Keep in mind that most of the "wall wart" power supplies do not have a 3rd
pin ground. I'm not sure the reason and I didn't research the NEC
regulations as to why, but we've all seen them this way. Now with that
said, there is always some amount of leakage current between primary and
secondary of any transformer. Although the leakage current is very small,
usually in the microampere range, the voltage being dependant on resistance
can be significant as B.J. has pointed out. Most solid state devices won't
like a potential difference of 50 volts.
One other respondent, Stuart, indicated that things should not be powered
up when making connections. Excellent advice. Specially true if there is
likelihood of the wrong connection being made. It is much safer to make all
the connections, check to see that they are correct as intended and then
apply power.
Rule of thumb: connect the grounds, connect the signal lines, connect the
power. This works in all cases and is the reason the 3rd pin ground
connector extends beyond the length of AC mains pins such that ground makes
contact first.
My CE at the broadcast station where I studied and learned always said,
"Safety first, funerals latter".
73
Bob, K4TAX
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