I get all the good ones - circuits interconnected i expect.

[jedisparks]

Hey folks

Doing a PIR for someone the other day. This is for a guy who did the wiring for his own extension. his story was that the electrical company sent to him by the local building control came to look at the 1st fix but wouldn't return for final test and sign off. Building control asked him to get a PIR done on his extension and that they would accept that.

He told me that he was a very experienced and succesfull engineer so electrical work was easy for him.

Anyway I agree to do a PIR on the extension. I thought it was a pretty sloppy install to be honest. Things like; connections to downlight transformers just left as exposed connector blocks and no enclosure, no earth sleeving, no identification of conductors, no grommets. Nothing major but a bit rough i thought.

I then went to do an insulation test on the new light crcuit he had installed, which he had just put into the same circcuit breaker as the existing upstair light circuit (the new circuit feeds the upstair lights in the extension). So I just disconnect the new cable that he has installed to do an IR test on it (agreed limitations are just for testing the new work). Anyway my tester wont allow me to do the the test - why? - because it was 200volts L-E & N-E. WTF! i thought. The only way to de-energise the new cable was to switch off the adjacent breaker which operated the downstairs lights.

This is as far as I go, my job there wasnt to trace and recity faults, only to report. My first thoughts though are; well this must be interconnected with the landing & hall lights, neutrals interconnected, that sort of thing. But what is weird is that when this cable is in the breaker for the upstair lights with the downstair light breaker switched on, the cable and top of the upstair breaker isnt live. Only when its removed from the breaker is the cable reading 200 volts. Weird.

Im sure my explanation is a bit difficult to follow here but any ideas? anyone?

 

[Kamikaze]

It’s the dreaded borrowed neutral again.

 

[jedisparks]

Oh also; I carried out continuity tests between the neutrals of this cable and the rest of the neutral bar (main switch off) and got greater than 2000 ohms. I did the same with the live of the cable and the other lighting lives, in their corresponding circuit breakers and also got greater than 2000 ohms. Weird, really weird.

 

[Knobhead]

Uncle Fester?

 

[MarkieSparkie]

Oh also; I carried out continuity tests between the neutrals of this cable and the rest of the neutral bar (main switch off) and got greater than 2000 ohms. I did the same with the live of the cable and the other lighting lives, in their corresponding circuit breakers and also got greater than 2000 ohms. Weird, really weird.

If as I suspect, you have a shared ("borrowed") neutral. The readings you have obtained are to be expected. You are measuring one or more loads on the shared lighting final circuits.
Concentrate your attention on the two-way switching of the landing lights where the neutral for the upstairs lamp has more than likely been derived from downstairs hall lamp or vice versa.
Remember that Safe Isolation of the whole installation at the main switch is the ONLY safe way of working when dealing with suspected shared neutrals.
Carry out continuity tests on the disconnected neutrals at the two way switching lighting points, looking for the neutral conductor that connects upstairs to downstairs.
Isolate the offending conductor and re-connected the others.
Re-test the lighting circuits they should now be separated, one circuit with no missing connections and the other with one lighting point with a missing neutral. If more than one lighting point is affected, you may need to re-connect the isolated conductor at that end only, as it probably loops in the other lighting points before going downstairs.
Run a new neutral conductor to the affected lighting point from the nearest lighting point on that circuit or from the appropriate DB neutral bar if more convenient.
Re-test the lighting circuits, both should now be fully connected and separated.
If so, re-energise the installation and let there be light.