"George" wrote in message news:TRZee.61608$WI3.54789@attbi_s71...
Hm... I do not advocate agains the GFCI.
I just said I do not expect it doing any change to his measurements!

If you read the whole thread you would found out he measured 48VAC
between tank water column and the cooper pipes for tap water...
I just know voltage measurements of this kind are worthless.
Yes, very similar to pH measurements of RO/DI water... ;-)

It will, in fact change his measurements completely, because 48 VAC will likely
carry significant amperage (more than 4 or 5 milliamps), which is more than
enough to trip the GFCI, and completely shut down everything. So after this
happens, he shouldn't read any voltage, and definitely no amperage.

I am taking from above that you did not read the original poster...
His 48VAC reading was even after he has unplugged all his devices.

Please read his original words
Could you explain then how GFCI could influence his voltage readings ?