Fires are classified by what is burning, and extinguishers are rated against those classes. Getting this right matters more than colour, because the classes are what the label on the extinguisher actually states. Electrical fire is deliberately not a class in the UK system, which surprises people and is the source of most of the confusion.
The classes
Class A is ordinary combustible solids: paper, wood, textiles, most rubbish. Class B is flammable liquids such as petrol, diesel, solvents and paint. Class C is flammable gases. Class D is burning metals including magnesium and lithium metal. Class F is cooking oils and fats at the high temperatures a deep fat fryer reaches. There is no class E.
Why electrical is not a class
Electricity is not a fuel, it is an ignition source, so a fire involving live electrical equipment is really a class A or B fire with a shock hazard attached. That is why extinguishers are not rated class E: instead, certain types are tested as safe to use near live electrical equipment, and the label says so. Carbon dioxide is the usual choice, and some foam and water types are dielectrically tested.
Class D and class F are specialist
Burning metals react violently with water and with the agents in ordinary extinguishers, so class D fires need a specific powder applied with an applicator, and nothing else. Cooking oil fires above their auto ignition temperature reignite through most agents, which is why class F wet chemical extinguishers exist and why a foam or powder extinguisher in a commercial kitchen is the wrong equipment.
Reading a rating, and what the fire extinguisher test fire means
An extinguisher label shows the classes it is rated for and a number against the A and B ratings indicating the size of test fire it passed. A higher number is a bigger capability, not a bigger cylinder. When a risk assessment specifies extinguisher provision it should express it in ratings rather than in cylinder counts, because two extinguishers of the same size can carry very different ratings.
A class B extinguisher, and what a class C extinguisher is for
Class B is flammable liquids, and foam is the usual choice because it forms a blanket over the surface and stops the vapour feeding the fire; carbon dioxide and dry powder also carry a B rating and both leave the liquid free to reignite from a hot surface. Class C is flammable gases, and here the honest answer is that the extinguisher is not the fix: putting the flame out while the gas keeps escaping leaves an unignited cloud, which is worse than what you started with. Isolate the supply, and use the extinguisher on what the gas has set alight. That is why a unit marked for class C is normally a dry powder carried for the surrounding fire rather than for the gas itself.
Questions people ask about fire extinguisher classes UK
What are the fire classes in the UK?
Class A solids, class B flammable liquids, class C flammable gases, class D metals and class F cooking oils and fats. There is no class E; electrical risk is handled by the extinguisher's own testing instead.
Which extinguisher is for electrical fires?
Carbon dioxide is the usual answer, and some water and foam extinguishers are dielectrically tested for use near live equipment. The label states whether a given extinguisher has been tested for it.
What does the number on a fire extinguisher rating mean?
It indicates the size of standard test fire the extinguisher extinguished, so a higher number is more capability. It is not the same as cylinder size.
Which deep fat fryer fire extinguisher is correct?
A wet chemical unit, class F. Water and foam make a fat fire worse and powder scatters it, so a kitchen with a fryer holds a wet chemical portable beside whatever suppression the canopy has.
Class b fires include which of the following: petrol, cooking oil or burning magnesium?
Petrol. Class B is flammable liquids, so petrol, diesel, solvents and paint belong to it. Cooking oil is class F, because above its auto ignition temperature it reignites through agents that would handle a petrol fire. Burning magnesium is class D, and it reacts violently with water and with ordinary extinguisher agents. The trap in the question is always the cooking oil, and it is the one that matters in a real kitchen.
What counts as a potential ignition source?
Anything that can deliver enough energy to start the fuel burning: naked flames and hot work, smoking materials, electrical faults and overloaded equipment, hot surfaces and heaters, friction and mechanical sparks, static discharge, and in some settings a chemical reaction or a lithium cell in failure. Electricity belongs on that list rather than on the list of fuels, which is why there is no class E extinguisher: a fire in live equipment is a class A or class B fire with a shock hazard attached, and the extinguisher is chosen for what is burning while its label decides whether it may be used near live equipment at all.