Fire Alarm and Emergency Lighting Circuits
It is a requirement of BS 7671 that fire alarm and emergency lighting circuits are segregated from all other cables and from each other in accordance with their respective British Standards, BS 5839 and BS 5266 (Regulation 528-01-04 refers).
The circuits of fire alarm systems need to be segregated from other circuits in order to minimize any likelihood of other circuits causing a malfunction of the fire alarm system. Such malfunctions may arise from:
- a breakdown of the cable insulation of the fire alarm circuits and/or other circuits
- a fire caused by a fault on another circuit
- electromagnetic interference to a fire alarm circuit as a result of the proximity of another circuit
- damage resulting from the need for other circuits to be installed in, or removed from conduit, ducts or trunking containing a fire alarm circuit.
Where a multicore cable is used for the connection of fire alarm circuits, it should be used exclusively for such circuits. None of the cores should be used for other types of circuit. This does not, however, preclude the multiplexing of signals of other systems with those of the fire alarm system.
Voltages in excess of ELV
Fire alarm cables carrying power at a voltage in excess of ELV (Extra-low voltage) should be segregated from ELV fire alarm circuits. In particular, cables connecting LV (low voltage) power to any fire alarm control, indicating or power supply equipment, should not enter the equipment through the same entry as cables for ELV circuits. Within equipment, LV and ELV cables should be kept separate from each other, as far as is reasonably practicable.
Damage by fire
The likelihood of a malfunction of the fire alarm system due to a cable being damaged by fire is minimized by installing the fire alarm system using fire resistant cables.
BS 5839 recommends that cables used for all parts of the critical signal paths, and for the final circuit providing low voltage power to the fire alarm system, should be one of the following types and should also meet the fire resistance requirements specified in BS 5839:
- mineral-insulated copper-sheathed (MICC) cables with an overall polymeric covering and conforming to BS EN 60702-1
- fire resistant cables having low emissions of smoke and corrosive gases when affected by fire and conforming to BS 7629
- armoured fire resistant cables having thermosetting insulation and low emissions of smoke and corrosive gases when affected by fire and conforming to BS 7846
- cables that provide the same degree of safety as that afforded by compliance with BS 7629.
Where fire resistant cables recommended by BS 5839 are used for the fire alarm system, segregation of fire alarm circuits from other types of circuit for the purposes of fire protection is not required, although segregation for other purposes e.g. electromagnetic interference, may be required.
Fire alarm signal circuits can be susceptible to electromagnetic interference. Therefore the advice of the manufacturer of the fire alarm equipment should be sought to establish which circuits are susceptible to electromagnetic interference due to their proximity to another type of circuit.
Where a circuit is susceptible to electromagnetic interference, the manufacturer’s recommendations to avoid such interference should be obtained and followed. The manufacturer may, for example, require a minimum separation distance between the cables of fire alarm circuits and those of other services.
Where such separation is required, the crossing of fire alarm circuits susceptible to electromagnetic interference and other circuits should be avoided. Where this is not possible the circuits should cross at right angles.
Mineral-insulated copper-sheathed (MICC) cables conforming to BS EN 60702-1 and steel wire armoured cables conforming to BS 7846 may be used throughout all parts of a fire alarm system without additional mechanical protection, except where there are particularly arduous conditions.
Where types of cable requiring mechanical protection are used, one means of providing mechanical protection, though not the only means, is to install the cables in conduit, ducts or trunking. However, where this method of installation is used the cables of the fire alarm circuits should be installed either:
- in a conduit, duct or trunking reserved exclusively for fire alarm circuits, or
- in a separate compartment of a duct or trunking, reserved solely for fire alarm circuits.
The segregation of fire alarm circuits in this way avoids the risk of mechanical damage to the wiring of such circuits, which may otherwise be caused when other circuits are installed in, or removed from, the same conduit, duct or trunking, or compartment of a duct or trunking.
Where the wiring of fire alarm circuits is installed in its own compartment of a duct or trunking, the partition separating the fire alarm circuit compartment from the compartment(s) of other circuits is to be rigid, strong and continuous.
Emergency voice communication systems
The cables of an emergency voice communication (EVC) system should be segregated from the cables, conduit, ducting and trunking of all other services including those of the fire alarm system and other safety services.
Where a multicore cable is used for the connection of EVC circuits, it should be used exclusively for the EVC circuits. None of the cores should be used for other types of circuit including those of fire alarm systems. This does not, however, preclude the multiplexing of signals of other systems with those of the EVC system.
Circuit segregation requirements
Circuits for escape lighting are required to be segregated from the circuits of other services. An emergency lighting circuit serving a luminaire that provides both standby and escape lighting is considered to be an escape lighting circuit.
Such a circuit is to be segregated from other circuits accordingly including any circuit serving a luminaire that provides standby lighting only.
The requirements for an acceptable means of achieving segregation are as follows.
(i) Wiring (single-core insulated non-sheathed cables)
Escape lighting wiring is to be used only for the escape lighting installation and any parts of the standby lighting installation which additionally serve as escape lighting. The escape lighting wiring is to be segregated from the wiring of any other circuits by separation.
Separation may be achieved either by installation in a separate conduit, ducting, trunking or channel, or by a mechanically strong, rigid and continuous partition of non-combustible material between the wiring of the escape lighting installation and the wiring of all other services.
Where escape lighting wiring is installed in ducting, trunking or channel, the ducting trunking or channel used should completely enclose the wiring and be marked to indicate that it is reserved for the installation of escape lighting wiring only.
(ii) Insulated and sheathed cables
Where the escape lighting installation is connected using suitable non-fire resisting insulated and sheathed cables, the cables should be separated from the cables of all other services by a minimum distance of 300 mm between the centre lines of the closest cables.
Where such separation is not provided, mineral-insulated copper-sheathed (MICC) cable complying with BS EN 60702-1 should be used. Alternatively cable assessed under the BASEC Certificate of assessment scheme as being suitable for use in an escape lighting installation without the need for separation may be used.
Where cables assessed under the BASEC Certificate of assessment scheme are installed without the required separation, they are to be of a type which have been certified to be in compliance with the appropriate test requirements of BS 6387 Specification for Performance requirements for cables required to maintain circuit integrity under fire conditions. Such cables should be at least category B.
A multi-core cable, where used for escape lighting, should be used exclusively for the escape lighting. None of the cores should be used for other types of circuit.