The fourth element involved in the combustion process is referred to as the 'chemical chain reaction'. Specific chemical chain reactions between fuel and oxygen molecules are essential to sustain a fire once it has begun. Essentially, fires are extinguished by taking away one or more of the elements in the fire tetrahedron. Classes of Fire Fires are classified according to the type of fuel involved. Thus, if the wrong type of extinguisher is selected the fire situation can be made worse, often threatening your own personal safety.
Some extinguishers are simply more effective than others on particular classes of fire.
Class A Fires involving carbonaceous solids, such as wood, cloth, paper, rubber and plastics. Class B Fires involving flammable and combustible liquids. Class C Fires involving combustible gases. Specialist advice should be sought. Where a fire, involving an electrical hazard, can be expected, the extinguisher must be electrically non-conductive, in addition to having the correct classification for the materials involved.