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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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    If the 210 volts neutral to earth is measured at the installation between the supply neutral and the local electrode it suggests that something has lifted the supply neutral above earth potential.  The transformer earth electrode could have a resistance to the general mass of earth of as much as 20 ohms.  A 10A earth fault current flowing back to the transformer via this electrode could raise the potential of the neutral wrt true earth.  This fault current could be coming from a different installation(s) connected to the same transformer.


    Regards


    Geoff Blackwell
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  • 0
    If the 210 volts neutral to earth is measured at the installation between the supply neutral and the local electrode it suggests that something has lifted the supply neutral above earth potential.  The transformer earth electrode could have a resistance to the general mass of earth of as much as 20 ohms.  A 10A earth fault current flowing back to the transformer via this electrode could raise the potential of the neutral wrt true earth.  This fault current could be coming from a different installation(s) connected to the same transformer.


    Regards


    Geoff Blackwell
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