Fire extinguishers are found in just about every commercial building, and for good reason—they can stop a small fire before it becomes a major disaster. But have you ever stopped to think: how does a fire extinguisher actually work?
Let’s break it down in simple terms, so you know exactly what happens when you pull that pin and aim at the flames.
Understanding the Basics
All fire extinguishers work on the same basic principle: they interrupt the fire triangle, which consists of:
- Heat
- Fuel
- Oxygen
To extinguish a fire, at least one of these elements has to be removed. Fire extinguishers do this in different ways, depending on the type of fire and the extinguishing agent inside.
Most modern extinguishers operate using internal pressure. When the extinguisher is activated, that pressure forces the extinguishing agent out through the nozzle, allowing it to cover the fire and stop it from spreading.
Key Components of a Fire Extinguisher
Here’s what makes up a typical extinguisher:
- Cylinder: The main body that holds the extinguishing agent.
- Pressure system: Either stored pressure (built-in) or a separate gas cartridge.
- Hose and nozzle: Directs the agent where it’s needed.
- Safety pin and seal: Prevents accidental discharge.
- Pressure gauge: Indicates whether the extinguisher is pressurised and ready for use.
The PASS Technique
Using a fire extinguisher in an emergency follows the PASS method:
- Pull the pin
- Aim the nozzle at the base of the fire
- Squeeze the handle
- Sweep from side to side
This simple approach is taught in fire safety training and is effective for most situations where an extinguisher is suitable.
Now let’s look at how different extinguishers actually tackle fires.
How Different Extinguishers Work
Water Extinguishers
Best for: Class A fires (wood, paper, textiles)
Water extinguishers cool the burning material, removing heat from the fire triangle. Some may contain additives to reduce surface tension, helping the water soak in better.
Water mist extinguishers, a newer type, use a very fine spray to smother flames and cool simultaneously—safer around electrics.
Foam (AFFF) Extinguishers
Best for: Class A and B fires (solids and flammable liquids)
Foam extinguishers form a blanket over the surface of a burning liquid, cutting off oxygen and suppressing vapours. They also cool the fire at the same time.
Dry Powder Extinguishers (ABC)
Best for: Class A, B, and C fires (solids, flammable liquids, gases)
Dry powder works by coating the fuel source and interrupting the chemical reaction at the heart of the fire. It’s highly effective—but messy—and not ideal for indoor use due to the dust cloud it creates.
Specialist Class D powders are also available for metal fires, though these are less common.
CO₂ Extinguishers
Best for: Electrical fires, some Class B fires
CO₂ (carbon dioxide) extinguishers displace oxygen and cool the area using freezing cold gas. They leave no residue, making them perfect for use around electronics—but they do reduce breathable air in enclosed spaces, so use with care.
Wet Chemical Extinguishers
Best for: Class F fires (cooking oils and fats)
These extinguishers use a potassium-based solution that reacts with hot oils to form a soapy layer (a process called saponification). This layer smothers the fire and cools the oil to prevent reignition—essential in commercial kitchens.
What About the Pressure?
Most UK extinguishers are stored pressure types. This means the propellant (usually air or nitrogen) is already in the main cylinder with the extinguishing agent. When you activate the unit, the pressure forces the agent out.
Others, particularly older or specialist models, may have a cartridge system, where pressing the lever releases gas from a separate chamber to expel the agent.
Safety and Maintenance
Fire extinguishers are life-saving devices, but only if they work when you need them. That’s why UK fire safety law requires:
- Annual servicing by a competent person (BS 5306-3)
- Clear placement and signage (BS 5306-8)
- Replacing or recharging after any use, even partial
- Extended servicing every 5 or 10 years, depending on the type
Failing to maintain extinguishers properly can leave your premises non-compliant—and unprotected.
How ESI Can Help
At ESI: Electrical Safety Inspections, we help businesses across Farnborough, Camberley, Reading, Guildford, and the wider Hampshire, Surrey, and Berkshire areas stay safe and compliant.
We offer:
- Expert advice on choosing the right fire extinguishers
- Full installation with correct mounting and signage
- Annual servicing and refilling
- Staff training on safe and effective use
If you’re not sure your extinguishers are fit for purpose—or you just want peace of mind—get in touch with our team. We’ll help you understand the systems you rely on and make sure they’re ready for anything.
