Instrumentation, signal, and telecommunications cabling should always ideally be separated from power cabling. This includes cabling used for fire alarm systems. James Hunt introduces this VoltiTECH by looking at some of the issues and system requirements...
As well as meeting minimum bend radii requirements, cable management systems are also usually required to carry wiring for mains power, lighting and other electrical services such as building controls, fire and security systems.
BS5839 Fire detection and fire alarm systems for buildings – Part 1 Code of Practice for system design, installation, commissioning and maintenance, in its 1988 edition stated that fire alarm cables should not be readily exposed to electromagnetic disturbance, possibly emanating from other cables. BS5839 still states in section 26.1 that ‘The circuits of fire alarm systems need to be segregated from the cables of other circuits to minimise any potential for other circuits to cause malfunction of the fire alarm system arising from:…electromagnetic interference to any fire alarm circuit as a result of the proximity of another circuit’.
Therefore, while it is recognised that this ideal situation is not always possible in practice, especially where alarm cables are retrofitted, none-the-less, fire alarm cables should, if possible, always be segregated from cables for other systems. Installation of cables should be in accordance with good practices recommended in the latest edition of the IEE wiring regulations.
Also, wherever possible, it is sensible to route fire alarm system cables through low fire risk areas. They should be PVC sheathed if installed in damp or corrosive environments, or where there is risk of mechanical damage. The minimum bending radius as defined for each type of cable has to be respected.
It is important to note that any later re-routing, or addition, of cables on to cable trays, conduits or other cable management system already occupied by electrical cables of a different kind may adversely affect an existing cable segregation arrangement. In turn, this could result in compromised fire prevention.
Other issues to consider include the following:
- Compliance with relevant BS and IEE Regulations for segregation of services.
- Conductor rating for alarm load / volt drop requirements.
- Earthing (in accordance with IEE Regulations).
- Mains voltage power supply to control equipment in accordance with the relevant regulations
- Protection provided for surface laid insulated and sheathed cables, where there may be mechanical damage, or attack by rodents, or where cables are installed less than 2.5m from the floor.
- Mineral insulated (MICC) cables to be provided with PVC sheaths in corrosive or damp situations.
More specifically, the 16th edition of the IEE Wiring Regulations (BS 7671) also demands segregation of electrical circuits. These need to be kept apart (segregated) and this is achieved through category classification. Amendments published in 1997 resulted in simplification, and there are now only two categories (Voltage Bands I & II).
Voltage Band I covers voltage levels so low that they cannot cause dangerous electric shocks. As such, Voltage Band 1 relates only to extra-low voltage (ELV) circuits, such as control, alarm and signalling circuits, as well as telecoms.
Voltage Band II covers all voltages - 230/400 V (240/415 V) - used in electrical installations not covered by Band I.
BS 7671 prohibits Band I and Band II cables sharing the same cable management system (trunking enclosure etc, or multicore cable). There are exceptions provided that every cable (or each conductor in a multicore cable) is insulated for the highest voltage likely. Otherwise, conductors relating to Voltage Bands I & II must be separated by an earthed metal screen or installed in separate trunking / ducting compartments. Foil screens have good screening properties and are light in weight, and are often ideal for use with fire alarm cables. Pliable fire alarm cables today unusually use metal foil / polyester tape or the drain wire screening system. For more on screening in this context, see David Oldfield’s article elsewhere in this VoltiBULLETIN.
Alternatively, such conductors must be laid in a partitioned cable tray, or separate conduit / ducting systems must be provided for each Voltage Band system.
Regulation 528-01-04 of BS7671 states: ‘Fire alarm and emergency lighting circuits shall be segregated from all other cables and from each other in accordance with BS5839 and BS5266’, and BS 58398 requires that that fire alarm cables must be separated from all others. Moreover, IEE Guidance Note 4 refers to BS5266, of which Section 8.2.1 states: ‘Cables used for the connection of an escape lighting luminaire to the standby power supply should…possess inherently high resistance to attack by fire…’. BS5266 also states that PVC cables in trunking (steel or PVC) and PVC/SWA cables will need additional fire protection.
In all cases, they should not be affected by electrostatic / electro-magnetic fields.
Another aspect is that where electric circuit outlets from both Voltage Bands share a common box or switchplate, the relevant connections must be segregated by a partition, and if the partition is metal, it must be earthed.
As always, whichever necessary cable segregation system is chosen, space should be left inside enclosures or compartments to allow the laying of further cables at a later dated if required.
In short, the gist of segregation, as far as BS5839 is concerned, goes far beyond EMC. It is a requirement to preserve the integrity of life safety systems from mechanical damage from other cables during installation, or from faults on other circuits during service.
Recommended cable types:
The new requirements introduced by the revision of BS 5839-1 ‘Fire detection and fire alarm systems for buildings – Code of practice for system design, installation, commissioning and maintenance’ in 2002 provided an impetus for change in the fire resistant cable market. Four years later, the changes driven by BS5839-1:2002 are now well understood, and many products meeting its requirements are now available. Indeed, says Prysmian, a review of the current LPCB ‘Red book’ listing, restricted to products listed specifically against BS 5839-1:2002, shows that the number of listed ‘standard’ cables has more than doubled, and the number of listed ‘enhanced’ cables has quadrupled in the last two years.
BS5839 (in section 26.1) states: ‘It is not normally possible to predict, with any accuracy, those areas of a building in which fire can or cannot occur. At the design stage, the exact routes that cables will follow may also be unknown. Therefore to ensure that cables used for critical signal paths (see 3.13) remain operational for an adequate duration, cables with an inherent ability to resist attack by fire need to be used throughout such critical signal paths’. Section 3.13 defines a critical signal path as a detection circuit.
However, only two types of cable may be used for fire alarm circuits where prolonged operation in a fire is required. These are mineral insulated copper sheath cables (MICC) complying with BS 6207, and cables complying with BS 6387 and meeting the requirements of categories AWX or SWX or better. The segregation of MICC cables with a plastic sheath is not as critical as for the segregation of ordinary twin and earth cable.
For hazardous areas, including combustible dusts, mineral insulated cable (with suitable approved fittings) is permitted in certain locations, and may also be installed in cable trays in certain instances.
Such cable tray systems must comply with regulations regarding current carrying capacity, earthing, fill, spacing and cable segregation. The latter can be provided by a separate cable tray system or by a divider within an existing cable tray.
Other cable containment systems, such as trunkings, are now available in zero-halogen, fire-resistant noryl material, as well as uPVC and GRP, plus steel and aluminium. For ease of on-site installation, trunking is available with pre-drilled centre compartments, which allow cables to pass through the dividing wall to the relevant outlets. Steel dividers are available with pre-punched knockouts, ensuring that screening is maintained. Single-, double- or three-compartmented trunking is readily available, and even four compartment segregation of cables is possible.
Though not, strictly speaking, a segregation issue, it is important that fire cables be fixed direct to a surface using metal ‘P’ clips. Plastic clips must not be used. Where cable is fastened to trays, especially to the underside, stainless steel cable ties must be used.
It is understood the Electrical Contractors Association (ECA) may – at some stage – publish a guidance document on the segregation of pliable fire cables.