Page 16


systems which define the level of sprinkler protection required. The hazard classification is mainly a function of the likely heat release rate (the units of which are kW) which in turn is related to the heat of combustion of the contents (and in many historic buildings, parts of the structure) (measured in KJlkg) and the rate at which these combustible materials will burn (measured in Kgls). Factors such as the building structure, structural fire resistance, height of ceilings and adequacy of heating will influence the sprinkler installation design and the performance required. Crucial design factors for a sprinkler system incIude details relating to the:

occupancies where the amount of combustible material is low, and is suitable for rooms and corridors not more than 126 m? in area bounded by fire resisting construction of not less than 30 minutes duration. This level of protection is now seldom used because of the area limitation and due to the fact that many occupancies for which it was originally intended now contain fire loads which exceed its capability (for example schools which have fire loadings which justify a higher hazard rating). The level of sprinkler protection provided by Light Hazard is: Assumed maximum area of operation - 84 m2 (AMAO)

assumed maximum area of operation (AMAO): this a worst case assumption about the number of sprinklers that may operate in a fire, expressed in m', minimum discharge density: the minimum allowed water application rate at the assumed maximum area of operation, expressed in mmlmin, the duration of the water supplies, maximum and minimum distances between the sprinkler heads, expressed in m, maximum distance of the heads from the boundary of the area, expressed in m, maximum area protected by a sprinkler head, expressed in m', clearance between the sprinkler heads and any obstructions such as joists or light fittings. In addition, in historic properties the installation should: involve the minimum of disruption to the fabric and structure of the building, have an aesthetically acceptable appearance,

To designate the hazard by BS 5306: Part 2 and the LPC sprinkler rules, the following classification system is used:

Light Hazard Light



intended for non


- 2.25 mm/min

Water supply duration

- 3 0 minutes

Allowable sprinkler spacings - 21 m2/sprinkler

Ordiitary Hazard The Ordinary Hazard classification includes nonindustrial occupancies where the fire loading is greater than that allowed in Light Hazard. It is also suitable for commercial and industrial occupancies such as manufacturing, and includes limited amounts of storage, providing they are not likely to develop intense fires early in the initial stages of combustion. The following design features are common to all Ordinary Hazard sprinkler systems:


Discharge Density

- 5 mmlmin

water supply - duration

- 60 minutes



Allowable sprinkler spacings - 12 m2/sprinkler Ordinary Hazard is subdivided into four sub groups, and the level of hazard that may be protected increases with increasing Group number:

be safe from all forms of accidental discharge. None of these latter points are addressed in the sprinkler rules, yet they are of vital importance when considering the installation of a sprinkler system in an historic building and should be uppermost in the minds of the clients, consultants and contractors involved. Constructive assistance may be provided by insurers in the form of advice and checking of initial proposals and design drawings.

Discharge Density


ordinary ~~~~~dcroup 1 AMAO

72 m?

Ordinary Hazard Group 2


144 m?

Ordinary Hazard Group 3


2 16 m'

Ordinary Hazard Group 3 s (Group 3 Special)


360 m'

High Hazard High Hazard occupancies include those that are the most difficult to protect such as process and storage risks where fires may develop rapidly. High Hazard protection of storage risks involves the use of a materials categorisation system that is also used where storage is found in Ordinary Hazard situations. It is