[Diabolical Artificer]

To check IP Z put a signal into your tfmr about 1v ish, then get a pot, put it across the tfmr or in parallel, adjust till your reading is halved. Remove pot and measure, that's you IP Z. Remember to use a frequency your tfmr will see in use, so 50hz for your bog standard mains tfmr, whatever khz for SMPSU types.

Andy.