Earthing Transformer (zigzag) Hi-pot Testing questing

[Latango]

Just a quick question, had a search through and couldn't find an answer:

By IEC60076, when doing a separate source voltage withstand test on a transformer, all terminals of the winding under test must be connected together, and all other windings / metal parts, etc have to be connected to earth. Does that hold true for ZN transformers too? Since the N of the winding is designed to be earthed, does it need to be earthed for hi-pot?

Thanks for any help,
Mat

 

[GuntherA]

No, you are purely testing the insulation of the winding to earth. If you earth the neutral terminal during SS you would be testing zero sequence impedance (and drawing a lot of amps!). Just be sure to check the rating plate for the insulation level. Some terminals which are designed to be earthed have reduced clearance, eg. SWER, you can usually tell by the size of the bushings.