You cannot post a reply to this discussion. If you have a question start a new discussion
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..
If there is a separate earth coming from the transformer, and that earth is connected to the star point of the transformer with the neutral conductor then tat is TN-S. There should be an earth electrode connected to the same star point which is not in the earth fault path.
If only 3 phases and neutral come from the transformer to the main panel and there is a link between the neutral in the panel and the installation earth then that is TN-C-S (PNB). PNB is one of the 2 types of TN-C-S.
If the supply to the building comes from the transformer and that supply may serve other users and the cable has earth electrodes along the cable route (the "M" in PME) then that is TN-C-S (PME), the other type of TN-C-S.
If the supply has no earth from the supply transformer and only the 3 phases (or a single phase) and neutral provided there should be an earth electrode connected to the installation MET to make the installation TT. In this case the earth fault path is through the consumers earth electrode through the general mass of earth and back to the star point of the transformer via the transformer electrode.
Some installation supplied from a cable coming from the street may look like a TN-S say with a connection from the lead sheath of a PILC cable to the MET but they are not because the DNO has jointed the cable and combined the earth and neutral in that joint. So the method of earthing cannot be verified by visual inspection. My local DNO (UKPN) have declared that where they have provided an earth from a cable in the street there whole network is PME. That PME cannot be undone by spitting the earth and neutral in a BNO network (which should be the case anyway). In this case you will see a separate neutral and earth at the supply head but the method of earthing is still PME.
I feel sure that my brief explanation will be disagreed with but my reference sources are BS 7430 and IET Guidance Note 8. GN8 has very clear drawings of the different earthing systems.
If you do not have a copy of Guidance Note 8 then I would highly recommend you get one. If Kirsty asked me if I could only take one GN to my desert island which one would it be I would say GN8. My luxury would be Kirsty herself, just of course for the convivial conversation to while away the time!
If there is a separate earth coming from the transformer, and that earth is connected to the star point of the transformer with the neutral conductor then tat is TN-S. There should be an earth electrode connected to the same star point which is not in the earth fault path.
If only 3 phases and neutral come from the transformer to the main panel and there is a link between the neutral in the panel and the installation earth then that is TN-C-S (PNB). PNB is one of the 2 types of TN-C-S.
If the supply to the building comes from the transformer and that supply may serve other users and the cable has earth electrodes along the cable route (the "M" in PME) then that is TN-C-S (PME), the other type of TN-C-S.
If the supply has no earth from the supply transformer and only the 3 phases (or a single phase) and neutral provided there should be an earth electrode connected to the installation MET to make the installation TT. In this case the earth fault path is through the consumers earth electrode through the general mass of earth and back to the star point of the transformer via the transformer electrode.
Some installation supplied from a cable coming from the street may look like a TN-S say with a connection from the lead sheath of a PILC cable to the MET but they are not because the DNO has jointed the cable and combined the earth and neutral in that joint. So the method of earthing cannot be verified by visual inspection. My local DNO (UKPN) have declared that where they have provided an earth from a cable in the street there whole network is PME. That PME cannot be undone by spitting the earth and neutral in a BNO network (which should be the case anyway). In this case you will see a separate neutral and earth at the supply head but the method of earthing is still PME.
I feel sure that my brief explanation will be disagreed with but my reference sources are BS 7430 and IET Guidance Note 8. GN8 has very clear drawings of the different earthing systems.
If you do not have a copy of Guidance Note 8 then I would highly recommend you get one. If Kirsty asked me if I could only take one GN to my desert island which one would it be I would say GN8. My luxury would be Kirsty herself, just of course for the convivial conversation to while away the time!
