Foam is the general purpose extinguisher for buildings that hold both ordinary combustibles and flammable liquids, which is most workplaces with a store cupboard. It is cream banded, rated for class A and class B, and it works by two mechanisms at once rather than one, which is where its versatility comes from.
How it works
Foam expands into a blanket that floats on a burning liquid, cutting the flame off from the air above it and stopping vapour rising from the surface. At the same time the water content cools, which is what gives it a class A capability on solids as well. On a liquid fire the blanket has to be laid gently rather than jetted into the surface, or it breaks up and the fire re establishes.
What it is for
Class A and class B: solids and flammable liquids. That combination makes it the sensible single extinguisher for a workshop, a garage forecourt office, a plant room, or any premises where solvents, paints or fuels are stored alongside ordinary combustibles. In many offices a mix of foam and carbon dioxide covers everything the risk assessment is likely to identify.
What it must not be used on
Cooking oil and fat fires: foam is not rated class F and the oil temperature will break the blanket down and reignite. Burning metals, where the water content reacts. And live electrical equipment, unless that particular foam extinguisher has been dielectrically tested and the label says so, which many are and some are not.
Foam and firefighting foam concentrates
Foam formulations have been changing as fluorinated firefighting foams are restricted for environmental reasons, and fluorine free alternatives have become standard in portable extinguishers. For a duty holder the practical effect is that replacement extinguishers may use a different formulation from the ones they replace, and disposal of the old ones is a regulated waste question rather than a bin question.
Questions people ask about what is a foam fire extinguisher used for
What colour is a foam fire extinguisher?
Cream. The body is red, as all extinguisher bodies are, with a cream band or panel identifying foam contents.
What should a foam extinguisher not be used on?
Cooking oil and fat fires, burning metals, and live electrical equipment unless the label states that extinguisher has been dielectrically tested.
Is foam better than water?
It covers more: class A and class B rather than class A alone. It costs more and, on a straightforward paper fire, water with an additive can carry a higher class A rating for the same size.