Job today installing a little wall mounted audio amp in the boardroom of
a factory. Turned on, got loads of hum when the input cable is in which
I put down to the crappy leads the gaffer buys (the screen is run
parallel to the signal rather than being wrapped round it. But they're
cheap he says. Grr.)
So I idly go to ground the floating input, touching it and an earthed
case, to see if it helps. And get stung.
Dig out the DVM and find mains earth floating at about 80V over, erm,
well me in rubber soles and fresh air. I mention this and they'll give
it to their sparks.
So, it got me wondering, with a machine shop of big lathes and mills on
the other side of the wall, is there any reason that earth should get
that high, apart from simply a high resistance connection somewhere?
Where are we going and why am I in this handbasket?
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