Fire ratings are a means of rating the fire-fighting capacity of a fire extinguisher by the maximum size of fire that has successfully extinguished under test conditions. All fire extinguishers that comply with EN3 and that are designed for use on class A and class B fires will carry a Fire Rating that indicates the size of fire that it has successfully extinguished. This rating consists of a number indicating the test size of the fire and a letter indicating the fire class.
The fire rating that a extinguisher carries is based on its fire-fighting capabilities, not on the size of the extinguisher. The higher the rating, the greater the fire-fighting capacity. While fire is divided into four principal classes under I.S. 291:2002 (A, B, C, and D), only classes A and B carry a fire rating.
Class A Testing Procedure
A wooden crib of sticks are stacked 560mm in height and 500mm in depth on top of metal frame support of 250mm in height to determine the Fire Rating of the extinguisher. The length of the crib in metres is ten times the class A fire rating that is to be achieved. For example, a fire extinguisher that is rated 13a has successfully extinguished a crib that is 1.3 metres in length. As the Fire Rating is ten times the length in metres, a fire extinguisher that is rated 43A is therefore capable of extinguishing a test wooden crib that is 4.3 metres in length. The ’43’ represents the 4.3 metres minus the decimal point.
Class B Rating
The Class B rating of a fire extinguisher is determined by the volume of flammable liquid in litres that it has successfully extinguished under test conditions. For example, a fire extinguisher that carries a 34B rating has successfully extinguished 34 litres of burning flammable liquid under test conditions. As provision of class B fire extinguishers under I.S. 291:2002 is a highly specialised subject, we will not cover the class B testing system or provision methodology here.
Class F Rating
Wet Chemical fire extinguishers for class F fires have a rating based on four benchmark tests using 5, 15, 25 and 75 litres of sunflower oil. The oil is heated to its auto-ignition point and allowed to burn for two minutes. Re-ignition must not occur within ten minutes after the fire has been extinguished. BS 7937 has now been replaced by BS EN 3-7 2004 + A1. Class F and the wet chemical fire extinguisher are not covered by I.S. 291:2002 as the Irish Standard pre-dates their widespread adoption.
Dual Fire Rating
AFFF foam type and ABC dry powder type fire extinguishers that are for use on class A and class B fires will have a dual fire rating. For example, a 9kg ABC dry powder fire extinguisher would typically carry a class A rating of 43A and 233B.
Provision of Fire Extinguishers
The fire rating system used in I.S.291:2002 allows the minimum provision of class A fire extinguishers for each storey of a building to be calculated by multiplying the floor area of each storey measured in square metres by 0.065. For example, a floor area of 500 sq.m. would require class A fire extinguishers with a minimum aggregate rating of 32.5A (500 m² x 0.065 = 32.5A).
This value then determines the minimum number of fire extinguishers that must be provided for that storey. The value can be achieved by providing one type of class A rated fire extinguisher or by combinations of different types with a class A rating and by class A extinguishers with different ratings to achieve the required value.
For example, 3 x 6ltr AFFF foam fire extinguishers with a fire rating of 13A each (providing an aggregate fire rating of 39A) or 1 x 2kg dry powder extinguisher with a fire rating of 13A and 1 x 6kg dry powder extinguisher with a fire rating of 27A (providing an aggregate fire rating of 40A) would achieve the minimum required value of 32.5A.
The storey must not be provided with less than two class A fire extinguishers with a minimum aggregate rating of less than 26A. This corresponds to a floor area of 400 m². Where the calculation returns a higher value, the higher value must always be provided.
However, if the building is in single occupancy with the upper floor areas not exceeding 100 m² then the minimum aggregate of the designation for these floors is 13A. The ground floor should always have a minimum of two fire extinguishers with a minimum aggregate rating of 26A. Where there are other classes of fire risk in the building, appropriate extinguishers for those classes should also be provided.
This is a minimum specification and the actual number and type of fire extinguishers to be provided will be determined by the fire load of the building and an appropriate fire risk assessment and by other factors specified in I.S. 291:2002 such as distance of travel (no one must travel more than 30 metres to reach a fire extinguisher) and proximity to special risks, etc.