C C CEILING(S) Suspended. Contents. 1. Introduction. 2. Earthing and bonding. 3. Supporting of electrical cables

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1 Contents 1. Introduction 2. Earthing and bonding 3. Supporting of electrical cables 4. Final connection to luminaires 5. Thermoplastic luminaire diffusers 6. Fire-protecting suspended ceilings 1. Introduction ceilings are often provided in commercial and similar premises. They generally consist of a grid of light metal bars, usually having an inverted T section, into which ceiling tiles, recessed luminaires (light fittings) and ventilation grilles etc are installed (see Fig 1). Surface-mounted luminaires, smoke detectors and similar items are sometimes installed to the underside of the ceiling. The void above a suspended ceiling, which accommodates the tension wires or other means by which the grid is suspended from the building structure, is commonly used for the routing of wiring systems and non-electrical services, such as pipework and ductwork. An example of a suspended ceiling, incorporating a ventilation grille and recessed luminaires Fig 1 NICEIC NICEIC TECHNICAL MANUAL 27 SEP 01 1 of 5

2 C49-13 C CEILING(S) 2. Earthing and bonding In practice, unless there are very exceptional circumstances (which would need to be taken into account by the electrical designer), the conductive parts of a suspended ceiling will be neither exposed-conductive-parts nor extraneous-conductive-parts. Consequently, in normal circumstances, suspended ceilings need neither to be earthed nor equipotentially bonded. Electrical equipment such as luminaires, lighting track, overhead busbars, air conditioning units and the like incorporated in a suspended ceiling will normally be of either Class I or Class II construction. The exposed-conductive-parts of Class I equipment are required to be connected to the main earthing terminal of the installation by a circuit protective conductor designed to conduct earth fault current. Class II equipment is designed such that any insulation fault in the equipment cannot result in fault current flowing into any conductive parts with which the equipment may be in contact. The conductive parts of a suspended ceiling incorporating Class I and/or Class II equipment are therefore not intended to conduct earth fault current, and so such parts need not be intentionally earthed. (Some conductive parts of a suspended ceiling may be earthed, however, by virtue of fortuitous contact with exposed-conductive-parts, including those of Class I equipment.) Unless there are exceptional circumstances, the conductive parts of a suspended ceiling will not introduce a potential that does not already exist in the space in which the ceiling is installed. In normal circumstances, therefore, there is no need to arrange for the conductive parts of the ceiling to be connected to either a main bonding conductor or to any supplementary bonding conductor. 3. Supporting of electrical cables Some of the requirements applicable to cables in suspended ceilings are embodied in Regulation Groups , and of BS In particular, Regulations and call for cables to be supported, either continuously or at appropriate intervals, such that no damage or undue strain will occur to the cables themselves or to their terminations. It is inadvisable for cables, including insulated and sheathed cables to BS 6004, to be laid directly on a suspended ceiling grid for a number of reasons, including: The cables are liable to be damaged, both during installation and later, by the sharp edges of the grid (Regulations and ). For example, a cable may come into contact with the cut edges of the grid at a notch where sections of grid intersect, or may become trapped between a ceiling tile and the grid, possibly when other trades are carrying out work in connection with the ceiling or other services routed above. The suspended ceiling grid may not have been designed to take the additional weight of cables, and may deform or collapse as a result. 2 of 5 27 SEP 01 NICEIC TECHNICAL MANUAL NICEIC

3 Adequate support for cables for fixed wiring within a suspended ceiling void can be achieved in a number of ways to ensure that the cables are kept well away from the grid and from other services. For example, the designer may consider: Providing a conduit or trunking system, or cable tray (metallic or non-metallic), fixed at suitable spacings to the building structure above the ceiling, or to walls or other structural elements. Providing a catenary system as shown in Fig 2, to which the wiring system may be attached using proprietary clips, cable-ties or cable-hangers at appropriate intervals. Attaching the cables by suitable clips or cable ties to ceiling support rods. Cross-section through a suspended ceiling void, showing cables supported by a catenary system Building structure Catenary wire Cable support (eg tiewrap) Cable(s) Luminaire suspension Final connection device (see item 4) Surface mounted luminaire ceiling grid ceiling tile Switches Note: ceiling supports omitted for clarity Fig 2 4. Final connection to luminaires All connections, including extra-low voltage connections, are required to be made in a suitable accessory or enclosure (Regulation ). NICEIC NICEIC TECHNICAL MANUAL 27 SEP 01 3 of 5

4 C49-13 C CEILING(S) Final connection of a luminaire can be achieved by the use of a luminaire supporting coupler (LSC) mounted immediately adjacent to the luminaire (for example mounted on a conduit box above it), keeping the final flex connection as short as practicable. Alternatively, a suitable plug and socket-outlet (correctly mounted) may be used. Both of these options will provide a suitable means of isolation for the connected luminaire. These and other methods of final connection are discussed in Topic L An advantage of the use of LSCs or socket-outlets is that it facilitates the completion of the fixed installation without the need to have prior knowledge of the exact locations of the luminaires. Moreover, the plug-in arrangement makes the final connection and testing, and subsequent maintenance or replacement of luminaires, particularly easy and quick to carry out. However, the ease with which final connections can be made may be an invitation to the lighting engineer to relocate the luminaires within the suspended ceiling in order to achieve a different lighting effect or intensity. From the contractor s point of view, such changes can result in longer lengths of flexible cords, which are often left to lie on the suspended ceiling for at least part of their length. In order to ensure compliance with Regulations , and , the following points should all be observed: Flexible cord connections to luminaires should be as short as possible (ideally, a vertical drop from the fixed connection to the luminaire). Flexible cords should not be allowed to rest on top of, or come into contact with the sharp edges of the inverted T section of the ceiling grid, for the reasons mentioned in item 3. Suitable cord grips should be provided to securely retain the sheath of the flexible cord if strain is likely to occur on terminations within a luminaire. For some types of luminaire, a cord grip may need to be purchased separately. Where the first and/or the second of the foregoing points cannot be observed, the contractor should consider the use of a type of flexible cord with a suitably reinforced sheath, or so-called curly flex, cut to the appropriate length. 5. Thermoplastic luminaire diffusers The use of thermoplastic lighting diffusers is subject to certain provisions concerning prevention of the spread of fire in buildings. These provisions, which are laid down in Approved Document B, relating to the statutory requirements of The Building Regulations, are the subject of Topic L Fire-protecting suspended ceilings Precautions that need to be considered where a recessed luminaire or other electrical equipment penetrates a suspended ceiling that is intended to contribute to the overall fire resistance of the floor/ceiling assembly are covered in Topic C of 5 27 SEP 01 NICEIC TECHNICAL MANUAL NICEIC

5 Topics referred to in this text: C49-15 CEILING(S):, fire-protecting, precautions in L105-1 LUMINAIRE(S): Connection of L LUMINAIRE(S): Thermoplastic diffusers in suspended ceilings Topics not referred to in this text, which are related and may be of interest: C93-5 CLASSES OF EQUIPMENT: Classes 0, I, II, III for the purposes of shock protection C165-1 CONNECTIONS, JOINTS AND TERMINATIONS: General L LUMINAIRE(S): Supporting couplers (LSC) S309-1 SUPPORTS: for cables Requirements for electrical installations (BS 7671): Some of the most important requirements are found in: Protection against indirect contact Section 413 Protection against thermal effects Chapter 42 Selection and erection in relation to external influences Section 522 Electrical connections Section 526 NICEIC NICEIC TECHNICAL MANUAL 27 SEP 01 5 of 5