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Shock Likelihood at Switch.

Zoomup
Mornin' All,


I have just inspected and tested a renovated  old flat's wiring. The original lighting wiring in places has no circuit protective conductor. The owner has installed metal plate light switches to two positions with wooden back boxes. At these two positions there is no circuit protective conductor. The flat has a new R.C.B.O. consumer unit and all other wiring is good.


I have recommended that the switches have a C.P.C. installed (difficult and disruptive) or be changed to all insulated types.


Just what is the shock risk at these two switch positions? What is the likelihood of the metal plates becoming live due to a fault? Has anyone every seen a metal plate switch break down so that the plates becomes live?


Thanks,


Z.

  • 0

    Zoomup:

    Just what is the shock risk at these two switch positions? What is the likelihood of the metal plates becoming live due to a fault? Has anyone every seen a metal plate switch break down so that the plates becomes live?




    That the line conductor is loose and comes into contact with the metal. If the switch has a metal toggle, that the toggle becomes live. A CPC would allow ADS to operate. Otherwise you would be relying upon the RCD to trip when somebody touches the switch.


    With one fault to danger, I would say C2. With a non-metallic switch, just a C3.

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    Has anyone every seen a metal plate switch break down so that the plates becomes live?



    Usually the switch mechanism itself will offer protection via double/reinforced insulation (as people touch the rocker/dolly directly) - I think the risk is more due to a faults in wiring insulation behind the plate - either directly or via the steel backbox (if present).


       - Andy.
  • Former Community Member
    0 Former Community Member

     (as people touch the rocker/dolly directly) 




    I tend to find the plate first when feeling around in the dark for the switch :)

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    I imagine that a simple risk analysis would result in a medium risk profile based on a low likelihood but a potentially high severity.  The latter would depend on many other things such as the combined floor and body resistance, area of contact and proximity of other earthy items. I wouldn’t argue with a risk assessment that concluded the risk to be tolerable but that is merely academic as it would be expected that relevant guidance was followed to ensure that basic control measures were present in the first place.
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    So, not 100 % directly related to the original question, but a pause for thought on the risks.


    Switches become dirty over time, including with salty grease from fingers.


    If someone's hands aren't perfectly dry, when using an old switch contaminated like this, there is a chance of current flow from the live parts inside the switch - but in reality the current flow will be very small hand to feet (less perhaps than 1 mA).


    Now, we had a metal-plate switch outside our bathroom - just at the top of the stairs. It was quite old. My son, about 12 at the time, came out of the bathroom, hands not quite dry (although, granted - at least he washed them ?). Switched the bathroom light off, and received a perceptive (but otherwise quite safe) shock across the end of his finger, quite simply because the metal plate was earthed!


    In this case, the secondary effects could have been much worse (fall down stairs).


    Conclusion ... sometimes  having earthed metal plate light switches is dangerous? ?


    (PS - did replace the switch after the incident, I dismantled it and found it to be full of dirt as predicted. Also checked the lighting circuit earths, and for voltage on earth just in case, but no faults found!)
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    We have a strange relationship with risk of electric shock. All over my house we have exposed live parts hanging out of the ceiling and sitting on top of bed side tables, well at least they are when the lamps are removed.
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    We have a strange relationship with risk of electric shock. All over my house we have exposed live parts hanging out of the ceiling and sitting on top of bed side tables, well at least they are when the lamps are removed.



    Indeed - and typical domestic toasters are perhaps even worse - much more live metal exposed and often more easily accessible. But all the same that's no excuse for not maintaining standards where they are sensible.

       - Andy.
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    The credible fault mode will be the wire insulation being damaged, perhaps under cut on installation, or pinched by the back box screws or the grub screw working loose and releasing the wire tail to flap about (I had this with a light switch on a particularly wobbly wooden partition ages ago, where presumably the slamming of the door and causing vibration, a few times it became intermittent.)

    Given we mitigate a similar risk of a loose meter tail flapping in a TT consumer unit by additional clamping, it may be possible to argue that you could achieve a similar level of safety in the same way - oversleeving and mechanical restraint, but it would be non-standard.

    adding  a CPC is ideal, and changing to a switch not requiring a CPC is a close second.


  • 0

    Zoomup:

    Just what is the shock risk at these two switch positions? What is the likelihood of the metal plates becoming live due to a fault? Has anyone every seen a metal plate switch break down so that the plates becomes live?

    Z.

     




    What does the regulations state? That should answer the question.


    As an aside, I once attended a house for an unrelated fault, the customer's wife mentioned that they had been receiving shocks from a lighting switch in the kitchen, this was a conduit installation with a plastic lighting switch on a metal box..Testing the lighting switch support screws, they were live. The owner didn't want more expense of repairs, so squished some bread in the screw holes! ....... Darwinian redundancy! The same house where new cheap, unpainted doors had been fitted throughout; these had knotty pine panels and some knots were missing, (as big as 25mm, so making it possible to see through the doors). I couldn't just walk away from that job ........ I ran!


    We don't get much money but we do get a laugh.


    Jaymack  

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    Chris Pearson:




    Zoomup:

    Just what is the shock risk at these two switch positions? What is the likelihood of the metal plates becoming live due to a fault? Has anyone every seen a metal plate switch break down so that the plates becomes live?




    That the line conductor is loose and comes into contact with the metal. If the switch has a metal toggle, that the toggle becomes live. A CPC would allow ADS to operate. Otherwise you would be relying upon the RCD to trip when somebody touches the switch.


    With one fault to danger, I would say C2. With a non-metallic switch, just a C3.


     




    Hello Chris,

                        the switch is an insulated rocker type not a metal toggle type. The back box is wooden.


    Z.

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