Just for information really. I re-wired the living room the other day as the 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.

Half an hour into the job, SWMBO came in and said the power was off upstairs (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.

So, the question: Was the RCD tripping because of a small current I was sending from N to earth when I was playing with the cables, or some other reason?

On 22/07/2012 22:15, GMM wrote:
Just for information really. I re-wired the living room the other day as the 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.

Half an hour into the job, SWMBO came in and said the power was off upstairs (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.

So, the question: Was the RCD tripping because of a small current I was 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.

Ron Lowe wrote:

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

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

On 22/07/2012 22:15, GMM wrote:
Just for information really. I re-wired the living room the other
day as the 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.

Half an hour into the job, SWMBO came in and said the power was off
upstairs (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.

So, the question: Was the RCD tripping because of a small current I
was sending from N to earth when I was playing with the cables, or
some other reason?

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.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Monday, July 23, 2012 1:48:26 AM UTC+1, John Rumm wrote:
On 22/07/2012 22:15, GMM wrote:
> Just for information really. I re-wired the living room the other
> day as the 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.
>
> Half an hour into the job, SWMBO came in and said the power was off
> upstairs (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.
>
> So, the question: Was the RCD tripping because of a small current I
> was sending from N to earth when I was playing with the cables, or
> some other reason?

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.



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.
--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 23/07/2012 14:19, GMM wrote:
On Monday, July 23, 2012 1:48:26 AM UTC+1, John Rumm wrote:
On 22/07/2012 22:15, GMM wrote: > Just for information really.
I re-wired the living room the other > day as the 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. > > Half an hour into the
job, SWMBO came in and said the power was off > upstairs (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. >
> So, the question: Was the RCD tripping because of a small
current I > was sending from N to earth when I was playing with
the cables, or > some other reason?

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.



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.

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.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/