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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.

Parents
  • 0
    Near misses are more common than fatalities, but that sounds like an unpleasantly  near one, the kind you remember for ever and learn from...

    In about 50 years of playing with electrons,  I have had two bad hand to hand shocks, one where the power was cut by an RCD, and without it my body would have been found in a loft far later, as no-let go was definitely active, and I can assure you that the tripping time for an 'instant ' RCD is quite long enough for a few 'life flashes before my eyes' thoughts. (I too had the right fuse, or so I thought, in my back trouser  pocket)

    The other was while I was at school and involved picking up a theatrical lighting lead with a 15A round pin  live plug, rather than a live socket. That, being pre-RCD, was saved by the prompt action of a friend sprinting for the mains switch, whereupon apparently I dropped on the stage like a rag doll, though most of what  I remember was not being able to breathe.

    So,  check and be safe, and if it seems odd, then double check and  even then, perhaps  have an assistant who knows where things isolate  and how to pull you off  safely. Of course in a perfect world no shocks would ever occur, but in a complex system where only zones can be switched off, and not the whole installation,  it is not always clear that the right thing has been isolated, and it is certainly possible to occasionally cut the wrong wire.

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  • 0
    Near misses are more common than fatalities, but that sounds like an unpleasantly  near one, the kind you remember for ever and learn from...

    In about 50 years of playing with electrons,  I have had two bad hand to hand shocks, one where the power was cut by an RCD, and without it my body would have been found in a loft far later, as no-let go was definitely active, and I can assure you that the tripping time for an 'instant ' RCD is quite long enough for a few 'life flashes before my eyes' thoughts. (I too had the right fuse, or so I thought, in my back trouser  pocket)

    The other was while I was at school and involved picking up a theatrical lighting lead with a 15A round pin  live plug, rather than a live socket. That, being pre-RCD, was saved by the prompt action of a friend sprinting for the mains switch, whereupon apparently I dropped on the stage like a rag doll, though most of what  I remember was not being able to breathe.

    So,  check and be safe, and if it seems odd, then double check and  even then, perhaps  have an assistant who knows where things isolate  and how to pull you off  safely. Of course in a perfect world no shocks would ever occur, but in a complex system where only zones can be switched off, and not the whole installation,  it is not always clear that the right thing has been isolated, and it is certainly possible to occasionally cut the wrong wire.

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