The combustion process
An extinguisher must interrupt the conditions sustaining fire
A fire continues when fuel, heat and oxygen or another oxidiser combine in a self-sustaining reaction. Extinguishing agents can remove heat, reduce contact with oxygen, interrupt the chemical reaction or separate fuel from the burning zone.
The visible flame is only part of the problem. Heated fuel, hidden fire, continuing electricity or gas and hot cooking oil can cause re-ignition after the apparent flame has gone.
Cooling
Water-based agents absorb heat and can reduce the fuel below its ignition temperature.
Smothering
Foam and wet chemical can form a surface layer that separates fuel from air.
Chemical interruption
Powder interferes with flame chemistry but may provide limited cooling.
Displacement
CO₂ reduces oxygen around the flame temporarily and leaves no residue.
Agent behaviour
The same mechanism can create different limitations
Water conducts electricity and can spread incompatible burning liquids. Powder can obscure vision, contaminate equipment and allow re-ignition. CO₂ can expose people to cold discharge and an asphyxiation hazard in confined areas. Foam compatibility depends on the liquid and product formulation.
Wet chemical is designed around hot cooking oils and fats. Water mist uses fine droplets, but each product still needs the correct certified rating and instructions.
| Agent | Main effect | Key limitation |
|---|---|---|
| Water | Cooling | Not for live electricity or incompatible liquids |
| Foam | Cooling and surface blanket | Liquid compatibility and environmental effects |
| CO₂ | Temporary oxygen displacement | Limited cooling and confined-space risk |
| Dry powder | Flame-reaction interruption | Visibility, contamination and re-ignition |
| Wet chemical | Cooling and a sealing layer | Selected principally for qualifying Class F hazards |
| Water mist | Fine-droplet cooling and displacement effects | Performance varies by tested product rating |
Tested performance
Fire classes and ratings connect the agent to a defined test fire
An extinguisher label identifies the classes and ratings for which the product has been tested. Capacity alone does not show capability: two units containing the same volume can carry different ratings.
The premises provision should combine certified ratings, travel distance, floor area, special hazards, user capability and escape arrangements. A generic product description cannot perform that calculation.
After use
Discharge is an incident, not the end of the process
Raise the alarm, call 999 where required and keep people away. A fire that appears extinguished can restart, and smoke or decomposition products may remain dangerous.
Record the incident, isolate affected equipment safely, replace or recharge discharged units through a competent provider and review the FRA where the event exposes a new hazard or failed control.