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The futility of believing the description of a fault.

alanblaby
I was kicking myself today.

I had a call to attend an office light in an ex-offender hostel building. Been there before, no problems, other than attentive 'inmates'.

It was a fluorescent light that was not working. The description was "the circuit breaker (rcbo) trips when this light is turned on".

OK. Try it, reset the rcbo, turn on the light, and after 5 seconds, the breaker would trip. What was unusual, was that the RCBO was marked as 'Heating'.

I completely missed this first clue.

Take the fitting apart, tested between L+E, sure enough it was off. 2nd fail there.

It was an old fitting, so I put up a new LED strip light. In doing so, I noticed a small spark when touching 2 of the 3 neutral cables. Strange, it must be a shared neutral, test it, and yes, one of the neutrals is live. OK, back to the board, and turn off the various light circuits, to find which one was making the neutral live. 

Back to basics now, started testing, and found there is no continuity from the board to the fitting. Switch off, and one wire was hanging out of the switch. 

Refix that, put it all back together, and find I'd been working on a live circuit.

The RCBO that was tripping was an old heating system that wasnt used, no idea why it tripped, that can be traced another day.


I was so peed off that I'd made such silly stupid mistakes. If an apprentice did that I would go mad, but, I think I may be getting sloppy, and needed this kick up the ***.

 Mitigating, was the staff saying that CB trips when the light is on (it did, but it wasnt related!), having to work in a cramped office with some slightly dodgy people around, and just assuming that a light is not working, as, I get maybe 5 a month of exactly the same thing where a light has failed.

Be careful, dont assume, and do the correct proof of dead tests.

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  • 0
    Yes I got caught by similar, which nearly ended it for me. Kitchen light fixture, 1970s fluorescent batten. Simple as can be. Junction boxed install, so just a single T&E to the fitting. Turned off the switch and removed the fuse labelled 'downstairs lighting' and pocketed same.  Tested L-N, dead. Tested with volt stick ... no response. Rubbed volt stick on shirt, red. Test again, dead. Test volt stick (I thought I was being rigorous.)


    Remove cable from fixture, remove fixture, place on floor. Still all fine.


    This was a 'stripout for a kitchen remodel' so used a 5A junction box (the traditional round type) to make the 3 connections safe.. Attached the Line to one terminal, no issues. Neutral to 2nd terminal no issues. Gripped junction box, and went to bend the CPC into place to terminal 3. My hands gripped both the JB and CPC firmly, and the world went very slow and very vibratey I could count the individual cycles of the AC, I guess I got maybe 20 cycles before my legs gave out and I fell off the fibreglass work platform.


    Things I learned. Never trust the switch (it was in the neutral). Never trust the labelling at the board (it was incorrect). And volt sticks require YOU to be referenced to a different voltage than the business end. (the shirt rubbing test works because you're charging the plastic tip with a fast rising voltage... the tester sees it as AC for a brief millisecond).  Like a safer and more sensitive neon screwdriver. Check from ALL wires to ALL OTHER wires with a proper tester. Never work with both hands on stuff you haven't PROVED dead and locked off. Don't get complacent.


    Edit: and wylex BS3036 carriers in your back pocket HURT when you land on them after a shock. But that was the least of my concerns
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  • 0
    Yes I got caught by similar, which nearly ended it for me. Kitchen light fixture, 1970s fluorescent batten. Simple as can be. Junction boxed install, so just a single T&E to the fitting. Turned off the switch and removed the fuse labelled 'downstairs lighting' and pocketed same.  Tested L-N, dead. Tested with volt stick ... no response. Rubbed volt stick on shirt, red. Test again, dead. Test volt stick (I thought I was being rigorous.)


    Remove cable from fixture, remove fixture, place on floor. Still all fine.


    This was a 'stripout for a kitchen remodel' so used a 5A junction box (the traditional round type) to make the 3 connections safe.. Attached the Line to one terminal, no issues. Neutral to 2nd terminal no issues. Gripped junction box, and went to bend the CPC into place to terminal 3. My hands gripped both the JB and CPC firmly, and the world went very slow and very vibratey I could count the individual cycles of the AC, I guess I got maybe 20 cycles before my legs gave out and I fell off the fibreglass work platform.


    Things I learned. Never trust the switch (it was in the neutral). Never trust the labelling at the board (it was incorrect). And volt sticks require YOU to be referenced to a different voltage than the business end. (the shirt rubbing test works because you're charging the plastic tip with a fast rising voltage... the tester sees it as AC for a brief millisecond).  Like a safer and more sensitive neon screwdriver. Check from ALL wires to ALL OTHER wires with a proper tester. Never work with both hands on stuff you haven't PROVED dead and locked off. Don't get complacent.


    Edit: and wylex BS3036 carriers in your back pocket HURT when you land on them after a shock. But that was the least of my concerns
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