Typically an ELV tungsten halogen lighting installation is supplied from the secondary output of a transformer, positioned remotely from the luminaires, at a nominal voltage not exceeding 50 V a.c. This article discusses some of the requirements of BS 7671 applicable to such an installation.
Tungsten halogen luminaires designed to operate at extra-low voltage (ELV) which have no provision for the connection of a protective conductor should be supplied from a SELV system (Regulation 715.414 refers). For a given power output, luminaires operating at ELV will demand a proportionally higher current compared with operation at 230 V. Should a loose or high resistance termination develop, significant heating (I2 R) can 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.
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). Generally, the transformer should be located as near as possible to the luminaires to minimise voltage drop.
Table 1 shows the minimum values of cross-sectional 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 or current carrying capacity requirements.
Lamp selection
The selection and erection of a luminaire should take into account the thermal effects of radiant and convected energy on the surroundings (Regulation 559.4.1 refers). Simply because a lamp can be inserted into a particular fitting does not necessarily mean that it is a suitable type or of the correct rating.
Two common types of halogen reflector lamps are generally interchangeable, but the intense heat produced by the lamps is reflected very differently. The dichroic (cool beam) lamp reflects visible light forwards but most of the heat is radiated backwards, whereas the aluminium lamp reflects both light and heat forwards. A consequence of inadvertently inserting a dichroic lamp into a recessed downlighter is that most of the heat would be projected backwards into the ceiling void, creating a risk of overheating. Especially where thermal insulation covers the luminaire, as is commonly the case in loft spaces.
Due to such risks, manufacturers place warnings on fittings. Table 55.3 of BS 7671 provides details for a range of BS EN 60598-1 symbols applicable to luminaires and controlgear (refer to Fig 1).
Transformers
Transformers should comply with the relevant product standard (Regulation 715.414 refers), and be positioned and mounted to minimise the risk of overheating. Where mounting on a flammable surface is required then a ‘class P’ thermally protected or temperature protected transformer should be used (Regulation 559.6 refers).
Regulation 715.422.106 requires that for thermal protection under fault conditions transformers supplying ELV luminaires should be either:
- protected on the primary side by a failsafe protective device that monitors the power demand of the luminaires (and automatically disconnects the supply in the event of one of a number of specified fault or failure conditions, as detailed in Regulation 715.422.107.2), or
- inherently and non-inherently short circuit proof.
Issues of compatibility
The operating characteristics of transformers, electronic converters and controls can vary considerably. Therefore, to prevent issues of compatibility, which can cause lamps to flicker or flash, unwanted operation of protective devices, or overheating of equipment, careful consideration should be given to the particular equipment manufacturer’s instructions (Regulation 134.1.1).
Among other factors, consideration should be given to the following equipment characteristics:
- the maximum VA rating of the transformer (this should not be exceeded)
- de-rating factors for control switches (switches should be rated for inductive loads)
- transformer voltage regulation (variations in voltage can reduce the lamp life)
- the minimum operating load (burden) of the transformer and associated control devices, such as dimmer switches (operation below this minimum value may not only lead to non-function but is likely to cause overheating and lead to component failure)
- the inrush current characteristic of the transformer/controls (such currents can cause unwanted operation of protective devices – nuisance tripping)
- whether lighting controls, such as dimmers, incorporate soft start features to limit the magnitude of switch-on current.
For guidance on the fire performance of down lighters refer to the Electrical Safety First Best Practice Guide No.5. www.electricalsafetyfirst.org.uk
