Loop Impedance Measurement from a second Transformer

You will probably get lots of relevant answers if you post the question in this forum instead: Wiring and the Regulations

No need to re-post, I've moved it for you... ?

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.

I am a little unsure what you are trying to do. Are you trying to re-create an American style supply for testing, but happy to work with 50Hz? If so, then you want a delta connected secondary, with the centre-point of one of the windings earthed. This will give you two phases at 120V to earth, and one phase 208V to earth. The phase to phase voltages should be 240V in all cases. 


Regards,


Alan.

Ah yes, good point, I had briefly forgotten that there are in fact far too many ways of doing 3 phase in the USA.  ?  (let's not even mention the options on 277V L-N / 480v  phase to phase. At least here it is always 3 equal legs, and usually 230V /400 unless it  is 400/690..)


You mention 208v, so consider the drawings below in one case the phase phase voltage is 208 and in the other it is 240.


In both cases line marked 'N' in these drawings needs to be earthed.  Only the upper kind, with 3 equal leg voltages can be generated  from UK 3 phase with an auto transformer. With a double wound system the world is your oyster..