"Dave Plowman (News)" wrote:

In article ,
Shaun wrote:
As someone who has experienced a shock from simultaneously touching a
metal screw retaining a double insulated switch into a metal backbox
that was contacting the live feed, to a metal carpet join strip that I
was standing on in my socks I can confirm it hurts.

Someone ought to provide M3.5 durable plastic screws - but I don't
think they do!

Pretty certain RS Components do nylon screws.

However, there's no reason a metal back box should ever be live if the
wiring was well done in the first place.

Yes I just noticed they do supply M3.5 but at the time they were
unavailable certainly from the main distributors (Farnell/RS/Vero/STC/
etc) and if I recall correctly one of the major nylon pillar/screw
manufacturers only did M3 and M4

Anyway the whole point was that if the backbox is not earthed then if
as in my case the screw connection to the switch was slightly high
resistance it degraded the insulation gradually (over a period of
10-15 years) This was sufficient to cause contact between the wire and
the backbox - this would have been trivial if non metal screws had
been used or if the backbox had been earthed the MCB/ fuse would have
blown.

I suspect the fault had been there for quite a while though and it was
only accidental simultaneous contact from the fixing screw via sweaty
feet to the carpet strip that was sufficient to be felt - badly
especially as it was a cross chest shock. If I'd been wearing shoes
or been fully on the carpet I doubt there would have been anything
felt at all.


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