Fire Ratings - What you need to know

In the past extinguisher requirements were based on type and size as performance was considered to be equal between makes and models. However this was not very scientific and failed to account for the effect of different extinguishing agent blends and different nozzle sizes etc and so in 1977 the system of fire ratings was born as part of manufacturing standard BS5423 - currently standard BSEN3.

The fire rating is the largest size of fire an extinguisher can successfully extinguish under test conditions with a skilled operator, and is used to determine extinguisher requirements in accordance with the standard for extinguisher provision BS5306-8

Class A fires – Flammable Solids

This test uses a crib made of pine sticks stacked 0.546m high and 0.5m wide with a variable length depending on the rating being tested; so, for example the common 13A rating uses a crib 1.3m long.

After an 8 minute pre-burn the operator must extinguish the whole fire so that no signs of re-ignition occur for at least 3 minutes after extinction.

Depending on the agent used, two otherwise identical extinguishers can achieve very different ratings – The Firechief XTR 6l water has a 13A rating, but the Firechief XTR Water Ecospray has a powerful 27A rating with twice the fire-fighting power.
An approximate rule of thumb is that for every 200sq.m. of floor area you need one 13A rated extinguisher, with the minimum provision being 26A over two extinguishers (or just one if under 50sq.m.)

The more exact calculation requires the floor area in sq. m. to be multiplied by 0.065 – rounded up to the nearest whole number, then this is the floor fire rating which you must provide enough extinguishers to meet.

An 800 sq. m. floor has a fire rating of 52A which means you could use 4 x 13A rated extinguishers or 2 x 27A rated extinguishers.

Also, the travel distance to an A-rated extinguisher should not exceed 30m

Class B fires – Flammable Liquids

This test uses a metal circular tray of increasing height and diameter depending on the test fire being tackled and the rating relating to the volume of fuel being used; the test being based on the exposed surface area being tackled.

For example, a 21B rating relates to a fire using 21 litres of Heptane and water in a tray of 920mm diameter with a surface area of 0.66m. A 183B rating uses 183 litres of fuel in a tray 2710mm diameter with a surface area of 5.75m.

Depending on the agent used two otherwise identical extinguishers can achieve very different ratings. The Firechief 1kg Powder has a 34B rating, but the Firechief Power Plus 1kg Powder has a powerful 55B rating due to the type of powder used.

Where a Class B risk is spillage based, provision should be based on the maximum foreseeable amount spilled and an extinguisher rated 10x that amount provided, so for example where up to 5 litres could be spilt and ignited you would need a 55B rated extinguisher.

Where the Class B risk is in open containers the calculations are more complex, being affected by the surface area of exposed containers, their proximity to each other and whether foam or powder extinguishers are to be used. Reference should be made to BS5306-8.

The travel distance to a Class B rated extinguisher should not exceed 10 metres

Class C Fires – Flammable Gases

There is no test rating under EN3 for gas fires and it is hazardous for gas fires to be extinguished other than by cutting off the gas supply. However, professional fire fighters can use powder extinguishers as these are considered suitable for snuffing a gas flame.

The travel distance to a Class C extinguisher should not exceed 30 metres.

Class D Fires – Flammable Metals

There is no test rating under EN3 for metal fires and specialist advice is required as to the agent required (usually graphite, copper, or sodium chloride) for a particular metal.

The travel distance to a Class D extinguisher is determined on a case by case basis.

Class F fires – Cooking Oils

This test uses a rectangular tray of increasing width and length (or for 5F a pot similar to a large chip pan) depending on the test fire being tackled and the rating relating to the volume of fuel being used; the test being based on the exposed surface area being tackled.

As an example, a 40F rating relates to a fire using 40 litres of sunflower oil in a tray of 600mm length and 450mm width with a surface area of 0.24m; a 75F rating uses 75 litres of oil in a tray 1000mm in length and 500mm in width with a surface area of 0.5m.

To determine provision the surface area of the fryer should be calculated and extinguishers selected from below

The travel distance to a Class F rated extinguisher should not exceed 10 metres

Electrical Fires

These do not have a rating system as electricity does not burn, but represent fires in classes A,B,C,D & F complicated by the presence of a live electrical supply. Non-conductive extinguishing agents such as CO2 gas, clean agent and powder can automatically be marked as safe for these fires.

Water based extinguishers have to undergo a conductivity test where the extinguisher is discharged from 1m onto a metal plate that has 35,000V running through it. An ammeter measures the current passing between the plate and extinguisher nozzle and where it is less than 0.5mA the extinguisher passes and can be marked “"suitable for use on live electrical equipment up to 1 000 V at a distance of 1 m" or use the electrically safe pictogram.

The travel distance to an electrically safe extinguisher should not exceed 10 metres.

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