ProDave Problem, Lamp becomes live when CU switched Off!

[ProDave]

Okay, the next "problem"

I was sent do do a simple job. a cable in the loft had got trapped by the loft hatch (because the original installer didn't clip any cables) and it has cut through the outer sheath.

It was a 3 core & earth cable, probably the landing light strapper, or possibly a feed to the bathroom fan. At this point I didn't bother to identify which circuit it was.

I was in the loft with a lead lamp plugged into a socket on the landing.

As it was accessible my plan was to just repair the damage by terminating it into a junction box and clip the cables so it wouldn't happen again.

So I went downstairs to the one and only CU, turned off the main switch and went back upstairs.

As expected the lead lamp had gone out, so I planned to make the repair by the light of my head torch.

As I closed my cutters around the damaged cable to cut it, my lead lamp came back on.

How and why? and what was the problem?

What tests did I do to arrive at the conclusion?

P.S this was discussed once before. If you remember it, don't take part, leave if for those that have not heard of it before, and don't cheat by searching for it.

 

[roys]

Somebody turned the main switch back on, as their kettle would not work.

 

[danmoriarty]

Main switch flicked back on?

*Tut too slow

 

[ProDave]

No, I was alone in the house, I went back and checked and main switch was still firmly OFF

 

[roys]

Maybe the seal fairy escaped out of your toolbox and switched the main switch back on and then off again when you came down the stairs

Was the loft one of those lofts that go through to the nextdoor house with either no dividing wall or gaps in dividing wall?

 

[steptoe]

there was a ghost!

I remember this, so I'll keep my trap firmly shut, :|

 

[m4tty]

No, I was alone in the house, I went back and checked and main switch was still firmly OFF

Locked off I hope lol

---------- Post Auto-Merged at 14:16 ---------- Previous post was made at 14:15 ----------

And I remember this too so I'll keep out of it

---------- Post Auto-Merged at 14:17 ---------- Previous post was made at 14:16 ----------

Maybe the seal fairy escaped out of your toolbox and switched the main switch back on and then off again when you came down the stairs Was the loft one of those lofts that go through to the nextdoor house with either no dividing wall or gaps in dividing wall?

Sidewinder likes cutting next doors cables in adjoining lofts

 

[ProDave]

No it was a modern house, no access to next doors loft and not a supply coming from next door.

Isn't it about time we started doing some testing? What would you like me to test?

 

[sparkytim]

Have a look in the CU and see if any cables coming from supply side of main switch

 

[ProDave]

Have a look in the CU and see if any cables coming from supply side of main switch

Good man, that's along the right lines. Can you think how that might cause the symptoms I described?

 

[sparkytim]

Cutters completed the circuit via cpc to earth?

 

[ProDave]

Cutters completed the circuit via cpc to earth?

Care to try and draw a diagram how that might work?

From where to CPC?

Why didn't the RCD trip?

 

[Evans Electric]

16th edit. Split load board, a faulty main switch that only breaks the Neutral, cutters make a circuit between L and Earth ?

 

[Guy]

An individual insulation test to identify which circuit has the damaged sheath and then inspect its L,N and CPC points of termination?

 

[ProDave]

An individual insulation test to identify which circuit has the damaged sheath and then inspect its L,N and CPC points of termination?

The damaged cable tested fine, even before the repair. It was also correctly terminated at both ends. No alterations to that circuit were done to fix the problem.

 

[Guy]

Is Evans on the right line with faulty or by-passed main switch. We have a supply, somewhere. Next I would voltage test on the demand side of the OFF main switch.

 

[ProDave]

You are on the right lines.

The load side of the main switch shows 0V, i.e the main switch IS functioning correctly and both contacts open when it's turned off.

I've been careful there to answer the precise question you asked.

 

[batty]

Has somebody been naughty with the old meter?

 

[Guy]

I like that Batty, there must be a supply before the main switch so your idea would make sense. Going to give some thought to the non tripping RCD with this theory though.

 

[ProDave]

No, nobody has been tampering with the meter. The fault lies in the installation. A fault that's probably been there since the house was built. The fault is in or downstream of the CU.