Another "impossible" fault thread

[ProDave]

Following on from other recent impossible faults, here's one I found this afternoon.

I will post just the initial symptoms of the fault and let you debate what it was. It took me less than 5 minutes of testing and inspecting to find the fault, then I had to draw out the circuit just to confirm my suspicions.

Here goes:

A new house, with a new install, completed in April 2007 (by an NICIEC electrician from Buckie). Hager Split load 16th edition CU, PME earth.

House is empty, just being prepared for sale, so nothing at all plugged in to any sockets in the house (except my lead lamp)

The initial fault I had to go and fix was a damaged wire in the loft. It was a 3 core & E, Landing 2 way switching, had got pinched in the loft hatch exposing bare copper wire. I was on my steps in the loft hatch, and I had plugged my lead lamp into the nearest 13A socket.

I decided the easiest repair was cut the damaged cable and re join with a junction box (it would remain accessible in the loft) then clip the cable (it wasn't clipped before) to prevent it getting trapped and damaged again.

So I go downstairs and turn off the CU Main Switch. Everything in the house is dead now, lights don't work, and my lead lamp went out. I'm now in the loft working just by my head torch.

As I close my cutters on the 3 core & E to cut it, my lead lamp lights up. Release the cutters, lead lamp goes off. Investigate a little, and just shorting the brown wire to the CPC brings my lead lamp back on.

So what's causing my lead lamp to light up in a dead house?

Without any testing results what do you think the fault was?

I'll be back later with the answer, the tests I did to find it, and a drawing of the fault path.

P.S my lead lamp was not faulty and did not contribute to the fault (but was the first thing to show there was a fault)

 

[sprocketflup]

Im carp at these, but that obviously means the cpc has 240volts on it, but the house is dead so its not from a short ?

---------- Post Auto-Merged at 16:46 ---------- Previous post was made at 16:44 ----------

So where wud the cpc be getting its voltage from? From the service head? Incorrect polarity at the incomer?

 

[ProDave]

Im carp at these, but that obviously means the cpc has 240volts on it, but the house is dead so its not from a short ?---------- Post Auto-Merged at 16:46 ---------- Previous post was made at 16:44 ----------

So where wud the cpc be getting its voltage from? From the service head?

Nope. CPC is at earth potential (linked to neutral at PME supply head). Try again.

 

[sprocketflup]

lol thats me spent im afraid! Unless there is something bypassing the CU main switch Im stumped

 

[m4tty]

Did it blow a hole in your cutters when cut ?

---------- Post Auto-Merged at 17:08 ---------- Previous post was made at 17:05 ----------

Was the consumer unit wired correctly ?

 

[Andy™]

all i can think of it isolator only isolated neutral, all 'appears' dead, until you cut through the cable, then fault path to earth (neutral)

EDIT: scrap that. otherwise big bang when you cut through the cable, unles you isolated the lighting MCB & main switch?

 

[ProDave]

Did it blow a hole in your cutters when cut ?---------- Post Auto-Merged at 17:08 ---------- Previous post was made at 17:05 ----------

Was the consumer unit wired correctly ?

No damage to cutters. Second point, you are getting warm. The CU had a correct polarity supply feeding it.

 

[Sidewinder]

ProDave,

Before you commenced work did you undergo full safe isolation testing to prove dead?

 

[ProDave]

ProDave,Before you commenced work did you undergo full safe isolation testing to prove dead?

Cough. No. The lights went off, and my lead lamp went out, so it was dead. Wasn't it? (wait for full explanation later)

 

[Andy™]

Cough. No. The lights went off, and my lead lamp went out, so it was dead. Wasn't it? (wait for full explanation later)

no. came accross many switch neutrals before.

personally, i never touch anything my tester hasnt. maybe slightly paranoid, but it keeps me alive. and out of habit, there is times where i have 'tested' a cable that isnt even connected to DB

 

[Sidewinder]

Dave,

I guess you can see why I asked a question rather than giving a guess...

I'll wait, not saying I know but perhaps my post and the answer are a clue?...

 

[steptoe]

by a 16th board I assume you mean only one RCD that feeds the sockets etc and lights are on main sw only?

 

[sprocketflup]

Hang on, sussed it.

Thinking back to previous posts by your good self ProDave, I reckon that yes you had isolated/made dead the house you were in, but the cable you were about to chop actually belonged to next door...... :^O

 

[ProDave]

Hang on, sussed it. Thinking back to previous posts by your good self ProDave, I reckon that yes you had isolated/made dead the house you were in, but the cable you were about to chop actually belonged to next door...... :^O

No, not even close. This is a newish detached house, so none of next doors wiring anywhere close. (that was my first thought until a reality check reminded me it was a detached house)

---------- Post Auto-Merged at 18:42 ---------- Previous post was made at 18:42 ----------

by a 16th board I assume you mean only one RCD that feeds the sockets etc and lights are on main sw only?

Correct. Only 2 socket circuits and the shower protected by RCD, the rest, lighting etc has no RCD protection.

 

[Chr!s]

If the isolate is double pole, and the cpc is at earth potential, then i would assume that something the split load kit has been incorrectly wired

 

[Revved Up Sparky]

Cpc's connected to neutral bar in CU and vice versa ?

 

[ProDave]

Cpc's connected to neutral bar in CU and vice versa ?

No. All CPC's and neutrals in the right place.

 

[kme]

Welllllllll

The only remaining question to be asked is: did the lead lamp come back on at full brightness?

I`m assuming yes; in which case I believe I know what the cause is......

 

[ProDave]

WelllllllllThe only remaining question to be asked is: did the lead lamp come back on at full brightness?

I`m assuming yes; in which case I believe I know what the cause is......

Yes, lead lamp came on at full brightness. If you know the answer PM me and I will confirm, just to let the speculation continue a bit longer before I give the full answer.

 

[m4tty]

C'mon Dave the suspense is killing. I cant work it out but defo think the split load wiring inside the cu is incorrect