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Solving an Electrical Fault from 10,000 Miles Away (Part 2)

Posted November 10, 2009 12:01 AM by PWSlack

"Why does the oven keep tripping the earth trip? It's so annoying. Can you sort it out?"

Editor's Note: Click here if you missed Part 1 of this two-part series.

Chapter 3: More Investigation

One thing that was curious was that the heritage cable presenting the fault at the distribution board was 2.5mm2 whereas the remainder of the lighting circuit was heritage 1.0mm2. So somewhere on this particular cable circuit was a joint between these two sizes that hadn't presented itself when the recent extension electrics went in. The other curious thing is that the earth fault presented itself at four places:

  • At the distribution board in the cupboard
  • At the kitchen lighting fittings
  • At the lounge lighting fittings
  • At a junction box on the lighting circuit on the landing upstairs (the box is tucked away behind a beam and ordinarily invisible).

So the faulty cable had four ends, which meant that there were two hidden T-splices in it!

The inherited electrical installation had not been carried out fully in pursuit of the UK's Wiring Regulations applicable at that time, whereas any professional Electrician is required to leave his/her work compliant with the edition of the Regulations, embraced within British Standard 7671, current at the time of the work. And whereas the work in the extension had been done to BS7671, the installation in the rest of the bothy had presented a lot of inherited faults, all of which had been corrected over time. This one, however, was the last one and a complete pain. The splices had to be found, for Neutral and Earth were still touching at one of them.

BS7671 requires that all cable ends are accessible for maintenance and testing. The two splices and the earth fault were clearly not.

A quick discussion with the Client: "Take the cupboard ceiling out and see if they're in there. It will be easy to hide the repairs when the ceiling goes back."

Plasterboard and glassfibre insulation make such a mess when they come down. Still, keeping the pieces makes sense, and drywall screws and short pieces of batten make a quick and easy solution to putting them back up again, while the saw gaps can be filled relatively easily before sanding down and making good prior to touching-up the paint.

No cables.

"Let me feel around in the ceiling". The Client's hands were somewhat smaller, and their squeezing into tight places discovered a "lego brick" cable connector hidden in the next bay along from the cupboard ceiling. Lo and behold: 2.5mm2 cable on one side and 1.0mm2 on the other. A piece of ceiling outside the cupboard had to come down to get to it to disconnect it, though.

Chapter 4: The Remedy

A 25m reel of replacement lighting circuit cable was purchased. The next step was to pull it in between the light fitting in the hall, which was fed from the other OK heritage cable on the lighting circuit, and the connection point just inside the garage loft space. This is where the extension works lighting cable had arrived from the other end of the house, and the remainder of the heritage lighting circuit cold be fed the other way round. A further two connections onwards to the kitchen and the lounge lights, and then the four-ended cable with two splices could be disconnected and abandoned, thereby bringing the whole bothy up to BS7671 current edition.

Then there was the access problem to consider.

Pulling the new cable in was easier said than done. The space above the hall is covered by a triangular-shaped pitched roof of gentle slope, which had been added above it during the extension works. There wasn't quite enough height to crawl. Further, 200mm of additional glassfibre loft insulation had been added to this space, some of which had to be moved aside to gain access.

"Cup of coffee and a biscuit?" Not now, for goodness' sake!

The space was a pot-holer's paradise, and the tight gap between a rafter and a brick chimney, some 8m away from fresh air, had to be squeezed through in some kind of crazy, itchy, sideways-horizontal limbo dance no fewer than 6 times before the new cable could be connected at both ends. It took an evening's effort just to get it in, and, as any reader that has handled the stuff will know, glassfibre loft insulation is very unforgiving to the human body.

The connections themselves were straightforward, as were removing the temporary supplies set up in Chapter 2. The bothy is now up to standard electrically, there are no more spurious earth trips, and the heritage 4-ended cable with the earth fault is now abandoned for good. All that remains is to tidy-up the ceiling in the hall and the cupboard and make good the decoration, hopefully before any relatives come over for tea and cakes.

Conclusion

The moral of the story is simple: if one is doing electrical work, make sure it is up to current local electrical codes at the time of installation, for it can be a very awkward and annoying job to sort out the problems after the final decoration is done.

The author (PWSlack) would like to thank CR4's Masu for offering advice and encouragement from another continent and for the incredibly kind offer of coming over to help find the fault and fix it, an offer that has not needed to have been taken up.

Postscript

The fault turned out to be a woodscrew, which had been used to screw a bracket to a wall to support a light fitting. There was a cable in the wall that had been buried in filler and remained undetected. The screw happened to have punctured the cable insulation and just kissed the Neutral conductor as it went through. When the light fitting had been added, the bracket became connected to the Earth conductor. Hence the Neutral-to-Earth fault. And as the Client stated, "So annoying."

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Power-User

Join Date: Jan 2007
Location: UK S.Northants
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#1

Re: Solving an Electrical Fault from 10,000 Miles Away (Part 2)

11/10/2009 6:47 AM

I had this recently when a previously correctly working lighting circuit started tripping the breaker after some plaster board had been fitted. The last screw in had shorted between conductors in a buried cable, in my case live and earth. How did you find it was the woodscrew? If you replaced the cable and disconnected the original you wouldn't have known.

So in your case the mechanism for tripping the breaker was due to some of the neutral current taking a parallel path through the short to earth, causing an imbalance between live and neutral >100 mA. Is this because the neutral is shared so with this increased current due to the oven there was a neutral voltage rise above earth potential causing some of the current to flow back up to the short and thence to earth?

The only other explanation I could come up with was leakage current from the heating element of the oven, through the powder insulation to earth. Re-reading part 1 I see how you ruled this out. Very interesting topic.

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Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#2
In reply to #1

Re: Solving an Electrical Fault from 10,000 Miles Away (Part 2)

11/10/2009 7:40 AM

<...find it was the woodscrew..>

The fault was present when the light fitting was in place, and disappeared when the light fitting, being bonded to Earth, was unscrewed from the bracket. The fault seemed at first to be something to do with the the light fitting being vertical and the beginnings of its attachment to the bracket, which was via a threaded attachment screw fitting that joined them both; once the attachment screw was partly-in, the fault appeared and withdrawing the attachment screw made the fault disappear.

The bracket, being inadvertently connected to Neutral while in contact with the mischevious woodscrew, was then tested for continuity between it and the Neutral conductor at the light fitting's terminals, and showed the fault; undoing the woodcrew a couple of turns made the fault disappear. Excavating the filler carefully with an old wood chisel revealed the buried cable and where the woodscrew had touched the Neutral.

<........some of the neutral current taking a parallel path through the short to earth, causing an imbalance between live and neutral >100 mA....this increased current due to the oven there was a neutral voltage rise above earth potential causing some of the [neutral] current to flow back up to the short and thence to earth?...>

Spot-on. Masu got very close just by talking about it via CR4's Personal Message system.

'Tis a wonderful thing, CR4.

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Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

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#3

Re: Solving an Electrical Fault from 10,000 Miles Away (Part 2)

11/10/2009 11:36 AM

A nice tale, well told.
My house has electrical stuff which needs sorting, nothing real bad tho' .
A classic I found when moving some light fittings and installing a shower room on the landing. A ceiling rose mounting screw was used as a termination to connect two cables effectively connecting live mains to one of the joists, good thing wood isn't very conductive!

Del

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