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What earthing arrangement is this?
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..
Not according to BS 7430, GN8 and the ECA Guide to the Wiring Regulations.
Well, with all due respect to the publications author, they are wrong - and there is a certain amount of copying in class going on anyway on this issue
Your argument is based on nothing more than the length of a conductor connecting to the means of earthing - ie, if you physically don't make a connection at the TX spill box, and make it at the switchboard then it must be TNC-S - which is complete nonsense.
If I give you the example of a switchboard with the transformer forming part of the board, your logic says TN-S (because you cannot see the short length of copper from the winding ends to the switchboard neutral)
If I sperate the switchboard and transformer by a short distance and use a bit of cable to make that link, then your logic says "Ahh - but it's TN-C-S"
Only if the neutral conductor is connected to earth at the transformer spill box and again at the consumer switchgear (or anywhere along that run) does it become TN-C-S. In most circumstances that would be unusual, because you cannot determine where Earth Fault current is going via the protection CT's - eg, you couldn't operate a Restricted earth fault scheme operating from the star point, including the LV windings and up to the primary circuit breaker
I suggest you apply the same logic to a generator as you do to the transformer and then tell me it's TN-C-S
Not according to BS 7430, GN8 and the ECA Guide to the Wiring Regulations.
Well, with all due respect to the publications author, they are wrong - and there is a certain amount of copying in class going on anyway on this issue
Your argument is based on nothing more than the length of a conductor connecting to the means of earthing - ie, if you physically don't make a connection at the TX spill box, and make it at the switchboard then it must be TNC-S - which is complete nonsense.
If I give you the example of a switchboard with the transformer forming part of the board, your logic says TN-S (because you cannot see the short length of copper from the winding ends to the switchboard neutral)
If I sperate the switchboard and transformer by a short distance and use a bit of cable to make that link, then your logic says "Ahh - but it's TN-C-S"
Only if the neutral conductor is connected to earth at the transformer spill box and again at the consumer switchgear (or anywhere along that run) does it become TN-C-S. In most circumstances that would be unusual, because you cannot determine where Earth Fault current is going via the protection CT's - eg, you couldn't operate a Restricted earth fault scheme operating from the star point, including the LV windings and up to the primary circuit breaker
I suggest you apply the same logic to a generator as you do to the transformer and then tell me it's TN-C-S
