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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..
. . . Assuming only one NE bond and one point of earthing for the LV side, it can be treated as TNS, just with the NE link outside the transformer tank. . .
With an ESI 35-1 standard transformer, the neutral - earth bond is always outside the tank.
Is there a separate HV earth ?
If not you must assume HV and LV earthing is shared and never interrupt the common CPC while the HV side is connected, even if the LV is off, or the transformer core and all the metalwork attached to it, will be driven to some nasty high voltage by the capacitance between the HV windings and the core and tank.
It is actually worse than that. To reduce losses, the HV winding is wound over the top of the LV winding (I might have that the wrong way round), which gives the opportunity for a flashover between the HV and LV windings. For this reason, the neutral to earth bond must never be removed with the transformer in service.
. . . Assuming only one NE bond and one point of earthing for the LV side, it can be treated as TNS, just with the NE link outside the transformer tank. . .
With an ESI 35-1 standard transformer, the neutral - earth bond is always outside the tank.
Is there a separate HV earth ?
If not you must assume HV and LV earthing is shared and never interrupt the common CPC while the HV side is connected, even if the LV is off, or the transformer core and all the metalwork attached to it, will be driven to some nasty high voltage by the capacitance between the HV windings and the core and tank.
It is actually worse than that. To reduce losses, the HV winding is wound over the top of the LV winding (I might have that the wrong way round), which gives the opportunity for a flashover between the HV and LV windings. For this reason, the neutral to earth bond must never be removed with the transformer in service.
