On Fri, 07 Aug 2009 09:53:07 +0100, Dave Plowman (News) wrote:

Don't automatically blame your local bench RCD. It is possible
that
there is enough leakage in the rest of your installation to be
near the
threshold of your split CU RCD and a few mA extra from your ATX
PSU
tripped that whilst being well below the threshold of your local
one.

Yes - although I don't get nuisance tripping from the main one. Oh - it
wasn't a few extra mA from the ATX - try an earth to neutral short via
the case due to a trapped cable. ;-(.

The additional leak doesn't have to be big. The RCD in the CU has the
leakage of *all* the circuits connected through it to contend with.
The one on your bench, down stream of the CU one, just the bench
circuits. If the house is sitting at say 20mA and the bench 5mA, 15mA
of extra leakage at the bench will trip most likely trip the CU RCD
(20+15 = 35mA) not the one on your bench (5+15=20mA)(*).

To give you the discrimnation the CU RCD needs to be a time delayed
one or feed the bench RCD on a radial taken before the CU RCD. I'd go
for the latter.

(*)Assuming a 30mA RCDs and they trip precisely at 30mA.

--
Cheers
Dave.