James asks this question, which concerns a borrowed neutral discovered during a site survey. It has been answered by the NICEIC (Theme – Regulations & Legislation):
Question: I've just carried out a site survey, where I discovered a 300mm x three-core SWA cable run from the main switch panel - a length of over 100 meters - to a three-phase distribution board.
When asking the site's in-house electrician how he had a three-phase distribution board run off a three-core cable (with no visible neutral), he quite proudly explained how he, as it was only a small neutral current involved, had run a small SWA cable to the local lighting and power distribution board 10 meters away and picked up the neutral there.
I have raised how severe a problem this is, and have now been tasked with rectifying the problem.
If a single-core armoured were run in parallel with the 300mm x three-core submain from the same source at the main switch panel, would I be correct in saying that eddy currents would be created because of the phase conductors being enclosed in the steel armour, and the neutral not being within this?
Would the correct solution be to replace the 300mm x three-core cable with a 300mm x four-core cable, and then bring the neutral conductor within the armouring, therefore alleviating the eddy current issue?
Answer: Indeed the regulations do not allow the neutral to run separately.
It should be contained in the same enclosure as the phase conductors and, therefore, a replacement supply would need to be put in place.
The arrangement you have described needs to be put right as this provision is inadequate.
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