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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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  • 0
    But the joy of TT is that so long as  neutral is properly insulated and treated (as '7671 requires..), with the same respect as a live wire. this is not immediately dangerous, no equipment correctly wired L-N sees any damaging over-voltages, and no one even feels any tingles. It is rather like the first fault on an IT system. A second fault however is not safe..


    This example also suggests the stables have a better connection to terra-firma earth than the DNO.


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


    I suppose if required you could make TT safer by having an alarm at the TX that calls in if there is significant current in the earth path in the manner of a sort of giant RCD for the whole street, but to raise a fault code, not to cut the power.

    Mike


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  • 0
    But the joy of TT is that so long as  neutral is properly insulated and treated (as '7671 requires..), with the same respect as a live wire. this is not immediately dangerous, no equipment correctly wired L-N sees any damaging over-voltages, and no one even feels any tingles. It is rather like the first fault on an IT system. A second fault however is not safe..


    This example also suggests the stables have a better connection to terra-firma earth than the DNO.


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


    I suppose if required you could make TT safer by having an alarm at the TX that calls in if there is significant current in the earth path in the manner of a sort of giant RCD for the whole street, but to raise a fault code, not to cut the power.

    Mike


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