In article .com,
writes
Hmm..... Does that mean that I could have a good RCD and just a bad
connection somewhere? The devices I was connecting, like TV, DVD etc.
don't have any earth wiring, so surely they wouldn't cause this short?




As I said it seems very likely that you have a short or low ohms between
the EARTH conductor and the NEUTRAL, any devices like DVD players or
TV's or bedside lights don't even come into this, with respect to earth
wiring.

What is happening is the RCD monitors the current into your house on the
LIVE line, and compares it with what goes OUT on the neutral line and
provided these are the SAME the AMOUNT does not matter (as long as its
within the current capacity of the unit) all will be well.

i.e. as long as 100 milliamps are coming in and 100 going out all is
well, and if say 50 amps are coming in and 50 are going out all is well
to.

But as soon as a fault causes 100 ma in but 70 ma out then this is
tripping time!, as is 50 amps in and 49.970 amps out trip time as well!.

Now suppose you or yours grab hold of a live wire somewhere on the
consumer side of the install, then the current in on the live is
diverted away, to earth, via you or yours!.

There now exists a current unbalance in that less current is now going
out via the neutral conductor in the RCD and provided that this current
is greater than its rated tripping current, usually around 15 to 20 ma
for a 30 ma device, then the trip will disconnect and remove the current
so you don't get a shock!!.

Now conversely if the earth and neutral are shorted !together! somewhere
the same thing in effect happens. Instead of current flowing back
through the neutral conductor of the RCD, it can go directly to earth
and thus is "shunted" away from the RCD neutral current going OUT.

Of course if nothing is switched on then no current will be coming in
the RCD and nothing will be flowing out either so no current UN_BALANCE
can occur.

Now when something is switched on the current that is coming in is
supposed to match the current going out. But because some of this is
going down the earth line and some through the neutral line via the RCD
then with a light load there isn't, as yet, enough current to cause the
trip level to be reached. As the current increases then there will come
a point when this reaches the trip level, and then the trip fires!.

Try to regard this as the live and neutral conductors in your instal
must NOT be connected to earth anywhere on the consumer side of the RCD
they can of course connect together on the supply side as with PME
systems, and it doesn't even matter whatever earthing system you use but
on the CONSUMER side of the RCD, no connections from live or neutral to
Earth FULL STOP!!!.....
--
Tony Sayer