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An interesting issue with a TT supply

Alan Capon
An interesting real life example of why you need double pole switching with a TT supply. The scenario is a small village with a few customers supplied by a single pole mounted transformer. 


We got a call from an electrician working in a property who had measured 400V phase to earth, 210V neutral to earth, and 241V phase to neutral, and was understandably worried. Any suggestions of what I tracked the fault down as? This can happen on any TT supply, but is fortunately very rare - I have come across it three times in 20 years. 


Regards,


Alan.
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    Had there been any PME users on that transformer it would have been another story altogether.

    I suppose that depends on how good an earth the PME'd installation's extraneous-conductive-parts happened to have. If it was the usual plastic pipes and so on, then the voltages would probably be very similar - but all their exposed- and extraneous-conductive-parts would have been a N voltage - so around 230V above true earth - so rather like a broken PEN situation (but the L-N voltage monitoring EVSE would have no chance). Probably not too much of a worry within the equipotential zone, but potentially a bit nasty on the edges...


     
    The advantage we had as the DNO, is that we had the ability to look at the whole village, and disconnect properties until we found out who had the problem.

    Would a simple clamp meter around all the conductors supplying a consumer indicate whether the problem lay in their installation - as there was likely a residual current of many amps? (Probably easier with ABC overheads...)


       - Andy.
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  • 0
    Had there been any PME users on that transformer it would have been another story altogether.

    I suppose that depends on how good an earth the PME'd installation's extraneous-conductive-parts happened to have. If it was the usual plastic pipes and so on, then the voltages would probably be very similar - but all their exposed- and extraneous-conductive-parts would have been a N voltage - so around 230V above true earth - so rather like a broken PEN situation (but the L-N voltage monitoring EVSE would have no chance). Probably not too much of a worry within the equipotential zone, but potentially a bit nasty on the edges...


     
    The advantage we had as the DNO, is that we had the ability to look at the whole village, and disconnect properties until we found out who had the problem.

    Would a simple clamp meter around all the conductors supplying a consumer indicate whether the problem lay in their installation - as there was likely a residual current of many amps? (Probably easier with ABC overheads...)


       - Andy.
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