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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.

  • 0
    While I have been misled many times over a problem I have only had one memorable near miss that I know about.


    I was disconnecting the lights on one gang of a two gang switch, only one lighting circuit and the fuse is in my pocket, noticed a separate supply to each switch and thought no more about it. Tested dead on the supply to the switch  I am disconnecting. When finished the customer asked if I could replace the two gang switch with a  single gang, she did not have much money so I was trying to do the job as cheaply as possible. Being a bit anal about testing dead I tested the supply to the second gang, it was live. I still had the fuse in my pocket.


    The second  switch for the main room light had been wired from the RFC! 


    Now apart from testing dead religiously I also wave a volt stick around as a further backup, I was going to say backstop but I may get reported. 


    Further to the post about a  death  from a non bonded installation, for a long time I have not touched any metal work in an installation I am unfamiliar with unless I have tested it. 


    I have found that as I have got older I have become far far more cautious and risk adverse, I probably needed to as at 18 I found myself sitting on a 4" RSJ over a 80 ft drop tying  a sling onto the RSJ and then pulling up a chain block and attaching it to the sling, no safety equipment, not even toe protectors! I thought nothing of it at the time.

  • 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
  • 0

    MHRestorations:

    I guess I got maybe 20 cycles before my legs gave out and I fell off the fibreglass work platform.




    Which demonstrates that the cannot let go rule may not always apply. Thank God!

  • 0
    Indeed, that was the most terrifying 400ms of my life,  My colleague said it seemed instant to him. Apparently i let out a wailing noise and 'flew'.  But I had time to register many things during that time. The fact the switch was still off. The fact the laser level was active (interestingly i think my eyes were crossing the beam, because the line was vibrating?), and 'so this is how it ends'. [yes i actually thought those words].  Since then I've been fastidious about testing, proving, and locking off. to the point of annoying people. Glad to be alive to annoy them

  • 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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