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What earthing arrangement is this?

Former Community Member
Former Community Member
The supply is from a private transformer in a four core cable 3ph + n. The cable armour is earthed and connected to the MET. However there is also a green and yellow cable connected to the neutral terminal at the main isolator going back to a the transformer casing. The transformer is only 5 or 6 metres away. I think this must have been intended to make it a tncs supply but seems to me to just create parallel neutral conductors. Or is it tn-s-c-s?  I have only been able to go off visual inspection because I could not disconnect the supply..
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    Chris Pearson:

    When I first read the OP, I assumed that the green/yellow cable was a protective conductor which was augmenting the armour of the 4-core cable, which I assume was insufficient on its own.




    There are two earths to a transformer. The windings are both earth free.


    There will be a bare earth run from the transformer tank to the switchgear and too the HV earth rods. There is probably also a loop of copper round the substation, also connected to the HV earth in an attempt to give the operator an equipotential zone to stand in (as far as possible). 


    There will be an insulated earth conductor run from the LV earth rods (sufficiently far enough away from the HV rods to be out of their influence), which will be connected to the star point (neutral) of the transformer. This would normally be connected at the LV fuse board in the substation, but with a single customer off the transformer may be connected at the incoming circuit breaker on the customer’s LV switchboard. This is TNS as there is no conductor that carries both neutral current and earth fault current. 


    Depending on the rise of earth potential for an HV fault, the HV and LV earths may be linked, but this is still TNS. 


    Regards,


    Alan. 

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  • 0

    Chris Pearson:

    When I first read the OP, I assumed that the green/yellow cable was a protective conductor which was augmenting the armour of the 4-core cable, which I assume was insufficient on its own.




    There are two earths to a transformer. The windings are both earth free.


    There will be a bare earth run from the transformer tank to the switchgear and too the HV earth rods. There is probably also a loop of copper round the substation, also connected to the HV earth in an attempt to give the operator an equipotential zone to stand in (as far as possible). 


    There will be an insulated earth conductor run from the LV earth rods (sufficiently far enough away from the HV rods to be out of their influence), which will be connected to the star point (neutral) of the transformer. This would normally be connected at the LV fuse board in the substation, but with a single customer off the transformer may be connected at the incoming circuit breaker on the customer’s LV switchboard. This is TNS as there is no conductor that carries both neutral current and earth fault current. 


    Depending on the rise of earth potential for an HV fault, the HV and LV earths may be linked, but this is still TNS. 


    Regards,


    Alan. 

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