2.