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
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.
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.