Caledonian Fire Alarm Cables for Fire Detection and Alarm Systems
DESCRIPTION
The following is a summary of the recommendations for the wiring of fire detection and alarm systems given in BS 5839-1: 2002
Cables for Fire Detection and Alarm Systems
The following is a summary of the recommendations for the wiring of fire detection and alarm systems given in BS 5839-1: 2002
Recommendations concerning the type and characteristics of a wiring system to be employed 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 are intended to ensure the integrity and reliability of fire alarm circuits before, and to a sufficient extent during, any fire.
The recommendations relate to the:
• protection of cables against mechanical damage
• protection of cables against damage by fire
• segregation of fire alarm circuits from other circuits
• identification of fire alarm circuits so that they can be distinguished from other circuits.
Fire alarm cables must also meet the requirements of BS 7671. There is therefore a need to consider conventional electrical engineering issues such as voltage drop, current-carrying capacity, impedance and external influences.
Protection of cables against mechanical damage
Types of cable
Four types of cable are recognized by BS 5839-1, namely:
BS EN 60702-1)
• cables conforming to BS 7629 (sometimes referred to as ‘soft-skinned’ cables)
• armoured fire-resistant cables conforming to BS 7846 (steel wire armoured or SWA cables)
• cables rated at 300/500V that provide the same degree of safety as that afforded by compliance with BS 7629. (This permits the use of suitable new cables.) Most designers specify either MICS or proprietary fire resisting cables complying with BS 7629. MICS cables are less likely to need additional mechanical protection and have a long history of performing extremely well in fires, often well beyond the duration necessary to ensure that the objectives of the fire alarm system are satisfied. On the other hand, the newer proprietary fire resisting cables complying with BS 7629 have now been in existence for many years without any evidence to suggest that their performance in fires is not adequate.
Conductor csa
All conductors should have a cross-sectional area (csa) of at least 1mm2 to ensure sufficient mechanical strength, whatever the type of cable employed.
Segregation
Fire alarm cables should not be installed within the same conduit as the cables of other services, to avoid the risk of mechanical damage.
Fire alarm cables may be installed within the same trunking as the cables of other services but, regardless of whether they are mineral insulated or soft skinned, a separate compartment should be provided solely for the fire alarm cables. The partition within the trunking should be strong, rigid and continuous. The recommendation is intended to avoid damage to the fire alarm cables when modifications are carried out to other circuits (for example, when other cables are stripped out or new cables are drawn in).
In certain cases, such as when a new fire alarm system is installed in an existing building, it may not always be possible to provide a separate conduit, trunking or compartment for fire alarm cables. In such cases, a departure from the recommendations of the standard should be recorded in the certification of the system. Where the segregation of cables is not reasonably practicable, care must be taken to avoid electromagnetic interference from other cables affecting the fire alarm circuits .
Additional mechanical protection
MICS and SWA cables may be used throughout all parts of a fire detection and alarm system without additional mechanical protection, except in particularly arduous conditions.
Mechanical protection should be provided for all other cables in areas that are less than 2 m above the floor level, other than in relatively benign environments in which a cable is clipped directly to a robust construction. The term ‘relatively benign’ is best illustrated by examples. In many situations in offices, shops and similar premises, it will be possible to install such cables without additional mechanical protection. In premises such as factories and warehouses where the environment is not ‘relatively benign’ additional protection to the cables will be necessary. In such cases, mechanical protection will be needed, for example, for at least part of each ‘drop’ to manual call points that are usually installed about 1.4 m above floor level. Mechanical protection should be provided for cables in all areas where physical damage or rodent attack is likely.
Additional mechanical protection may be provided by means such as:
• running the cables on cable tray
• ‘chasing’ the cables into the building structure, or
• by installation of the cables in conduit, ducting or trunking. Where particularly arduous conditions are likely to be experienced (such as impact by forklift trucks or goods trolleys), cables should either be buried in the structure of the building or protected by substantial metal conduit or trunking, or by robust steel capping.
Cable joints
Cables should be installed without external joints, wherever practicable. Where jointing of cables is necessary, other than where joints are made within components of the system, the terminals used to join the cables should be constructed of materials that will withstand a similar temperature, and duration of temperature, as the cable itself. Plastic connector blocks are unlikely to be suitable. All joints should be enclosed within junction boxes clearly labelled with the words ‘FIRE ALARM’.
Protection of cables against damage by fire
Fire resistance
Wiring systems used for critical signal paths, and for the low voltage mains supply to the system, should be fire resisting. The cable itself should be fire resisting. It is not sufficient to provide fire protection for a non-fire resisting cable by burying it in the building structure. The entire length of the mains supply cable to the system, from the point at which the circuit is first dedicated to the fire alarm system, should be fire resisting.
Fire resisting cables are divided into two types according to their level of fire resistance, namely ‘standard fire resisting cables’ and ‘enhanced fire resisting cables’. Performance levels are given for both groups of cables. The performance levels and associated tests relate to a new standard, EN 50200, and also to
BS 5839-1. The performance levels were developed largely by determining the level of performance that can reasonably be expected of:
• an MICS cable for enhanced fire resistance, and
• an existing proprietary fire resisting cable that has been certified as complying with BS 7629, for standard fire resistance. Since the publication of BS 5839-1: 2002, a number of new generation ‘soft skinned’ cables have been launched by the cable industry that are able to achieve the performance required for cables of enhanced fire resistance. In addition, two new standards, BS 8434-1 and BS 8434-2, set out the tests specified in BS 5839-1 so that, in future, BS 5839-1 will be able to refer to these standards rather than including all the test details.
Cable supports
Methods of cable support should be such that circuit integrity is not reduced. The methods of support should withstand a similar temperature, and duration of temperature, as the cable itself, while maintaining adequate support. Plastic cable clips, cable ties or plastic conduit and plastic trunking, where these products are the sole means of cable support, are unlikely to be suitable.
Segregation of fire alarm circuits from other circuits
Fire alarm cables should be protected against any detrimental influence from other circuits. Where a multicore cable is used for the interconnection of fire alarm circuits, none of the conductors should be used for circuits other than those of the fire alarm system. To avoid electromagnetic interference with the fire alarm signals, care needs to be taken that any recommendations of the manufacturer of the fire alarm equipment in respect of separation of fire alarm cables from the cables of other services are followed.
Low voltage fire alarm cables (e.g. 230 V circuits) should be segregated from extra-low voltage fire alarm cables (e.g. 24 V circuits). Separate cables (selected from the four permitted types listed at the beginning of this article) are themselves a suitable form of segregation, subject to compliance with any recommendations of the manufacturer of the fire alarm equipment in respect of separation for the purposes of avoiding electromagnetic interference.
The mains supply to any control, indicating or power supply equipment should not enter the equipment through the same cable entry as cables carrying extra-low voltage. Low voltage and extra-low voltage cables should be kept separate as far as practicable within equipment.
Identification of fire circuits so that they can be distinguished from other circuits
Having segregated the fire alarm circuits from the other circuits and, in the case of trunking, kept the fire alarm cables within a separate compartment from other circuits, it is important that this situation is maintained. It is also important that there is no interference with fire alarm circuits as a result of confusion between the fire alarm circuits and other circuits. Accordingly, all fire alarm cables should be of a single, common colour that is not used for cables of general electrical services in the building. The colour red is preferred. However, it would be possible to comply with the recommendations of BS 5839-1 by using another colour, provided the same colour is not used for cables of other electrical services in the building.