In article , Stefek Zaba s-nilnews-
writes
Roger wrote:


Well I certainly managed to trip the RCD in my sisters house while
working on a disconnected ring main. I put it down to the fact that the
MCB only isolates the live and the neutrals are all linked thus a
neutral/earth short anywhere on the system will give an alternative
return path for any device drawing current.

Exactly so. Congratulations: you've managed to say in 5 lines what
usually takes me several screenfuls - and just took Andy W nearly as
long, but with more cases more thoroughly considered - on the frequent
occasions that this FAQ comes up!

Stefek

Just to chuck my 2 penny worth in Stefek has this right and Roger has
put the same thing the same way.

All you need to remember is the FUNDAMENTAL thing about a RCD is that it
compares the current IN to the current OUT if they are unbalanced by
more that the rated trip current nominally 30 or 100 ma then the device
will trip.

Remember the word !!CURRENT!! flow thats the important part.

It matters not whether the RCD is supplying a single table light or a
shower and cooker on the go together. As long as the CURRENT IN matches
the current OUT then all will be well. Offer the electricity an
alternative path and this balance is upset and off the trip will go.

That is all there is to it.

Let me try to put something else in perspective.

Imagine an RCD connected to the incoming supply. Nothing is connected to
the output of it at all.

Now take a wire thats connected to a good earth like a stake in the
ground outside in very damp soil and after its been raining. Touch this
to the live and the trip will immediately fire off.

Now do the same with the neutral output side of the trip and it won't
trip. So why does this happen?. Remember CURRENT flow. In the first
instance from the live to your earthstick current is flowing from the
incoming live line, through the RCD and then via your stick and then
through the earth back to the substation where one side of the supply
the neutral is connected to earth so you can see that current flows
through the live side of the RCD back through the earth connection to
the supplying transformer. It is NOT going back through the RCD so a
current imbalance has taken place.

The important thing is that there was an !!imbalance!! because more was
going OUT of the RCD than what was coming BACK through it!.

However when you touched the earth stick to the neutral NO CURRENT was
flowing through the live side, so no current was or could have been
flowing back!.

Now suppose you have a one bar electric fire connected to the output
terminals of the RCD. Normally what is going out is going back through
the RCD, so all is well.

Now connect that earth stick to the neutral again and it will trip.
Reason is that the CURRENT has an alternative way back to the earthed
side at the substation, some current is flowing through the RCD back to
the substation , but some is now taking the alternative route home via
the earth stick.

So an imbalance is caused, their being current flowing OUT through the
RCD but it has an alternative route back and if its going back and its
NOT through the RCD so its not the same as what's coming IN, then off
will go the trip.

Simple as that.

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