Fire Extinguisher Ratings Explainedbarriehol
We are asked this question so many times. These mystical codes that are printed on all quality-approved fire extinguishers are there to indicate, not just the type of fire that particular extinguisher can combat, but also the size of the fire the contents can effectively put out.
So for example, a water additive stored pressure extinguisher coded 13A will extinguish a Class A fire of freely burning combustibles of size 13. What the devil is size 13 you may ask? Well it’s definitely not a shoe size, but a specific arrangement of combustibles used to test the extinguisher.
Different tests apply to each Class of fire (from A,B,C, D & F) but the rule is the larger the number before the Code letter the bigger the fire (of that Class) that can be extinguished.
For a more detailed explanation, read on….
Fire Ratings Explained
Most fire extinguishers have a fire Rating printed on the canister. This code, for example 13A/55B, defines the Class (or type) of fire and the size of fire this type of fire extinguisher is capable of extinguishing.
Thus in our 13A/55B example this extinguisher will combat a Class A fire (typically freely burning materials like wood, cloth and coal) of Size 13 or a Class B fire (flammable liquid) of size 55. For Class A fires the Size test is conducted using a fire comprising an open latticework of wooden sticks approx 40mm square x 500mm long piled 14 high. The number 13 comes from the number of sticks in each row as the example below. Fire extinguishers that can put out a fire up to this size are rated 13A. A 34A would extinguish a stack with 34 sticks and so on, so the higher the number the bigger the fire it can extinguish.
In practice you should estimate the volume of combustibles at risk against this benchmark and select the most applicable size for the job. Always err on the side of caution. Having decided a 13A rating is appropriate you can then compare Ratings on the different types of extinguishing agents and available sizes to decide the most appropriate for your application.
Available 13A rated extinguishers*
Plain Water 6 litre
Water with Additive 3 litre
Foam (AFFF) 3 litre
ABC Dry Powder 2kg
(CO2 is unsuitable for this type of fire)
In our example 13A/55B we then have to look at 55B
Class B fires involve burning liquids like petrol, oils (not cooking oils) heptane etc. Only AFFF foam, CO2 and Dry Powder extinguishers are suitable for this application. (See Extinguisher Chart)
The prefix 55 again defines the size of fire and in this case is easier to visualise as in this case the test uses 55 litres of burning fuel. A 175B rating uses 175 litres and so on.
By the way, there is no such thing as a Class E rating in the UK – often thought to cover Electrical fires. Many fires are caused by electrical faults which in consequence ignite other flammables. If an extinguisher is described as “safe to use” on fires involving live electricity it means the suppressant is not inherently electrically conductive so you don’t risk electrocution when combating the fire. In any event, a sensible approach is to cut off the power supply to any area where a fire occurs.