The following information is provided for guidance purposes, and should not be relied upon for the purposes of a fire risk assessment, in which case, British Standard 5306-8:2000 should be referred to.
Fire extinguishers are ‘rated’ on their ability to extinguish test fires. In the case of class A, a wooden crib of specific size and length is ignited, and the amount of burning crib that can be extinguished is measured.
Traditionally, the ‘basic’ extinguisher for general use is the 9-litre water, which can extinguish 1.3m of the standard wooden crib fire, and is rated 13A (the decimal place is dropped). Ratings are marked on extinguisher bodies, e.g. 13A, 70B.
British Standard 5306 contains formulae for calculating the number of class A extinguishers as follows: floor area (m2) x 0.065 divided by the extinguisher rating.
This can be a little complicated for the layman, so to simplify things, as a rule of thumb in factories, offices and shops etc, one 13A extinguisher covers 200 square metres of floor area.
To calculate how many 13A extinguishers are required, divide the floor area by 200, and round up.
|Example: floor area 1300 square metres:
1300 ÷ 200 = 6.5
Round up to 7
7 x 13A water based extinguishers.
There should be a minimum of 2 extinguishers per floor, unless the upper floor area is very small i.e. below 100m2, and in single occupancy, in which case, only one extinguisher is required on the upper floor.
Where it is desirable to have smaller, lighter extinguishers, foam, or water with additives, can be used to reduce weight whilst maintaining fire fighting capability. e.g. 6 Litre AFFF foam spray extinguishers are usually rated at 13A, but are approximately 30% lighter than 9 litre water.
Specialised extinguishers should also be provided to cover classes B (Liquids), D (Metals), E (fires involving electrical apparatus) and F (cooking oils and fats), e.g. foam, carbon dioxide or powder.