The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 138

June 26, 2023 10:24 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 137, LWF began to look at the classification of fire alarm systems. In part 138, we continue to discuss the classification and sub-classification of fire alarm systems as per BSI and NFPA.

BS 5839-1 provides sub-divisions for category P fire alarm systems, as follows:

P1 fire alarm systems

A P1 fire detection system provides fire detector coverage in all areas of the building, excepting the following:

  • Some voids – usually less than 800 mm in height, unless the spread of fire could take place through the void. Additionally, any floor voids in data processing rooms are normally protected irrespective of depth.
  • Cupboards
  • Toilets
  • Bathrooms
  • Some small lobbies

P2 fire alarm systems

A P2 fire alarm system needs detectors in defined areas of the building, which are deemed to have a high fire risk. Examples follow:

  • Any area containing an ignition source and easily ignitable materials
  • Any area with a high consequence of fire

Any area of the building which does not have protection should be separated by fire-resisting construction to avoid the spread of fire from one area to another.

BS 5839-1 treats any void within a building with a depth greater than 1.5 m as a room. This applies whichever type of fire alarm system is required.

NFPA 72 does not divide systems into classifications. The standard anticipates that designers and fire engineers will have sufficient knowledge and experience to determine the level of coverage required from a fire alarm system. Once the required level has been ascertained, the standard gives guidance.

Total Coverage (Complete Coverage)

The NFPA guidance states that total coverage relates to all rooms, storage areas, lofts, attics, ceiling voids and other sub-divisions and accessible spaces, with the following exceptions:

  • Where an inaccessible area does not contain flammable materials
  • Where a small concealed space over a room does not exceed 4.6 m2 in area
  • An area with open grid ceilings does not require detectors, subject to specific recommendations
  • As per NFPA 90A, concealed accessible spaces above suspended ceilings used as a return air plenum do not require detectors
  • Areas below open loading docks or platforms do not require detectors, subject to specific recommendations

In part 139 of LWF’s series on fire engineering, we will continue looking at the different types of property protection fire alarm system classification as per NFPA 72, before beginning to discuss systems for life protection. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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