Loop Impedance Measurement from a second Transformer

Assuming you mean there is a spec to be met for the Zs on the 208V side ,and between one of your 120V lives and local earth, rather than the Zs on the primary side (which wont affect the outcome very much, unless it also means the L-N loop is high resistance)


How is the transformer wired ? If it is double wound with a star connexion and bonded so that the outgoing neutral is strapped to the local CPC then measure the Zs in the normal way, with nothing special connected or removed - the path is out of the transformer secondary, along the CPC, and back to the bond point and then to the neutral point on the TX.

Iff double wound it matters not if it is configured as star star or delta star, but the secondary side must be star, to do the bonding of the star point to CPC.


If it is an auto transformer, then it needs to be so the star point goes to the incoming neutral ,but not the CPC, and here you rely on the Earth neutral bond of the 230/400 system, so fault currents have to go all the way back to the 400V substation - here the incoming Zs matters, both the L and E impedance.


If there is no hard wired neutral,  if it is double wound, it would be usual to earth the centre of 3 inductors one to each secondary phase, configured as a zero-sum winding, recreating a star point.

IF there is no neutral and it is an auto transformer then not to worry, as the 400V side NE bond will do it for you.


It will only become TT, if there is no metal CPC from transformer to the load, and you rely on a pair of electrodes, and a path in the ground (terra-firma sort of ground) to complete the earth path.

Generally it is normal to make things TN-S for double wound systems, and whatever the incoming is for auto transformers.


If its not one of those cases, or if it needs a bit more explanation feel free to keep asking - my explanation is necessarily terse, and I do not know what exactly you have or what you already know.

Assuming you mean there is a spec to be met for the Zs on the 208V side ,and between one of your 120V lives and local earth, rather than the Zs on the primary side (which wont affect the outcome very much, unless it also means the L-N loop is high resistance)


How is the transformer wired ? If it is double wound with a star connexion and bonded so that the outgoing neutral is strapped to the local CPC then measure the Zs in the normal way, with nothing special connected or removed - the path is out of the transformer secondary, along the CPC, and back to the bond point and then to the neutral point on the TX.

Iff double wound it matters not if it is configured as star star or delta star, but the secondary side must be star, to do the bonding of the star point to CPC.


If it is an auto transformer, then it needs to be so the star point goes to the incoming neutral ,but not the CPC, and here you rely on the Earth neutral bond of the 230/400 system, so fault currents have to go all the way back to the 400V substation - here the incoming Zs matters, both the L and E impedance.


If there is no hard wired neutral,  if it is double wound, it would be usual to earth the centre of 3 inductors one to each secondary phase, configured as a zero-sum winding, recreating a star point.

IF there is no neutral and it is an auto transformer then not to worry, as the 400V side NE bond will do it for you.


It will only become TT, if there is no metal CPC from transformer to the load, and you rely on a pair of electrodes, and a path in the ground (terra-firma sort of ground) to complete the earth path.

Generally it is normal to make things TN-S for double wound systems, and whatever the incoming is for auto transformers.


If its not one of those cases, or if it needs a bit more explanation feel free to keep asking - my explanation is necessarily terse, and I do not know what exactly you have or what you already know.