Fire Extinguisher Ratings – Find Out About Fire Classifications and Fire Extinguisher Ratings

Fire extinguisher ratings are designed to indicate the type of fires they should be used on, and to give an indication of how effective they will be.  Each fire extinguisher rating in expressed as a series of numbers and letters which indicate the classes of fire they can be used on the size of fire they are suitable for.

Different types of fires are classified by different letters, and fire extinguishers ratings may show several letters because they can be used on more than one type of fire.  In terms of the size of fires the extinguisher can be used on, the number gives an indication of this, and generally speaking, the larger the number, the larger the fire that extinguisher can deal with.

The numerical part of the fire extinguisher ratings relate directly to tests carried out to measure their effectiveness.  A wooden crib test is one test that is used to measure how large a fire an extinguisher can deal with.  A fire extinguisher rating of 13A, for example, indicates that the extinguisher was able to put out 1.3m of the fire.  The number on a fire extinguisher rating is therefore a very accurate indication of what each extinguisher can cope with.

As another example of the numerical rating, the test for measuring the effectiveness of extinguishers for use on flammable liquids refers to the approximate area of fire in square feet that it is able to extinguish.  The specific tests vary with the different fire types, but the basic system of accurately measuring the extinguisher’s effectiveness is the same.

It would seem obvious to think that the number on a fire extinguisher would increase with the size of the extinguisher.  This is generally true, but size is not the only factor that effects the number in the fire extinguisher rating.  With water fire extinguishers, additives are often mixed with the water to increase their effectiveness, therefore you could have two extinguishers which are the same size, but which have different numerical ratings, because one is more effective than the other.

Fire Classifications – The UK and US Differences

As already mentioned, the letters used in fire extinguisher ratings refer to the classes of fire that they can be safely used on.  We therefore need to look at these fire classifications first, before looking at the actual fire extinguisher ratings for the various types of fire extinguisher.

1. The following are the main classifications of fires, with US classes shown in brackets:

Class A Fires (also Class A in US)

Any fire involving solid materials such as wood, card, paper, fabric, etc.

Class B Fires (also Class B in US)

Class B fires involve flammable liquids including kerosene, oils, petrol, paints, diesel, etc.

Class C Fires (comes under Class B in US)

A separate classification except in US, this category is for flammable gases, including butane, methane, etc.

Class D Fires (also Class D in US)

The D Classification is for combustible metals or alloys, such as potassium, magnesium or titanium.

Electrical Fires (Class C in US)

Electrical fires are sometimes known as Class E in the UK, but this is an error as there is no separate classification for electrical fires in the UK.

Class F Fires (Class K in US)

Specifically relates to fires involving deep fat fryers (fats and oils).

Fire Extinguisher Ratings:

The following information shows the classes of fires covered by the different ypes of fire extinguisher:

Main fire extinguisher ratings shown refer to UK, with US ratings in brackets.

Water Fire Extinguishers:

Classes of fire: A

(class A in US)

Carbon Dioxide Fire Extinguishers:

Classes of fire: Electrical fires, A, B and C

(A, B and C in US)

Dry Powder Fire Extinguishers:

Classes of fire: A, B, C and Electrical fires

(A, B and C in US)

Wet Chemical Fire Extinguishers:

Classes of fire: A, F

(A and K in US)

Foam Fire Extinguishers:

Classes of fire: A and B

(A and B in US)

Fire Extinguisher Ratings - Conclusions

Understanding fire classes and fire extinguisher ratings is important in order to be sure of using the correct type of fire extinguisher on any fire in your workplace.  It can be very dangerous to use the wrong type of fire extinguisher on some fires, one of the most obvious examples being the use of water on an electrical fire.  For this reason proper staff training is important for any staff who are required to use fire fighting equipment.