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Fully charged The installation of extralow voltage tungsten halogen lighting Objective Although a lighting installation designed to operate at extra-low voltage (ELV) may not present a risk of electric shock to the user, as outlined in this article unless correctly installed in accordance with BS 7671 it may not only fail to function properly but, in some cases, it could be a fire hazard. For these reasons, particular requirements for extra-low voltage lighting installations are contained in Section 715 of BS 7671 – Special installations or locations.
T
his article considers some of the factors that should be taken into account for the installation of a typical ELV tungsten halogen lighting installation, supplied
Fig 1 ELV suspended lighting system
from the secondary output of a transformer positioned remotely from the luminaires. Luminaires supplied by electronic drivers, such as those used for LEDs, are not considered. For an ELV1 lighting installation the protective measures extra-low voltage provided by SELV or PELV should be used; however, where the luminaires have no provision for the connection of a protective conductor, they should be supplied from a SELV system (Regulation 715.414 refers). It should be noted that a FELV system does not satisfy the safety requirements of SELV or PELV and so is prohibited by Regulation 715.411.7.201. It should be recognised that compared with operation at 230 V, a load operating at ELV will demand a proportionally higher current; for example, a 50 W lamp will demand over 4 A when operating at 12 V. Should a loose or high-resistance termination develop, significant heating (I2 R) will occur, which, if undetected in concealed areas such as loft spaces, may ignite surrounding combustible materials. To minimise such risks all terminations and connections of an ELV lighting installation should be made in accordance with Regulation 526.2, and except for appropriate systems that use visible bare conductors complying with the requirements of Regulation 715.521.106, they should be suitably enclosed in accordance with Regulation 526.5. For suspended systems, terminations and connections of conductors should be made using screw terminals or screwless clamping devices complying with BS EN 60998-2-1 or BS EN 60998-2-2 (Regulation 715.521.107 refers). Voltage drop Excessive voltage drop can adversely affect the performance of an ELV lighting installation, therefore, careful consideration should be given to the rating and number of luminaires supplied and the lengths of circuit supplied from an ELV source. For compliance with BS 7671, the voltage drop between the transformer and the furthest luminaire should not exceed 5% of the nominal voltage of the ELV installation (Regulation 715.525). Table 1 shows the minimum values of crosssectional area (copper conductors) permitted by Regulation 715.524.201 for an ELV lighting installation; however, these values may need to be increased to comply with voltage drop. 1 ELV – having a nominal voltage not exceeding 50 V a.c. (or 120 V ripple-free d.c.)
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