Foam is the agent most often chosen when a building has both ordinary combustibles and a flammable liquid risk in the same space, because it is the only common portable that is rated for both without the discharge problems of powder. Its band is cream. In practice most units sold in the UK are aqueous film forming or fluorine free foam in a water base, and they behave like a water unit with an extra capability.
The two mechanisms
On class A the foam behaves as water does and cools. On class B it does something different: the foam floats on the surface of the liquid and seals it, cutting the fuel off from the air above it and stopping the vapour that is actually burning from leaving. That is why foam is applied to a liquid fire indirectly, off a wall or the far edge of a tray, rather than driven straight into the surface.
Why the application technique matters more than with water
A jet driven into a pool of burning solvent splashes it. The blanket has to be laid, not injected. Training on foam therefore has a step that water training does not, and a building that has chosen foam for a liquid risk has taken on a training obligation with it.
Where foam is refused
On cooking oil, because the temperatures in a fryer break the blanket down and the fire re-establishes. On live electrical equipment, because the carrier is water, unless the unit is specifically tested and marked otherwise and the marked safe distance is kept. A foam unit is not an electrical unit by default and should not be described as one.
Fluorine free and what changed
The industry has moved away from fluorinated foams for environmental reasons and the replacements behave slightly differently on hydrocarbon fires. For a duty holder the practical consequence is that a like for like swap at service time can change what the provision is rated for, so the rating on the new cylinder has to be read rather than assumed.
Questions people ask about foam fire extinguisher UK
Foam fire extinguisher used for what type of fire?
Class A solids and class B flammable liquids. It is not a class F unit and it is not an electrical unit unless its own label says so.
What does a cream fire extinguisher used for in an office mean?
It usually means the office has a store or a plant room with solvents, paints or fuels in it, and one provision has been chosen to cover both that and the paper.
Is foam better than water for general use?
It is more capable and more expensive, and it covers a risk many buildings do not have. Where there is no flammable liquid, foam is buying a capability that the assessment has not asked for.
What is the right foam fire extinguisher use, and where is it wrong?
Right on ordinary solid combustibles and on flammable liquids, which is most of an office, a workshop, a storeroom and a common part, and it is why foam has become the default unit. The technique matters more than with water: aimed at a surface behind or beside a liquid fire so the agent flows over it and forms its film, never straight into a container of burning liquid. Wrong on live electrical equipment, because the agent is water based and conducts, and wrong on cooking oils and fats, where it does not hold the surface and the attempt throws burning oil out of the pan.