So thats why sometimes when you have an Earth to Neutral fault, it
doesn't show up until some "current" is flowing through the RCD through
the alternative path back to the substation, as until their is some
current flowing "through" the RCD, then it won't trip. Course that only
needs to be more then the rated tripping current but as long as thats
exceeded then off it will go, but it CAN'T trip unless there is some
current actually flowing through it in order to exceed this rated trip
imbalance level...

Tony Sayer


Correct !!! But can I ask what makes the current flow through these
conductors back to where ever?

Why didn't the RCD trip when the neutral was touched with the earth stick
whilst no load was connected across it?

Then why did it trip when the earth stick was touched to the phase terminal,
even though there was still no load connected across it?

What makes this all happen?

Is there something somewhere that creates these flowing currents?

If so, what is it?


Its because the supply transformer has one side connected to earth which
forms the neutral line..

What are the actual mechanics behind the generation and supply of this
electrickery stuff?

I think I'm meeting my full potential on this one guys. :-) LOL !!!


What you need to consider is that the mains is tied to EARTH when it
comes out of the substation transformer. There is a drawing of this on
the TLC website.

Consider the circuit as follows. Say we have a mains transformer up a
power pole say 200 metres away from the subscribers premises. Now trace
the circuit from lets say the live side of that supplier transformer.
It then goes along the live line to the supply fuse through the meter
then through the RCD then through the fuse board isolator switches etc
then if the "earth stick" was connected to the live output side of the
RCD the circuit would continue back through earth to the earth plate at
the substation so there would be a "load" placed across the supplying
transformer, and hence current would be flowing through the live
conductor of the RCD but not back through the neutral side of the RCD so
the RCD will now be unbalanced and thus trip.

Now consider the same circuit from the earthed side of the supply
transformer trace it along the subs premises through the isolator switch
and then out through the neutral side of the RCD. Now consider the same
earth stick connected this time to the Neutral conductor this will
provide a path through earth back to the supplying transformer.

No current will flow and you should be able to see that no current will
flow 'cos we've "connected" one side of the supply transformer back to
itself so no current is flowing save a "very" small amount of induced
leakage!..

The problem arises with the neutral short to earth when the current that
comes in on the live line now has another path back to the earth plate
at the supplying transformer as well as the one, and only one, it should
have through the neutral side of the RCD.

So suppose we have 10 amps flowing through the live side of the RCD we
don't have the full 10 amps flowing back through the neutral as some of
its being diverted back through the neutral-to-earth short of the
consumer side, or protected side of the RCD, so as long as that current
is greater than the 30 milliamps difference that the RCD is set to, then
off goes the RCD.

--
Tony Sayer