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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..
i see your point, but everyone calls it TNC-S that I have come across. which probably means PNB is a better term for it.
But it is actually TN-S - all that's happened is that the N-E Bond has been expressed out to the first consumer switchgear position
Usually, the multiple earth rods and tape is used to explain why its TNC-S
It's just an electrode - the fact that there are multiple electrodes in the array does not make it a multiple earthed neutral - the complete electrode has a single connection to the system neutral
its not combined though, Ill have to check but I think its got a link between N and the transformer frame in the spreader box too
There will be a HV earth (or so called steelwork earth) and the LV earth, which is the mechanism that ensures the neutral is solidly and reliably at or around earth potential - it should usually have a single N-E bond. The connection or otherwise of the HV and LV earthing together is a function of the effective resistance - calculated normally to limit the rise of earth potential under a HV fault (usually interwinding) and limt the rise of voltage on the neutral to 2kV - historically this would have been a limit of 1 Ohm, but today we calculate the actual value of ROEP depending on the speed of protection. (eg 430V for "normal" and 630V for so called high speed disconnection. Earth connections to the transformer casing are usually bonding rather than earthing - depending on the physical make up of the receiving switchgear, that connection is from the MET - but may physically be at the neutral conductor depending on how the link is arranged on the incoming side of the de facto main switch (ie even when the switch is open, the N-E bond and equipotential bond of the transformer frame should remain in place
i see your point, but everyone calls it TNC-S that I have come across. which probably means PNB is a better term for it.
But it is actually TN-S - all that's happened is that the N-E Bond has been expressed out to the first consumer switchgear position
Usually, the multiple earth rods and tape is used to explain why its TNC-S
It's just an electrode - the fact that there are multiple electrodes in the array does not make it a multiple earthed neutral - the complete electrode has a single connection to the system neutral
its not combined though, Ill have to check but I think its got a link between N and the transformer frame in the spreader box too
There will be a HV earth (or so called steelwork earth) and the LV earth, which is the mechanism that ensures the neutral is solidly and reliably at or around earth potential - it should usually have a single N-E bond. The connection or otherwise of the HV and LV earthing together is a function of the effective resistance - calculated normally to limit the rise of earth potential under a HV fault (usually interwinding) and limt the rise of voltage on the neutral to 2kV - historically this would have been a limit of 1 Ohm, but today we calculate the actual value of ROEP depending on the speed of protection. (eg 430V for "normal" and 630V for so called high speed disconnection. Earth connections to the transformer casing are usually bonding rather than earthing - depending on the physical make up of the receiving switchgear, that connection is from the MET - but may physically be at the neutral conductor depending on how the link is arranged on the incoming side of the de facto main switch (ie even when the switch is open, the N-E bond and equipotential bond of the transformer frame should remain in place
