"David WE Roberts" wrote in message
...
Just set off to do my good deed of the day.
Replacing the PIR outside light by the front door which had given up the
ghost.
New light half price from Homebase was the incentive :-)

Skip to the bottom if you wish to avoid the detailed route to disaster.

All going swimmingly at first - took the old fitting off the wall and
noted that the screw holes looked to be the same spacing as the new back
plate, so no drilling needed.

Noted that there were two cables coming in, with the red wires joined.
I knew where one cable went - switch by the door.
I assumed

That's where you went wrong........




normal lighting circuit - one cable is power in, other cable is to
and from switch.
In this case the two reds commoned, so the two blacks are power when the
switch is closed.
Checked with my trusty analog multimeter and this seemd to be the case.
Confirmed on the downstairs lighting circuit by turning off circuit
breaker at the fuse box and finding no current afterwards.

Unwired and rewired and then tested, but the circuit breaker for the
downstairs lighting wouldn't reset.
Bugger - what did I do wrong?
Took the fitting off the wires - no change.
Took the back plate off the wall - no change.
Uncoupled the two red wires and the two earth wires and pushed the two
sets of wires apart.
This seemed to fix it.
??????
Double checked - one set of wires are power, others are to the switch.

O.K. - start again but slowly.
Wound the two earth wires together and slid the sleeving over.
Checked breaker - wouldn't reset.
Took off sleeving and seperated the two cables and spread the three wires
in each cable at 120 degrees to each other.
Breaker wouldn't reset.
Checked for obvious wire damage on live cable - no obvious sign.
Joggled cables.
Breaker wouldn't reset.

[Noted that whilst mucking about with cables and lighting breaker out, I
did get a belt off something.]

Back to the trusty multi-meter.

Checked all the wires for 240V a/c - nothing.
Checked switch cable with the 1ohm setting - switch open nothing, switched
closed open circuit between red and black.
O.K. - seems right.

Check between wires in power cable.
There seems to be open circuit between live and earth, neutral and earth,
and live and neutral.

My conclusion is that somehow in moving the wires around I have induced a
short between live, neutral and earth somewhere in the power wire which
disappears into the cavity wall to gawd knows where.
Have I missed anything which might be good news?

I assume the best result I can get is to lift the bedroom floor above the
cable exit in the wall and find a cable coming out of the inner wall and
going across to the light fitting under the centre of the floor.
Of course, it could go to the outside light or the hall light.
Or the lighting power may not be wired directly into the light fittings.

Whichever way it goes, I forsee a world of hurt.
To get at the light fittings from below, I have to deal with thoughfully
applied Artex which probably means breaking the light fittings to get at
the internals because the cover will not unscrew.
To get at the light fittings from above involves lifting carpets and
floorboards, and in the case of the front bedroom draining and
disassembling a super king size water bed.

So please tell me there is something simple and obvious I have overlooked
which is easy to fix.

Cheers

Dave R
--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

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