Quote:
Originally Posted by Penthode
The isolation transformer won't help.
Look at it this way: you want to differentially measure a small dc voltage that is not referenced to the chassis ground. A three pronged plug on your VTVM, one os hot, one is neutral and the round pin is case ground. The metal case is grounded via the third pin to protect against you getting a shock if the case is not at ground potential.
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I know this is a settled matter, but there's one thing I find odd. Your assumption that an isolation transformer will pass it's plugs ground through to its outlet and that the VTVM would have a 3 prong cord... While I wouldn't doubt there are ones that do out there somewhere, I've never seen an isolation transformer built that way out of better than half a dozen I've had. Most isolation transformers I've seen don't have 3 prong cords and many that do don't ground the outlet or use a 2 prong outlet.
Likewise the vast majority of VTVMs made are only 2 prong...It wouldn't surprise me if some lab or mil spec units or some last gasp 80s units might have been built with 3 prongs, but that seems like assuming the exception rather than the rule.... Better to assume the rule and warn to check for the exception in most cases.