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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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    Yes a fault on another phase to Earth is possible along with an open or high resistance TX Earth connection. To get the measured results 210V to Earth needs 11A or so across 20 Ohms, or a lot more current if it is lower. The only point about that is that pole pigs are usually single phase. Bigger transformers have 2 poles, but perhaps Alan was a little "quiet" to make the problem more difficult! I am a bit confused because a few customers would only get a single phase. But otherwise, I reckon I may be correct.
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    Yes a fault on another phase to Earth is possible along with an open or high resistance TX Earth connection. To get the measured results 210V to Earth needs 11A or so across 20 Ohms, or a lot more current if it is lower. The only point about that is that pole pigs are usually single phase. Bigger transformers have 2 poles, but perhaps Alan was a little "quiet" to make the problem more difficult! I am a bit confused because a few customers would only get a single phase. But otherwise, I reckon I may be correct.
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