UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

Diabolical Artificer · 15th Jul 2019, 2:18 pm

I'll admit to being a bit confused about isolating tfmr's, does it stop you getting a belt if you touch live or not? I just got my DMM, set to AC volts, connected live to the red lead and stuck the black test lead tip into the concrete lintel of my workshop and read 60v AC RMS this is with my isolating tfmr/variac set at 200v. Is the meter reading wrong? Sometimes the meter will give a low reading when not directly connected to an AC source.

If an isolating tfmr doesn't protect against mains shock then what's the point of using one? I'm also not 100% clear whether it is safe to connect a scopes probe black ground lead anywhere but on the ground/chassis when the DUT is powered from an iso tfmr. If the DUT is sitting on a wooden workbench with no insulating mat then it's earthed to some extent yes? Why are scopes X IP circuit ground connected to earth anyway, why not left floating?

Some clarification for us neophytes would be handy. I've read several threads on the subject here ont forum non of which clarify the situation much. Some sort of list of rules for numptys would help.

Andy.

15th Jul 2019, 2:18 pm	#17
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	Re: Isolation/isolating transformers I'll admit to being a bit confused about isolating tfmr's, does it stop you getting a belt if you touch live or not? I just got my DMM, set to AC volts, connected live to the red lead and stuck the black test lead tip into the concrete lintel of my workshop and read 60v AC RMS this is with my isolating tfmr/variac set at 200v. Is the meter reading wrong? Sometimes the meter will give a low reading when not directly connected to an AC source. If an isolating tfmr doesn't protect against mains shock then what's the point of using one? I'm also not 100% clear whether it is safe to connect a scopes probe black ground lead anywhere but on the ground/chassis when the DUT is powered from an iso tfmr. If the DUT is sitting on a wooden workbench with no insulating mat then it's earthed to some extent yes? Why are scopes X IP circuit ground connected to earth anyway, why not left floating? Some clarification for us neophytes would be handy. I've read several threads on the subject here ont forum non of which clarify the situation much. Some sort of list of rules for numptys would help. Andy. __________________ Curiosity hasn't killed this cat...so far.