Changes in Amendment 3 to BS 7671: 2008 relating to final circuits supplying lighting points
Amendment 3 to BS 7671: 2008 introduced changes in how the regulations relating to certain types of lighting installations are arranged. This article identifies these changes.
Prior to the introduction of this last amendment, all regulations relating exclusively to the particular requirements of luminaires and lighting installations were included in Section 559 (Luminaires and lighting installations) of BS 7671.
However, due to the increased level of danger associated with certain types of lighting installations, Amendment 3 now classifies outdoor lighting installations and extra-low voltage lighting installations as special installations and in addition to the general requirements of BS 7671, are subject to the requirements of Sections 714 and 715 respectively.
Lighting in household premises should be served by one or more designated final circuits, according to the size and layout of the property. It could be argued that, where practicable, more than one lighting circuit should be provided, even in small premises, so as to avoid the loss of all lighting should one circuit be disconnected due to the operation of its circuit protective device (Regulation 314.1).
Similarly, it might be desirable for areas such as a hallway and stairs to be served by separate circuits so that some lighting for the stairs is still available should one of the circuits be disconnected.
Regulation 559.5.1 lists acceptable means of final connection that should be provided at each lighting point. In domestic premises, this final connection is likely to be made via a ceiling rose, a batten lamp holder or a pendant set or by direct connection to the terminals of a luminaire.
Each radial final circuit for lighting connects a number of such lighting points to the circuit overcurrent protective device within a consumer unit or a distribution board (see Fig 1).
Ceiling roses are used to connect a flexible cable from a suspended luminaire (a pendant luminaire) to the fixed wiring installation. A typical arrangement using a ceiling rose is shown in Fig 2.
Fig 3 shows a pendant set which comprises a ceiling rose and a lampholder connected by a length of flexible cable. Where a luminaire is installed suspended by a pendant set, the method of fixing the ceiling rose to the fabric of the building should be capable of carrying a mass of not less than 5 kg, and must be in accordance with the manufacturer’s instructions. The weight of the luminaire and its accessories must be compatible with the mechanical capability of the ceiling or suspended ceiling or supporting structure where installed (Regulation 559.5.2 refers).
Where ceiling roses are used, Regulation 522.214.171.124 requires that only one outgoing flexible cable be connected to each ceiling rose, except where the ceiling rose is specifically designed for more than one such pendant connection. Any cable between the fixing means and the luminaire must be installed so that any expected stresses in the conductors, terminals and terminations will not impair the safety of the installation (Regulation 559.5.2 refers).
It should be noted that Regulation 5126.96.36.199 precludes the use of ceiling roses in circuits that normally exceed 250 V in operation. Ceiling roses complying with BS 67 have maximum current and voltage ratings of 6 A and 250 V respectively, and are intended for use in final circuits having a rating not exceeding 16 A where the ceiling rose has screw-type supply terminals (see Figure 2), or not exceeding 10 A where the supply terminals of the ceiling rose are of the screwless type.
Ceiling roses to BS 67, commonly used in household premises have a rated current not exceeding 6 A, and lighting circuits for household premises are generally protected by an overcurrent protective device with a rated current or current setting (In) of 6 A. When considering the design of lighting circuits of domestic premises; the number of lighting points connected to a single circuit depends upon the wattage of the lamp.
Traditionally, each lighting point was considered to have a rating of 100 W. This gave a typical number of 13 lighting points served from a 6 A protective device (6 A x 230 V = 1380 W). Today, where the wattage rating of an energy efficient lamp suitable for the domestic market can be as low as 5 W, the potential number of lighting points served by a 6 A protective device has increased considerably (1380 W ÷ 5 W = 276!).
With reference to Table 52.3 of BS 7671, the minimum conductor size for a radial final circuit for lighting, protected by an overcurrent protective device with a rated current or current setting (In) of 6 A, is 1.0 mm2 for thermoplastic (PVC) or thermosetting insulated cables having copper conductors.
The flexible cable between the ceiling rose or similar and the lampholder is permitted to have a minimum cross-sectional area of 0.75 mm2 (see Regulations 433.3.1(ii), 524.1 and Table 52.3). This flexible cable should preferably have 90 °C thermoplastic insulation.
As for any other circuit, the voltage drop between the origin of the installation and the terminals of the luminaire needs to be checked in order to verify compliance with Regulation Group 525 and Appendix 4 of BS 7671.
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