Today`s call out to an "impossible" scenario.

[kme]

I`m going to give you the same spiel I had from the customer; and tell me what occurs to you.....Snakehips is disallowed, `cos he already knows about it.

Domestic end-terrace house. "Sudden reverse polarity on a lighting circuit" was the description over the `phone. So off I went to have a look.

Customer had left the house for approx 1 hr; and returned to find his combi boiler wouldn`t start up; so he calls out "Swalec", who provide him with breakdown cover.

When the engineer arrives, he determines boiler has reverse polarity. So does fused spur supplying, and double socket on ring.

So does incoming supply.

However, when the MCB for the upstairs lighting circuit is switched off; the reverse polarity "rights itself"!!!!

So I begun testing.

The results are EXACTLY as described above. The upstairs lighting is a standard 1.0mm FTE B6 60898; and causes the incoming supply to read as reverse when switched on. It didn`t do this 2 days ago.

Anybody want to guess?

( I`ll give you the details of the earthing, CU layout etc...later on. Might help)

KME

 

[m4tty]

Glad you got that one mate and not me. Wouldnt have a clue lol. Interested to see what happened

 

[Sidewinder]

Well I could tell you but I would have to kill you!

 

[gram1980]

Live to earth fault on lighting circuit, tt supply with high resistance or non existant main earth and a lack of/or faulty rcd ? Regards gram

Sent from my iPhone using Tapatalk

 

[sprocketflup]

Gotta be quantum innit

---------- Post Auto-Merged at 17:59 ---------- Previous post was made at 17:58 ----------

Im with Matty on this one, not got a Scooby

 

[S60TEM]

mmmm, Neutral to earth fault on a TNC-S with no RCD protection present, non split load board?

funny one that, i'd be baffled for a while to be honest.

 

[steptoe]

borrowed neutral from next door on the upstairs lights,

with what info your sharing so far

:C

 

[SLIPSHOD & SLAPDASH]

Stolen supply on different phase from next door

 

[Phoenix]

Of course, the lighting MCB reversing the polarity is as you say impossible... but its possible for it to 'appear' as if thats whats happening

TT supply, knackered (or no) RCD/VOELCB, and a phase to earth fault on the lighting... what do I win?

 

[kme]

No mate - Gram got there first

There is a 13 ohm phase/earth fault; TT; 16th type split board with no RCD to front end, no main bonding, 199.6 ohm Zs(DB).

So....

Any meter has to be measuring with reference to something; on a 3 lead test, earth is taken as reference, as it should be constant.

However, in this case, the fault will cause the earth conductors of the installation to rise nearer to phase voltage than neutral; the meter therefore determines that they are suddenly "reversed" with respect to earth. Ergo the fault.

The lighting (spider type) and associated junction, are somewhere under the extensive flooring in the loft, which is covered in lots of loft-dwelling things

Its going to be a bit of a buggr, methinks.

Insult to injury: borrowed neutral to landing light; services enter building at opposite end to CU; earthing conductor disappears into concrete pavement outside front door........

But......its work

Well done to those who guessed correctly. Scoobs have been awarded. Phoenix; you can`t have a prize - Gram got it right before you

KME

 

[Sidewinder]

I have to add my congrats to those who got it right.

I can never diagnose faults over the phone or on the forum it just ain't in me!

So well done to gram, I'm well impressed mate to be honest, also well done to Phoenix too, just pipped at the post.

BTW, KME & I are not one in the same it's just we are in the same area and do some work with each other.

Where are you 2 based, there may be some work kicking around!

 

[Andy™]

wish i was here a little earlier to answer the question....

there was a similar thread about this not too long ago

 

[Sidewinder]

Andy,

I'm sure KME will be back in a bit, but this was the scanario with several testers MFT included as far as I know.

I may do some controlled tests in a bit, (not tonight, longer timescale than that!).

 

[Andy™]

Andy,I'm sure KME will be back in a bit, but this was the scanario with several testers MFT included as far as I know.

I may do some controlled tests in a bit, (not tonight, longer timescale than that!).

well, that saves me offering

 

[Sidewinder]

Nah Andy,

The more makes etc. of MFT tested the better!

What about a "knowledge base" of this?

 

[Andy™]

Nah Andy,The more makes etc. of MFT tested the better!

What about a "knowledge base" of this?

if i get time in the next few days ill give it a go

now, how should i accidently get wrong polarity on a TP board.....

then agin supplt is TNS....

looks like ill be sticking a rod somewhere for this

 

[Sidewinder]

I was going to write a missive on how to defeat earthing, bonding and RCD's & thought better of it!

We can discuss this safely I "think" you know where, or PM me!

I actually got to about 6 lines of text before I realised what I was typing on an open internet forum!!!

 

[Andy™]

i hope you didnt delete it. make a new thread? ive already got my experiment planned, following KME's info

 

[gram1980]

Wots my prize then had this fault a couple of times, 1st was due to chippie's putting a ceiling up & screwing thru cable, next was a rotten class 1 switch & backbox. I'm in swansea if you've any work going

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[Sidewinder]

gram,

You ARE F'In jokin!