Fire alarm cables for use in thin walls or partitions

BS5839-1 'Fire detection and fire alarm systems for buildings - Code of practice for system design, installation, commissioning and maintenance' requires that the installation of 'the entire system should conform to the requirements of BS 7671'. It notes that the requirements of BS5839-1 supplement those of BS7671 and in general do not conflict but where conflict might exist; the requirements of BS5839-1 should take precedence.

The issue of the new 17th Edition IEE Wiring Regulations / BS 7671:2008 which was published in January 2008 and came into effect in July 2008 has impacted UK fire alarm installations. In particular, the new series of Regulations given in clause 522.6 concerning cables concealed in a wall or partition has led to a number of questions. These new Regulations are specifically aimed at the issue of penetration of concealed cables by nails, screws and the like.

For the mains supply to the fire alarm system, BS5839-1 warns that not all cable types complying with its requirements will provide an adequate level of mechanical protection to meet the clause 522.6 requirements. Advice in the commentary to Clause 26 'Cables, wiring and other interconnections' regarding this clause of the 17th Edition IEE Wiring Regulations is given regarding the need to choose both 'standard' and 'enhanced' cables that will provide an adequate level of mechanical protection when concealed in a wall or partition. Further advice states that solutions using RCD protection should be avoided wherever possible in order to meet BS5839-1 requirements.

Although one may question the necessity, ELV circuits are not excluded from the clause 522.6 requirements. In order to ensure complete compliance with BS7671, the same consideration needs to be given to all fire alarm cabling including the ELV circuits. Whilst it is likely that this matter will be corrected by a future amendment to BS7671, the easiest solution for BS5839-1 may be an interpretation in the revised PD 6531 'Queries and interpretations on BS5839-1' which is proposed for publication in 2009.

The solution:

A number of solutions are allowed by BS7671 for cables to be installed in thin walls or partitions but not all are appropriate for fire alarm installations. Possible methods of installation are:

• Enclose cable in earthed conduit complying with BS EN 61386.
• Enclose cable in earthed trunking complying with BS EN 50085.
• Install mechanical protection to prevent penetration by nails, screws and the like.
• Install within a defined zone with RCD protection as necessary.
• Use a cable incorporating an earthed metallic covering.

If using the last option, which is likely to be the most suitable and cost effective for fire alarm installations, the following choices are available:

• Use an armoured cable (BS 5467, BS 6346, BS 6724 or BS 7846).
• Use a cable complying with BS EN 60702-1.
• Use a cable meeting the requirements of BS 8436 and in particular the nail penetration test.

BS 8436 was specifically written to describe screened small power cables suitable for such installations and includes the required nail penetration test. Whilst it does not specifically cover fire resistant cables, the general constructions given are essentially similar to those for screened fire resistant cables to BS7629-1, albeit without the requirement for an insulation material giving fire resistant properties.

Whilst many fire alarm cables to BS7629-1 employ a thin foil screen which cannot meet the nail penetration test given in BS8436 necessary to prove adequate protection, cables with a more robust screen, the same as used for BS8436 compliant cables, such as FP200Gold or FP PLUS are able to comply with the requirements of the nail penetration test and can be selected for such applications.

This is another matter that could usefully be clarified in the revision of PD 6531.