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, 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, the rating relating to the volume of fuel being used and 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 in 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 firefighters 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, the rating relating to the volume of fuel being used and 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 of 1000mm length and 500mm 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.
Ratings for Mobile Extinguishers
Mobile extinguishers (that have a total mass over 20kg and are manoeuvred by an operator on foot only, usually on fixed wheels) are covered by EN 1866 as opposed to EN3 and operate a slightly different rating system based on extinguishing agent and fire class.
Class A Fires - Flammable Solids
There is no rating on mobile extinguishers for Class A fires however, the following basic performance requirements are imposed:
Extinguishers containing water, water with additive, foam or other liquid: The agent must already be approved as being able to extinguish an EN3-7 13A test fire when tested in a 9-litre portable extinguisher.
Extinguishers containing dry powder: The agent must already be approved as being able to extinguish an EN3-7 34A test fire when tested in a 9 kg portable extinguisher.
Class B Fires - Flammable Liquids
Dry Powder extinguishers undergo rating tests that use a combination of 233B & 21B EN3-7 test fires as laid out in this table:
| Fire Rating | No. of 233B trays | No. of 21B trays |
| IB | 1 | 1 |
| IIB | 1 | 2 |
| IIIB | 1 | 3 |
| IVB | 1 | 4 |
The 21B trays are placed directly alongside the 233B tray.
The arrangement is as follows, and set out in the image below:
- II B at 0° and 180° of the 233B;
- III B at 0°, 120° and 240° of the 233B;
- IV B at 0°, 90°, 180° and 270° of the 233B.
The extinguisher must extinguish one of the types of B fire at least two times out of a series of three fire tests, one of the types of B fire.
Water Additive, Foam or other liquid agents do not undergo the IB – IVB tests and do not carry a Class B rating. The agent must already be approved as being able to extinguish an EN3-7 183B test fire when tested in a 9 litre portable extinguisher.
Class C Fires - Flammable Gases
There is no test rating under EN1866 for gas fires and it is hazardous for gas fires to be extinguished other than by cutting off the gas supply. Professional firefighters can use mobile powder extinguishers as these are considered suitable for snuffing a gas flame.
Class D Fires - Flammable Metals
There is no test rating under EN1866 for metal fires and specialist advice is required as to the agent required (usually graphite, copper, or sodium chloride) for a particular metal and the quantity required.
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 must undergo the EN3-7 conductivity test as detailed above.
Carbon Dioxide and Clean Agent Mobile Extinguishers
These are not covered by EN1866 and have no test or rating system. Carbon Dioxide mobile extinguishers are usually used for larger enclosed risks (particularly helicopter and aircraft engines and engine bays) and the size is usually based on the volume of the area being protected. 1kg of liquid CO2 usually yields approximately 0.5 cubic metres of gas.
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