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Shock Likelihood at Switch.

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 Legh Richardson over 5 years ago

mapj1:

An RCD will not provide the necessary shock protection even though by fortuitousness it may operate due to a minor current imbalance and current flow to an earthing point.

If this was true, RCDs could be omitted without making the installation less safe, and all the agonising over risk assessments for 32A sockets would be moot.

I'm afraid this is not really true, a 30mA RCD is chosen for exactly this reason. The shock current is set by the environment, things like how well earthed you are and how much contact area, but the whole point of an RCD is indeed to limit that shock current to either be less than 30mA, or less than half a heart beat period, so you are very likely to survive. Without it, and absent some other ADS you are very likely to die.

You and I may prefer a 10mA limit, or a shorter operating time, (well I would, having once been saved by one, I seemed to have plenty of time to realise what was happening and think about what had gone wrong before it tripped) but they are indeed set to limit the shock; to a very painful but survivable level.

What is true is that the regs in the UK do not recognise an RCD as the sole means of protection against shock, and like to see it backed up by insulation over the live bits and/ or earthed enclosures.

I appeciate that class 2 gardening electrical equipment has no cpc but requires a RCD to provide supplentary basic and fault protection in the case of a fault with the cable.

From the details given to us by the OP. There is no cpc connected in the lighting circuit. Therefore there will be no earth return path back to the MET and thus no out of balance currents through the RCD. So we are now reliant on possible current flow through the person touching the unearthed live part and then through various parts of the building fabric. This method of disconnection is not guaranteed, It maybe fortuitous but not guaranteed as it is specific for this installation due to other factors, so it is not regarded as a general method of disconnection.

So I don't think this is a valid arguement for using a class 1 accessory without a connected cpc.

Legh
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0 Zoomup over 5 years ago

Mornin All,

I can see that I have been remiss in my duty. I regret that deeply. The new metal ASTA certified plate switches, made to BS EN 60669 are not up to standard, even with the self extinguishing all plastic encapsulated live switch parts just mounted onto a metal plate. The switch mounted onto a wooden box in a brick wall is wrong. I have told the flat owner that the switches need to be earthed. So worrying.

So I am going to contact the flat owner and say that as live parts can be accesses without the use of a tool I shall regard the installation as "Danger present. Immediate remedial action required" C.1.

The live parts (as another forum member pointed out) accessible without a tool can be touched and a shock risk is present. The risk of burns or worse is present, and the R.C.D. will not even notice it. I shall advise the removal of all pendant lamp holders. I shall disconnect the lighting circuit and the flat viewers and letting agent can view in the dark. The stairs are narrow and steep though, so perhaps torches will be needed.

C.1. it is.

Z.
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Shock Likelihood at Switch.

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