Ask the experts/Technical
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Fully charged Safe isolation and borrowed neutrals Those planning to safely isolate a final circuit generally approach the task assuming that the circuit has been correctly wired. However, this assumption can leave a person working on the supposedly isolated circuit exposed to danger, if the circuit shares its neutral with another final circuit. This condition is sometimes called a borrowed neutral.
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or compliance with Regulation 314.4 of BS 7671, each final circuit of an installation should be connected to a separate way in a distribution board, and the wiring of each final circuit should be kept electrically separate from that of every other final circuit. This is essential to prevent a final circuit intended to be isolated from being indirectly energised. One of the most potentially dangerous errors an installer can make is to ‘borrow’ the neutral of another final circuit. ‘Borrow’ is a
Fig 1 Typical borrowed neutral condition
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In this example, instead of running the neutral of circuit No. 1 back to the distribution board, a short cut has been taken and the neutral has been ‘borrowed’ from circuit No. 2.
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misnomer, as there is no intention to later disconnect (or ‘return’) the borrowed neutral. More accurately, the neutral is being ‘shared’ or ‘crossed’ (particularly in the case of adjacent ring final circuits). For example, NICEIC occasionally finds cases where a neutral conductor from an upstairs lighting circuit is ‘borrowed’ for a ground floor staircase luminaire, or vice versa. Similarly, where the wiring of two radial circuits supplying socket-outlets is installed in the same trunking or conduit systems, a neutral conductor is occasionally found to be ‘shared’ between the circuits. Fig 1 shows a diagrammatic representation of a typical borrowed neutral condition. Contractors should be aware of the dangers to electrically-skilled operatives of such malpractice. Borrowed neutrals create a dangerous trap for any operative carrying out maintenance work or other installation alteration or extension work in the future. Electricians have been electrocuted whilst working on circuits they believed to be completely isolated. In one such incident, having been engaged to carry out an extension to an existing electrical installation, an electrician carried out what he believed to be all the necessary isolation procedures and completed tests for voltage to prove dead that part of the installation on which he was about to work. However, he was unaware that the neutral conductor from another circuit was connected to the circuit he was working on. His work necessitated the disconnection of the circuit neutral conductor, which had been tested for voltage when connected to the switchboard and showed only a typically small voltage to earth – a not unusual circumstance. Having unbolted the neutral conductor, he proceeded to remove it from its termination and immediately received an electric shock from which he was unable to break contact: he had disconnected a neutral that was connected to two circuits, the second of which remained live. Fig 2 refers. As shown in Fig 3, a hazardous situation can also exist where two circuits share the same neutral where the line conductor of a seemingly isolated circuit remains live. A borrowed neutral discovered during periodic inspection and testing of an installation would warrant a Classification code C2 (potentially dangerous) being recorded in the Electrical Installation Condition Report. In such cases, the client should be advised of the danger that this defect presents to those working on the circuit. Provided the client is in agreement, the Connections Summer 2013
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