The fire triangle is the three things every fire needs: heat, fuel and oxygen, with the chemical chain reaction added as a fourth element in the modern fire tetrahedron. Fire extinguisher classes (A, B, C, D and K) group fires by what is burning, and each extinguisher type is listed for the classes it can put out safely.
This guide explains how each extinguisher breaks the fire triangle, why the letter C means different things in the US and European systems, and which agent suits each class. AZEIDK supplies Falcon Gold fire extinguishers in Saudi Arabia and inspects and services them under preventive and periodic maintenance contracts.
Key takeaways
- A fire goes out when one side of the triangle is removed or the chain reaction is broken: cooling, smothering, fuel removal or chemical inhibition.
- US (NFPA) classes: A ordinary solids, B flammable liquids and gases, C energized electrical equipment, D combustible metals, K cooking oils and fats.
- European (EN 2) and ISO 3941 classes: C means flammable gases, electrical is not a separate class, and cooking oils are Class F.
- ABC dry powder covers A, B and C; CO2 is a class B/C fire extinguisher; wet chemical is for commercial kitchens; metal fires need a special powder.
- Read the letters on the label and datasheet. An extinguisher is approved only for the classes marked on it.
In this article
- What is the fire triangle?
- How does an extinguisher put a fire out?
- Fire extinguisher classes: A, B, C, D and K
- Why does Class C mean different things in the US and Europe?
- Which extinguisher for which class?
- Falcon Gold extinguishers at AZEIDK by fire class
- Common mistakes when matching extinguishers to fire classes
- After selection: readiness and periodic inspection
- Frequently asked questions
- Conclusion
What is the fire triangle?
The fire triangle is a simple model of ignition: a fuel that burns, oxygen from the air, and enough heat to start and sustain combustion. Without one side, a fire cannot start. Remove one side during a fire and it goes out.
- Fuel: any combustible material, solid (wood, paper, cardboard), liquid (petrol, diesel) or gas (LPG).
- Oxygen: normal air contains enough to sustain most fires.
- Heat: an electrical spark, open flame, friction or a hot surface.
From fire triangle to fire tetrahedron
Training material from the US extinguisher manufacturer Amerex notes that fire was modelled as a triangle until newer extinguishing agents arrived, and that it is now represented as a tetrahedron: heat, fuel, oxygen and chain reaction. The fourth element explains why dry chemical knocks down flames so quickly, even though it does little cooling and does not fully exclude air.
How does an extinguisher put a fire out?
Every agent attacks one or more elements of the tetrahedron. Knowing which one explains why an extinguisher works on one fire and fails, or becomes dangerous, on another.
| Method | Element removed | Typical agents |
|---|---|---|
| Cooling | Heat | Water; wet chemical solution on cooking oil fires |
| Smothering or separation | Oxygen | CO2, which displaces air; foam, which blankets a liquid surface |
| Fuel removal | Fuel | Closing a gas or fuel valve; a foam blanket that holds back vapours |
| Chemical inhibition | Chain reaction | Dry chemical powders and halogenated agents such as FM-200 |
Amerex also points out that foam is the only agent that can be applied to a flammable liquid to prevent ignition, because it suppresses vapour release. That is why foam is preferred around fuel storage and spill areas.
Fire extinguisher classes: A, B, C, D and K
Fire classification depends on what is burning, not on the size of the fire. The definitions below follow the US system used by NFPA 10 and match the OSHA definitions in 29 CFR 1910.155.
Class A: ordinary combustibles
Wood, paper, cloth and some rubbers and plastics: anything that leaves embers or ash. Cooling works best, so water, foam and multipurpose ABC powder all suit Class A.
Class B: flammable liquids and gases
Petrol, diesel, solvents, oil-based paints and greases, plus flammable gases under the US system. Water spreads a burning liquid, so use foam, CO2 or dry chemical. On a gas fire the first step is to shut off the supply if it is safe to do so, because putting out the flame while gas keeps leaking can lead to a build-up and re-ignition.
Class C: energized electrical equipment
Panels, motors and equipment still connected to power. The agent must be non-conductive, such as CO2, dry chemical or a clean agent. That is why a US extinguisher rating carries no number in front of the C: it only confirms the agent does not conduct electricity. Once the power is isolated, the fire is treated by the material actually burning.
Class D: combustible metals
Magnesium, titanium, zirconium, sodium, lithium and potassium. These fires are rare in ordinary buildings but possible in some factories and machine shops. Amerex states that agents for other classes do not work on them and that water can react with burning metal. Class D powders are selected for the specific metal and its form.
Class K: cooking oils and fats
Vegetable oils and animal fats in commercial cooking appliances such as deep fryers. The oil is so hot that it can re-ignite after the flame is knocked down. The answer is a wet chemical extinguisher, covered in detail in our kitchen fire extinguisher (Class K) guide.
Why does Class C mean different things in the US and Europe?
Extinguishers built to both systems are sold in Saudi Arabia, so two labels can use the letter C for different hazards. Under EN 2 and ISO 3941, Class C is flammable gases, there is no separate electrical class, and cooking oils are Class F rather than K.
| Fire type | US (NFPA) | Europe (EN 2) |
|---|---|---|
| Ordinary solids | A | A |
| Flammable liquids | B | B |
| Flammable gases | B | C |
| Combustible metals | D | D |
| Energized electrical equipment | C | No separate class |
| Cooking oils and fats | K | F |
Many datasheets based on the European system add an E to show the extinguisher can be used on electrical equipment. The official Falcon Gold datasheet for the 6 kg dry powder unit, for example, lists classes A, B, C and E: here C means gases and E means electrical.
