RCD tripping question

[GMM]

Just for information really. I re-wired the living room the other day as t= he floor was out so it was easy. A simple ring just for the one room, wire= d in red and black (although the CU has an RCD for the rings). Knocked the= mcb off, checked the L vs N - 0 volts, L vs earth - 0 volts and N vs earth= - 0 volts.

Half an hour into the job, SWMBO came in and said the power was off upstair= s (which I'd promised wouldn't happen). Cause - RCD tripped. Re-set, stil= l no voltage in the cables I was working on. Started wondering where the p= romised cup of tea was - answer, no power. Cause - RCD tripped. This repe= ated itself until I had finished the job. Since then, no trips, no problem= s.

So, the question: Was the RCD tripping because of a small current I was se= nding from N to earth when I was playing with the cables, or some other rea= son?

[Ron Lowe]

floor was out so it was easy. A simple ring just for the one room, wired in red and black (although the CU has an RCD for the rings). Knocked the mcb off, checked the L vs N - 0 volts, L vs earth - 0 volts and N vs earth - 0 volts.

(which I'd promised wouldn't happen). Cause - RCD tripped. Re-set, still no voltage in the cables I was working on. Started wondering where the promised cup of tea was - answer, no power. Cause - RCD tripped. This repeated itself until I had finished the job. Since then, no trips, no problems.

sending from N to earth when I was playing with the cables, or some other reason?

The short answer is: Yes.

'Knocking the MCB off' is not good enough.

THe circuit you were working on still had a N and E connection to the CU, and any E-N faults you created would trip the RCD.

To properly isolate the circuit you were working on, you needed to lift the corresponding N out of the neutral block too.

[Andy Burns]

Such as snipping through the old cables with sidecutters ...

[John Rumm]

Yes, spot on.

Basically if the power is on and other circuits are live, then a neutral to earth short can (although not always will) cause a trip. It only takes a tiny rise in voltage on the neutral to allow enough mA to flow to earth and cause a trip.

To prevent this, disconnect the neutral of the circuit you are working on at the CU first.

[GMM]

Well it's gratifying that my thoughts were correct! I suppose there will always be a little juice in the neutral as things run, though not enough to cause any problems (apart from this) unless was something shorted without popping its mcb.

[John Rumm]

You can in theory get a reasonable current flow from neutral to earth in the right set of circumstances (assuming there is no RCD there to prevent it!) But not normally enough to be a problem.