In uk.d-i-y, smb wrote:
On 17 Jan 2004 23:09:25 GMT, wrote:

massive snip

Thanks Stefek, for your comprehensive reply.

Cheers. Radio silence from me is because I'm in the US for a week; just
chewing through uk.d-i-y backlog while considering wording of major document
I need to start on tonight ;-)

Just to fill in a couple of blanks, the cable (10mm2), the RCD and the
40amp MCB were all installed at the same time as the shower. There was no
previous electric shower. There is definately nothing else on the same
circuit. The total length of the cable run is about 6 metres.

OK, so the more worrying possibilities become less likely (phew!) It's
possible then that the positioning of the MCB between other loaded MCBs
is the problem - they're warming one another up and making the shower one
trip. But that's not nearly as likely as a fault in the shower, methinks...

I will get my brother to test his RCD. Is the built in test button a
sufficiently good test to rule out a problem with the RCD?

Sadly not: if the test button doesn't make the RCD pop, the RCD *is* faulty
for sure. But if it does pop, the RCD could still be out of spec: too
sensitive, not sensitive enough, take too long to pop (not as if you can
reliably press the test button for exactly 20 milliseconds, is it!). But
in your brother's case it's an individual MCB which is popping, not the
RCD, right? To "properly" test an RCD needs a "proper" RCD tester, which
full-time electricians can spread the 700-quid cost of over years of work,
but which thee and me aren't going to shell out on! It'd make a good
school *design* project, though, using suitable resistors to make a 10mA,
15mA, and 30mA load, and a gravity-dependant timing element such as a steel
ball running down an inclined channel with a conductive section: lots
of good simple applied maths working out how to do various short timing
intervals. Course, Health-n-Safety being wot it is, you couldn't ever
*build* one with the kids :-(

I believe the shower is still within guarantee, so if the RCD works and
all the connections look sound, we'll get someone out to have a look at
it.

Hope you/he get it all sorted... Stefek