Paging Sparkies

Lord Frag emerged from their own little world to say

had a similar problem some years ago

the fault was traced to a nail that had been put through a 25mm cable
when refitting a floorboard. The nail had gone through the outer
insulation but not shorted any wires.

It took about 6 months of people walking across the floor before the
nail broke through the insulation on the live wire. We were getting
intermittent earth trips as the nail periodically touched the bare earth
wire. [1]

[1] years before RCCBs became standard

 

Slight problem...

Fuse box is a traditional fused variety.

Sitting downstairs, watching TV.

Living room lights on, no other downstairs lights on.

Lights go off of their own accord.

Fuse has blown. And I mean *blown*. Like evaporated the whole length of
fuse wire.

Replace fuse. Plug in. *BLOWS* again.

Turn off all lights downstairs. Replace fuse. Blows again.

Today I replaced both dimmers downstairs, checked continuity between
live feed coming in and neutral in one light switch socket (i.e.
testing between live feed for whole of lighting circuit and neutral).
No short, approx 3~4M ohms with all lights off, each bulb is about 50
ohms resistance, all switches working fine.

So this indicates to me that the feed from the fuse box to the
downstairs lighting circuit has come adrift somewhere and is shorting
to neutral/earth? (the lighting circuit is testing out OK, so that
can't be blowing the fuses)

I'm off to pull the upstairs landing up tomorrow to try to find the
problem, or disconnect the original feed cable and bung a new one in if
I can't find the short.

Anyone got any vague ideas what else it could be before I start serious
probing? The continuity was measured with a brand new multimeter bought
today, LCD, auto ranging, one thing that occured to me is that there
might be a short to the mains AC voltage but not to the meters testing
voltage? Or some insulation only breaking down under 240V AC load? (but
I wouldn't expect that to consistently blow the fuse in such a big way
instantly?)

Voltage measured across the fuse box terminals is 240V AC.

<puzzled>

 

Lord Frag pretended :

Wrong instrument for checking possible breakdown of insulation. You
need an instrument commonly known as a 'Megger', which tests the
insulation resistance value at 500v.

Did you not check the resistance between live and earth?

Most problems occur at joints in the cables... Ceiling roses, joint
boxes and switches. Another likely possibility is that a cable has been
nailed in the process of fixing floorboards.

 

the cont known as Lord Frag says...

I had this a few years back. Traced it back to a maggot in the toilet
light switch.

Check all light switches in the affected area.

 

I thought it was a kneeslider thread.

--
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I thought it was a kneeslider thread.

ITYM kneetrembler