In 2026 a revision of the standard, ISO 3941:2026, added Class L for lithium-ion cell and battery fires where no metallic lithium is present, as reported by the UK Fire Protection Association. Exponent notes that NFPA does not currently recognise this class. Do not select an extinguisher on an L marking until test methods and approved products exist.
Which extinguisher for which class?
The table maps common extinguisher types to the US classes. The final reference is always the letters printed on the extinguisher itself.
| Extinguisher type | How it works | A | B | C | K |
|---|---|---|---|---|---|
| Water | Cooling | Yes | No | No | No |
| AFFF foam | Separation and cooling | Yes | Yes | No | No |
| ABC dry powder (monoammonium phosphate) | Chemical inhibition | Yes | Yes | Yes | Not adequate |
| BC dry powder (sodium or potassium bicarbonate) | Chemical inhibition | No | Yes | Yes | Not adequate |
| CO2 | Smothering | Not usually listed | Yes | Yes | No |
| FM-200 clean agent | Inhibition and cooling | Model dependent | Yes | Yes | No |
| Wet chemical | Cooling and saponification | Yes | Model dependent | No | Yes |
Class D units are not in the table because they are chosen for the metal itself. A “class B/C fire extinguisher” usually means a CO2 or BC powder unit. It suits electrical rooms and liquid stores, but it does not replace a Class A extinguisher in offices and warehouses. For the difference between the two powders, see our ABC vs BC dry powder guide.
Falcon Gold extinguishers at AZEIDK by fire class
These examples come from AZEIDK product pages. Classes are taken from the manufacturer’s official datasheets (Emirates Fire Fighting Equipment Factory) and use European lettering:
- ABC dry powder: the Falcon Gold 6 kg dry powder extinguisher (FDP6), 70% ABC powder, classes A, B, C and E. The range also includes 1 kg and a 50 kg wheeled unit.
- CO2: the 6 kg CO2 extinguisher (FCD6), classes B, C and E, leaving no residue on panels and equipment.
- AFFF foam: portable foam extinguishers in 6, 9, 10 and 12 litre sizes for classes A and B; the 6 litre model is rated 13A 113B on the datasheet.
- Ceiling-mounted clean agent: the 6 kg automatic FM-200 unit (FFMA6), classes A, B, C and E, self-actuating at 68°C.
For cooking oil fires, the site also lists a 6 kg FG wet chemical extinguisher for Class K/F, rated 13A 75B on its datasheet.
Common mistakes when matching extinguishers to fire classes
- Using water or foam on an electrical panel that is still energized.
- Relying only on CO2 in a cardboard warehouse, although CO2 is not usually listed for Class A.
- Throwing water on a burning pan of oil, which flashes to steam and scatters burning oil.
- Reading a C on a European label as “electrical” when it means gases.
- Using ordinary powder on a metal fire in a magnesium or aluminium machining shop.
- Knocking down a gas leak flame without closing the supply.
After selection: readiness and periodic inspection
The right extinguisher is useless if it is empty, low on pressure or far from the hazard. NFPA 10 sets monthly inspection, annual maintenance and hydrostatic testing for each type, as explained in our NFPA 10 portable extinguisher guide. The AZEIDK team inspects and services extinguishers under periodic maintenance contracts. For staff training on correct use, see how to use a fire extinguisher with PASS.
Frequently asked questions
What are the elements of the fire triangle?
Heat, fuel and oxygen. The modern fire tetrahedron adds the chemical chain reaction. Removing any one element puts the fire out.
What are the fire extinguisher classes?
In the US system: A for ordinary solids, B for flammable liquids and gases, C for energized electrical equipment, D for combustible metals and K for cooking oils and fats.
Is an electrical fire always Class C?
Under NFPA, yes. Under EN 2, Class C is flammable gases and electrical is not a separate class; some datasheets use the letter E to show suitability for electrical equipment.
What is a class B/C fire extinguisher?
An extinguisher listed for flammable liquids and energized electrical equipment, such as a CO2 or BC dry powder unit. It is not suitable on its own for wood or paper fires.
What is a class B fire extinguisher used for?
Flammable liquids such as petrol, diesel and solvents, and flammable gases under the US system. Foam, CO2 and dry powder units can carry a B rating.
Can an ABC powder extinguisher be used on a kitchen fire?
It is not adequate for cooking oil fires in commercial fryers, because the oil can re-ignite. Use a wet chemical Class K extinguisher.
Conclusion
The fire triangle explains how each extinguisher works, and fire extinguisher classes tell you which one belongs where. Start with what could burn in each room, choose an extinguisher listed for that class, read the letters according to the system on the label, and keep it ready with periodic inspection.
Need extinguishers matched to the hazards in your facility? Send us your activity and floor areas, and we will propose suitable Falcon Gold extinguisher types with datasheets and a periodic maintenance offer. Talk to the AZEIDK team
AZEIDK supplies fire extinguishers of all common types and sells them to individuals and businesses from its branches in Saudi Arabia.
