Mains Insulation 'Resistance', Multiple PCs and Load Problems Question

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Whole house RCD trips - fall down stairs in dark or (the previousy mentioned case) family trapped behind locked door in dark looking for key as fire raged.
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Isn't there something in queens regs that sez there have to now be separate trips for each circuit?.....

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Tony Sayer


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wrote:

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.
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Peter Parry.
http://www.wpp.ltd.uk/
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tony sayer wrote:

I did . But you weren't there ....;-)

Zero ?
Steve
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On Sun, 11 Jan 2004 12:05:25 +0000, David Longley

Probably not, however www.cda.org.uk/megab2/elecapps/pub142lo.pdf , www.cda.org.uk/megab2/elecapps/pub141lo.pdf and www.iee.org/Publish/WireRegs/article.pdf might explain it all to you.
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Peter Parry.
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Peter Parry wrote:

Oh yeah? Numbers/source? RCD type?
J.B.
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On Mon, 12 Jan 2004 06:30:26 -0600, "Jerry Built"

Message-ID: < and associated thread.

Any, makes little difference.
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Peter Parry.
http://www.wpp.ltd.uk/
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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.
M.
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When a significant earth leakage current is expected, as in this case, a high integrity earthing system is required. What you describe is one way of doing this.
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Andrew Gabriel

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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.
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Many thanks.
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.
--
David Longley

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