A brief introduction to fire cable and fire alarm cable

Fire cable having been designed with a standard fire resistance is specified for general use and is suitable for most installations requiring fire-rated cable. Fire cable having 'enhanced' fire resistance is specified for installations where the fire alarm would be expected to work for a longer period in a fire.

Recommendations concerning the type and characteristics of a wiring system to be used for a fire detection and alarm system in a building (other than a dwelling) are given in Clause 26 of BS 5839: Fire detection and fire alarm systems for buildings Part 1: Code of practice for system design, installation, commissioning and maintenance.

The recommendations were designed to ensure the integrity and reliability of fire alarm circuits before, and to a degree during, any building fire. They relate to the protection of cables against damage by fire and against mechanical damage, as well as the segregation of fire alarm circuits from other circuits and the identification of fire alarm circuits (to make them distinguishable from other circuits).

Fire resisting wiring systems should be used for critical signal paths and for the low voltage mains supply to the system. The full length of the mains supply cable to the system, from the origin of where the circuit first becomes the fire alarm system, should be fire resisting. Such fire resisting cables are divided into two types according to their level of fire resistance: 'standard fire resisting cables' and 'enhanced fire resisting cables'. The performances of these relate to EN 50200 and BS 5839-1.

Since BS 5839-1: 2002 was published, modern generation cables have been introduced that achieve the performance required for the 'enhanced' category. See information about Prysmian's guide to the selection of cable type for particular building applications in accordance with BS 5839-1:2002 'Fire detection and fire alarm systems for buildings' elsewhere in this VoltiTECH.

With regards to this VoltiTECH, the important part of BS 5839-1 is the recommendations for the fire resistance of cables. These were simplified by reliance on tests, now specified in the relevant European test standard for fire-resisting cables.

BS 8434-1 and BS 8434-2 set out the tests specified in BS 5839-1 so that BS 5839-1 can refer to these standards rather than including all the test details. BS 8434-1 concerns methods of test for the assessment of the fire integrity of electric cables, testing for unprotected small cables for use in emergency circuits, and BS EN 50200 with the addition of water spray. BS 8434-2:2003+A2:2009 covers the methods of test for assessment of the fire integrity of electric cables, testing for unprotected small cables for use in emergency circuits, and BS EN 50200 with a 930° flame and with water spray.

'No' to plastics as sole means of containment:

Fire cables and fire alarm cables are not the only items covered; cable containment and support systems are too.

In 2005, there was a fire in a block of flats that resulted in the deaths of two firefighters and one other person. During the incident, one of the firefighters became entangled in fire alarm cables, support for which had failed as a result of fire damage to the supporting plastic trunking.

The use of plastic trunking as the sole means of support for fire alarm cables does not satisfy the recommendations of BS 5839-1, primarily to prevent loss of circuit integrity. BS 5839-1 now makes reference to the hazard for firefighters.

This further emphasises the need to avoid plastic components as the sole means of support. The new edition also cites inadequately supported cables as a significant departure from the recommendations of BS 5839-1, to which a newly appointed maintenance contractor should draw the attention of the user. Designers might also want to draw the topic of fire resistance of cable support to the attention of installers.

Some of the changes to the 2008 edition of BS 5839-1 are, therefore, quite significant, so that it is important that users ensure that they are referring to the latest, 2008 edition, which is annotated on the cover with the reference '+A2: 2008' after the reference BS 5839-1: 2002.

Cable support should be designed so that the cable's circuit integrity is not compromised. This, therefore, precludes the use of plastic clips and ties. For this aspect, please see the article by Draka on installing fire alarm cables in ceiling voids, and entitled 'Installing fire alarm cables in ceiling voids' elsewhere in this VoltiTECH.

Some of the above information has been taken from an article in Fire Safety Engineering magazine (www.fseonline.co.uk) - 'Fire Detection and Alarm systems - BS 5839-1 a trusty companion' published November 2008 and written by Colin Todd. Colin Todd is a specialist fire safety consultant and principal of CS Todd & Associates. He is a member of the BSI technical subcommittee FSH/12/1 and was responsible for drafting the 2002 version of BS-5839-1.

Other general information on fire cable and low smoke cable:

Installation of fire alarm cables in ceiling voids - David Oldfield, Product Development Manager, Draka UK, points out, regarding fire cables and fire alarm cables, that BS 5839-1:2002 makes specific reference to recommendations regarding alarm cable installation and support.

Fire resistant cables - the importance of standards - International and national standards are important in setting down good practice and providing a basis for product and system definition to ensure safe and effective operation. This is particularly true in the area of fire performance where incorrect functioning could have life safety implications, as Prysmian points out.

Fire alarm cables for use in thin walls or partitions - Terry Journeaux of Prysmian Cables & Systems takes a look at one aspect of fire cables and fire alarm cables for buildings, which is fire alarm cables for use in thin walls or partitions.