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 a foam extinguisher should 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.
AFFF foam extinguishers, and what AFFF foam actually is
Aqueous film forming foam is a water based agent with a surfactant that spreads a thin film across a burning liquid, so it cuts off the oxygen as well as cooling, and that film is the whole reason foam beats water on a liquid fire. In a red bodied unit with a cream label it is the commonest general purpose extinguisher in British offices, rated for ordinary combustibles and for flammable liquids. The chemistry is changing: fluorine free foams are replacing the older fluorinated ones as those are restricted, and the replacements can behave slightly differently in use, which is a reason to read the body of the unit on your wall rather than the one you were trained on years ago.
Other questions people ask
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.
Is an AFFF foam extinguisher the same as the older foam ones?
Not always, and the label is where the answer is. Older units used fluorinated surfactants; newer ones increasingly use fluorine free formulations because of restrictions on those chemicals, and while both carry the same fire ratings the technique and the performance at the edges can differ. What does not change is how it is applied: onto a surface behind or beside the fire so the agent flows over it, never fired into a pool of liquid, which throws the liquid out of its container. If your estate has a mix, make sure the training uses the units people will actually reach for.
What is a foam extinguisher used for, and where should it not go?
Ordinary solid combustibles such as paper, card, wood and textiles, and flammable liquids such as petrol, solvents and paint. That combination is why it has become the default unit in offices, workshops and common parts. It should not go near live electrical equipment, because the agent is water based and conducts, and it must not be used on cooking oils and fats, where it will not hold the surface and the attempt ejects burning oil. Those two exclusions are the reason a kitchen carries a wet chemical unit and an electrical intake carries carbon dioxide.