The latest advice is that lighting circuits should not be on an RCD
at all (unless the house earthing system requires it in which case it
should be a 100mA RCD AFAIK). The change came about when someone
finally realised the "safety" improvement was actually killing more
than it saved.
It's not "imperfect insulation", it's surge protection in the switch-
mode power supplies used in computer equipment. These consist of
devices called MOVs (metal oxide varistors) connected between live and
earth and neutral and earth. Normally these devices only conduct when a
surge arrives down the wire; the surge is then carried away to earth.
The devices do have a very small leakage current, which means that in
installations where large numbers of computer equipment are connected to
the same supply, it can cause nuisance tripping of the RCD. Similarly,
if the earth were to become disconnected, the surge protection of all
the connected kit combined could cause the potential of metal-cased
equipment to rise high enough to deliver a nasty shock.
We had the downstairs office refurbished recently here (uni in NW
England) and as we expected to install a lot of computer kit, the
electricians installed special sockets with two earth tags on the back,
and ran a second earth circuit. Can probably take a photo and put it up
somewhere if it's of interest.
There could be several things going on:
Computers use switching mode power supplies, which have two quirks:
(1) They draw a big spike of current near the peak of the sine wave. The
average current may be a nice sedate 2 amps or so, but in actuality it may
be drawn as a tall spike, maybe 10 amps for 1/10th of each half-cycle.
Power loss goes up as the square of the current, so all the associated wires
and circuit breakers are going to heat up more than expected.
(2) Each power supply has rather studly line-filter capacitors, to keep
these and other spikes from exiting back thru the power lines. A few of
these on the same circuit may lead to excess ground currents, which might
confuse or trip a GFI.
I've seen this at home where my workshop has like 8 to 10 items of old
electoinic equipment. Put these all on one circuit and the GFI trips.
One of the first responses said there were capacitors between the two
wires and that there were new regulations. This, plus your post, plus a
little bit of natural folk physics on the part of my friend, provides a
pretty good idea of what was probably being said or alluded to.
If you have a photo conveniently to hand, by all means send it to me as
an e-mail. If you have one of the MOV inside a PSU, please let me have
one of those too.
Thanks again to you all.
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