Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins". And why would
interference harmonics cause a leak?


Its usually capacitors connected between Neutral and earth and Live and
earth. Not a lot of current is intended to flow but some does. These are
mainly used to stop or alleviate interference (radio frequency) problems
often referred to as EMC compatibility. That's it in brief..

Doesn't in all circumstances "what goes in must come out" apply? In
other words why should the live current ever (at any instant) differ
from the neutral current (except in direction)?

Thats the idea .. what goes into the "house" on one conductor must come
out on the other, any discrepancy then thats "leakage" sometimes caused
by capacitors in equipment, sometimes caused by insulation breakdown and
sometimes caused by YOU when you might grab hold of a live conductor and
the leakage makes its way to Earth by you .. not a good thing then the
RCD trips out and disconnects you from Earth..


Or is it that there is never a real leak, but the breaker is fooled into
thinking there is, in which case what is the mechanism for fooling it?


No theres no fooling at all as such, what comes in must be the same as
what goes out more then 30 milliamps difference - then trip out time.

Sometimes an Earth Neutral short can be rather difficult to comprehend
as normally with no current flowing then nothing will happen as Earth
and Neutral are bonded together at the incoming point on most supply
instances. However when some current starts to flow then as the current
increases what is going in on the live isn't all coming out on the
Neutral thru the RCD as the current thru that increases due to load then
there will come a point when it gets past the trip value then out will
go the trip!..


If no current is being consumed in the connected circuits then the
current imbalance isn't enough to fire off the trip. Switching anything
on that will cause sufficient current to flow will .. after the tripping
current point is reached then out it goes.


Put that in a simplistic way with nothing switched on then nothing will
be flowing thru the live to come out on the neutral so no imbalance.

However switch something on then the current flowing Into the RCD on the
live it should see an equal current flowing OUT on the Neutral which it
won't as its flowing away through the Earth system, not necessarily to
earth as such, just the earth part of the incoming supply connected to
the neutral now flowing around and past the RCD on the wrong conductor

When there is sufficient current flowing thru the live conductor and not
out of the neutral side then once that reaches the tripping current then
it will trip out...
--
Tony Sayer