This article aims to provide clarity on the requirements of BS 7671 concerning Band I and Band II circuits.
Electrical installation work, particularly in commercial and industrial premises, can often include a range of installation systems. This article looks at some particular requirements of BS 7671concerning the routing of the following:
- Band I installations, which typically cover circuits such as telecommunications, alarm installations, extra-low voltage and voice and data systems. Extra-low voltage is defined as a voltage not exceeding 50 V a.c. or 120 V ripple-free d.c. (whether between conductors or to Earth).
- Band II installations which contain the voltages for supplies to household and most commercial and industrial installations, such as low voltage circuits for power and lighting. Low voltage is defined as a voltage exceeding extra-low voltage but not exceeding 1000 V a.c. or 1500 V d.c. between conductors (or 600 V a.c. or 900 V d.c. between conductors and Earth).
Requirements of Regulation 528.1
Unless the segregation requirements of Regulation 528.1 are satisfied, circuits of voltage Band I and voltage B and II should not be contained in the same wiring system as a circuit exceeding low voltage, and a Band I circuit should not be contained in the same wiring system as a Band II circuit.
To minimise the risk of interference between cables of differing voltage levels, every cable or conductor should be insulated for the highest voltage present. Where a multicore cable contains Band I and Band II circuits, as shown in Fig 1, all conductors should be insulated for the highest voltage carried by the cable. One example of this requirement might be where certain cores of a multicore armoured cable to BS 5467 supply SELV equipment, such as a cooling unit having an external condenser requiring a low voltage supply (Band II) and a control circuit (Band I).
Other methods detailed in 528.1 include, among others, cables insulated for their system voltage and installed in a separate compartment of a cable ducting or cable trunking system, see Fig 2; for example where twisted pair cables (say Category 5 ‘Cat 5’) supply telecommunications equipment and single insulated cables to BS 6004 supply low voltage circuits, such as socket-outlets.
A separate conduit, trunking or ducting system may be required; for example where it is desired and/or required to segregate cables supplying computer equipment, fire detection and alarm systems to Grade A and low voltage final circuits.
Requirements of Regulation 528.2
In circumstances where cables are either buried underground or are installed in ducts, a minimum clearance of 100 mm shall be maintained between power cables and telecommunications cables. Where this is not possible, either of the two options detailed in Regulation 528.2 of BS 7671 should be fulfilled at the relevant point. These options are:
- a fire-retardant partition is provided between the cables. This might be, for example, bricks, shaped blocks, protective cable conduit and the like, or
- where cables cross, mechanical protection between the cables is provided. This mechanical protection might include conduit, concrete cable protecting caps or shaped blocks.
Where it is necessary to take account of potential interference, reference should be made to manufacturer’s data and, where necessary, BS 6701 and BS EN 50174¹ (Notes 1 and 2 attached to Regulations 528.2 refer).
Requirements of Regulation 528.3
A wiring system should preferably be located away from non-electrical services. However, where a wiring system is located in close proximity to a non-electrical service the following conditions should be met:
- the wiring system is to be suitably protected against the hazards likely to arise from the presence of the non-electrical service in normal use, and
- fault protection is to be afforded in accordance with the requirements of Section 411 of BS 7671.
Wiring systems should not be installed in the vicinity of a service that produces heat, smoke or fumes likely to be detrimental to the wiring, unless the wiring is protected against such harmful effects by shielding. The shielding should be positioned such that it does not affect the dissipation of heat from the wiring.
Requirements of Regulation 521.11.201
Irrespective of the type of system – Band I and/or Band II – being installed, electrical contractors should be aware that the extent of Regulation 521.11.201 of BS 7671 is not restricted to any particular cable type or voltage, but applies to all wiring systems within the scope of the Wiring Regulations.
In accordance with Regulation 521.11.201, any installation within the scope of BS 7671 should be protected from premature collapse in the event of fire. For such purposes, non-metallic cable supports (such as plastic conduit saddles, cable clips and ties) and non-metallic trunking should not be used as the sole means of support for wiring systems (Note 2 of Regulation 521.11.201 refers).
It should be noted that an escape route includes not only defined routes such as corridors, stairways and hallways, but also includes open areas.
1 BS EN 50174: A European Standard in three parts to assist those persons designing, specifying, installing and operating information technology cabling.
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