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A final connection to a motor in a factory or a plant room frequently presents a problem because the supply will often be from overhead and there are no walls or other structural features to provide support for the conduit drop.
NICEIC Area Engineers have reported conduit drops of 5 m or more with one right-angled set and often a second, more open, set for which the only support is provided by the conduit entries at each end. A knock from a ladder, vibration, or the use of the conduit as a hand-hold could cause damage or slackening of conduit joints leading to damage to the wiring or a high resistance connection or an opencircuit protective conductor.
The conduit must be adequately supported, have secure fixings and be protected from mechanical damage (Regulation 522.8.1). The conduit must be protected from mechanical deterioration and electrical continuity must be assured (Regulations 543.2.2, 543.2.5 and 543.3.1 refer). Vibration of the motor must be taken account of by a means such as flexible conduit but the flexible or pliable conduit must not be used as a protective conductor (Regulation 543.2.3 refers).
Whether or not steel conduit is used as a circuit protective
conductor, it is always required to provide reliable electrical continuity along its length and to be effectively and permanently connected to the Main Earthing Terminal (with certain permitted exceptions such as where SELV conductors only are installed within the conduit). Every joint in conduit must be electrically and mechanically continuous (Regulation 543.3.6 refers).
An alternative approach is to use a flexible cable, preferably metal-sheathed, to supply the machine with a short length of conduit at the lower end. This conduit should be firmly fixed to the machine or a support, as shown. The conduit should rise to about 2 m above floor level and should terminate with a ring bush or cable gland.
This method will accommodate discrepancies in the relative locations of the machine and supply point, and will provide mechanical protection adjacent to the machine. The flexible cable must be adequately supported.
A wiring system shall be selected and erected to avoid during installation, use or maintenance, damage to the sheath or insulation of cables and their terminations. The use of any lubricants that can have a detrimental effect on the cable or wiring system are not permitted.
Where a metal enclosure or frame of a low voltage switchgear or controlgear assembly or busbar trunking system is used as a protective conductor, it shall satisfy the following three requirements.
(i) Its electrical continuity shall be assured, either by construction or by suitable connection, in such a way as to be protected against mechanical, chemical or electrochemical deterioration
(ii) Its cross-sectional area shall be at least equal to that resulting from the application of Regulation 543.1, or verified by test in accordance with
BS EN 60439-1.
A gas pipe, an oil pipe, flexible or pliable conduit, support wires or other flexible metallic parts, or constructional parts subject to mechanical stress in normal service, shall not be selected as a protective conductor.
The metal covering including the sheath (bare or insulated) of a cable, in particular the sheath of a mineral insulated cable, trunking and ducting for electrical purposes and metal conduit, may be used as a protective conductor for the associated circuit, if it satisfies both requirements of items (i) and (ii) of Regulation 543.2.2.
A protective conductor shall be suitably protected against mechanical and chemical deterioration and electrodynamic effects.
Every joint in metallic conduit shall be mechanically and electrically continuous.
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