On 02/08/2015 05:35 PM, Bob van der Poel wrote:
Just wondering on all this ... where the grounding is "suspicious" does
the use of a ext. chord with a built in GFI switch add any safety? Probably
doesn't solve grounding loop problems.
Wikipedia has an article on residual-current devices (RCD):
http://en.wikipedia.org/wiki/GFCI
Whether using an RCD is more or less safe depends upon what you're
doing. If you're using a power tool with bad insulation while standing
on wet concrete, a RCD could save your life. But if the RCD is powering
something critical (ventilator, chemical reactor, etc.) and the RCD
trips, bad things could happen.
The only interaction between RCD's and ground loops I can think of under
normal conditions (e.g. no ground fault) would be whether or not the RCD
passes or omits the equipment grounding conductor. I seem to recall
seeing early residential GFI receptacles with only two prongs (hot,
neutral). Everything I've seen in the past ~20 years has all three
(hot, neutral, ground). The former would break ground loops and the
latter would allow them.
Something to understand is that RCD's can nuisance trip -- e.g. trip
when you don't want them to. For example, there is one GFCI receptacle
in my kitchen that protects all the receptacles. It trips whenever I
plug or unplug a particular counter-top appliance; sometimes
immediately, sometimes later. If I forget to reset the GFCI, my wife's
phone doesn't charge and I hear about it.
Here is an audio example -- I was doing live stage sound an at outdoor
event with many acts each day. One 120/240V "spider" box with GFCI
circuit breakers and receptacles powered everything -- sound, lights,
and whatever the acts connected. A band took the stage, started
performing, and the power started cycling in and out. I had to stop the
show and trouble-shoot it. The problem turned out to be the keyboard
player's surge suppressor outlet strip (and possibly my equipment
rack-mount outlet strip). Those strips contain three non-linear
semiconductor devices connected across hot, neutral, and ground. At low
voltages, say ~200 VAC, they have high impedance. But at high voltages
(e.g. a surge), they conduct:
http://en.wikipedia.org/wiki/Surge_protector
The problem is that there is always leakage current, and the spider box
GFCI's saw that as a ground fault. It was curious that the power cycled
in and out, rather than simply tripping off. The solution was to remove
the keyboard player's surge suppressor outlet strip and use a plain
outlet strip.
David
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