Water fights all "type A" fires (solids) e.g. wood, paper, plastic, upholstery etc. It works mainly by cooling and removing the heat. These are also the cheapest and most widely used fire extinguishers and are not suitable for fires where electricity is involved.
Foam fights all "type A & B" fires, including wood, paper, plastic, upholstery etc. as before, plus any flammable liquids. Not only does it cool and remove heat, but forms a film over the surface of the liquid, excluding any oxygen. Like Type A however, they are not recommended for fires
involving electricity - but they are safer if inadvertently sprayed onto live apparatus.
Electrical & Type B
Carbon Dioxide fights electrical fires and flammable liquids. It is an asphyxiate and removes all oxygen, effectively smothering the fire. The main advantage of this extinguisher is the complete lack of mess after use, however it does have no post fire security and the fire could re-ignite.
Wet Chemical is specifically designed to fight fires involving cooking oil and fat and employs a method called soponification (a chemical reaction between vegetable oil/animal fat and a strong alkali, producing soap). It can also have a cooling effect (similar to type A).
Type A, B, C & Electrical
Powder is a multi-class extinguisher and as such, is extremely popular. It is often specified due to its multiple class capability, fighting solids, flammable liquids, flammable gasses and electrical fires. Although it is a very fast acting extinguisher, it has very little or no cooling effect so re-ignition can be a danger. Not only this, but it can obscure vision, damage goods and leave a very big mess behind.
British Standard 5306-3:2000 covers the maintenance of portable fire extinguishers, and recommends that “regular visual inspections of all portable fire extinguishers be carried out by the user or user’s representative. The frequency of inspections by the user should be not less than monthly and, when circumstances require, inspections should be carried out more frequently”.
Employers have a legal responsibility under the Fire Safety Order (FSO) to ensure that all Fire Extinguishers within the premises are regularly serviced and maintained, so that they are kept in good repair and working order. (Fire extinguishers are pressurised vessels that can explode when corroded or damaged, causing serious injury and even death).
To ensure compliance with this legislation, a 'responsible person' should carry out a visual inspection to confirm that the extinguisher has not been damaged or accidentally discharged. These inspections should be carried out at monthly intervals at least.
If your organisation employs five staff or more, you must also hold written records of all services, maintenance and inspections in a Fire Log Book.
You extinguishers should be properly serviced once a year by a suitably-qualified engineer who are BAFE-registered and can offer a full maintenance package.