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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..
. . . In a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.
No it doesn’t! The star point is within the transformer tank and is never brought out apart from split phase transformers or some specialist traction transformers. The star point will be brought out to a neutral terminal in the LV connection box. There will be no connection to earth inside the tank. Whether the single connection to earth is in the connection box or 100m away, it is still TNS. In order to be TNC-S, you would need a cable core that provides the neutral and earth function, in this application there isn’t one.
Yes, I do understand that the connexion need not be made inside the transformer itself and it is self evident that there could be no connexion to the general mass of the earth within the tank. Of necessity in any system there must be at least a short length of cable internally from the star point to the connexion box (and similar cables for the lines at the other ends of the windings) and that cable must serve both as a neutral and a fault path. We can ignore that.
My point is that in a TN-S system the PE must have its own connexion with the star point - you can have as many junctions as you like; but if the PE sprouts off the N at some distant point (as Andy drew it in his first diagram) then there must be a section of cable which serves both as N and PE - it cannot be any other way.
Moreover, if we take a TN-S system, there are two cables which are connected to the star point: the N and the PE. If they are joined anywhere downstream of the transformer, they now become two cables in parallel and therefore the system becomes TN-C-S.
If I am mis-reading Figures 3.8 and 3.9, I apologise for being thick.
. . . In a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.
No it doesn’t! The star point is within the transformer tank and is never brought out apart from split phase transformers or some specialist traction transformers. The star point will be brought out to a neutral terminal in the LV connection box. There will be no connection to earth inside the tank. Whether the single connection to earth is in the connection box or 100m away, it is still TNS. In order to be TNC-S, you would need a cable core that provides the neutral and earth function, in this application there isn’t one.
Yes, I do understand that the connexion need not be made inside the transformer itself and it is self evident that there could be no connexion to the general mass of the earth within the tank. Of necessity in any system there must be at least a short length of cable internally from the star point to the connexion box (and similar cables for the lines at the other ends of the windings) and that cable must serve both as a neutral and a fault path. We can ignore that.
My point is that in a TN-S system the PE must have its own connexion with the star point - you can have as many junctions as you like; but if the PE sprouts off the N at some distant point (as Andy drew it in his first diagram) then there must be a section of cable which serves both as N and PE - it cannot be any other way.
Moreover, if we take a TN-S system, there are two cables which are connected to the star point: the N and the PE. If they are joined anywhere downstream of the transformer, they now become two cables in parallel and therefore the system becomes TN-C-S.
If I am mis-reading Figures 3.8 and 3.9, I apologise for being thick.
