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Shock Likelihood at Switch.

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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.

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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.


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  • 0
    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.


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