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RCD's as fault protection?


Robin Spark

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Just thinking about the situation of using an RCD on lighting circuit where customer puts up their own metal light fitting and Line conductor gets trapped on body of fitting.

Now I am thinking that as the casing is not earthed and its attached to plaster board/wood then nothing is going to happen until someone touches fitting again ie to clean it or change lamp.

Is this right? Does anyone have data on resistivity of a person fully clothed and dry, so as to work out fault current?

Thanks in advance

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Just thinking about the situation of using an RCD on lighting circuit where customer puts up their own metal light fitting and Line conductor gets trapped on body of fitting.

Now I am thinking that as the casing is not earthed and its attached to plaster board/wood then nothing is going to happen until someone touches fitting again ie to clean it or change lamp.

Is this right? Does anyone have data on resistivity of a person fully clothed and dry, so as to work out fault current?

Thanks in advance

Surely any metal fitting that could allow line conductor to get trapped would not be a ClassII fitting, and thus would have an earth connection to the metal part.. thus the RCD would operate as soon as you switch the light on ?

Either way fault current if RCD protected will only ever reach 30ma

which at 230v would imply a total resistance inc the body part of the path.. 230/0.03 7.666kohm?

There are some tables of typical body resistances dependant upon path through body..

e.g. hand to hand...

hand to foot...

foot to foot...

hand / foot diagonal & same side of body!

but I cant recall a link to it at the moment!:(

that said it is a very non-precise science due to many variables associated with individual body size's etc..

:|

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Sorry Specs,

forgot to say i was referring to lighting cct with no cpc. Was also just wondering what the fault current would have been without use of rcd but suppose there are lots of variables.

Need to get off to bed now as tired and work in morning, have a good week:)

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Sorry Specs,

forgot to say i was referring to lighting cct with no cpc. Was also just wondering what the fault current would have been without use of rcd but suppose there are lots of variables.

Need to get off to bed now as tired and work in morning, have a good week:)

The added calculation problems come with the body resistance changing as the electricity make a path through it...

e.g. burning cells along the path the electric takes!

Apache probably knows a bit more about the passage of electricity though living bodily cells!?

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The added calculation problems come with the body resistance changing as the electricity make a path through it...

e.g. burning cells along the path the electric takes!

Apache probably knows a bit more about the passage of electricity though living bodily cells!?

maybe! It's not good on the whole, but defibrillators and EST have saved many lives. Oh and TENS machines. And pacemakers. mmm. Electricity can be good for us........

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I think any person putting a light fitting up that is not earthed probably deserves what they get if the worst happens. But it probably won't be them that gets the belt.

Batty

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Another case of a person not versed in the Black Arts, fitting metal fitting with no earth , where are the Electric Police when you need them ??

Deke

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Just thinking about the situation of using an RCD on lighting circuit where customer puts up their own metal light fitting and Line conductor gets trapped on body of fitting.

Now I am thinking that as the casing is not earthed and its attached to plaster board/wood then nothing is going to happen until someone touches fitting again ie to clean it or change lamp.

Is this right? Does anyone have data on resistivity of a person fully clothed and dry, so as to work out fault current?

Thanks in advance

Darwin Award?

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Just to clarify about this post.

I went to property other day to quote for some alterations. additions to lighting cct, new customer ( well hopefully) and there was no cpc on lighting cct and they had old wylex CU with rewirable fuses.

Customer had already got metal class 1 fittings installed (including in the kitchen which was recently done) and when i asked them who put this up (thinking it may have been themselves) they replied an electrician had donethis for them who at the time mentioned to them that there was no earth.

With regards to the use of RCDs in this situation this comes after i had read the Electrical safety Councils guide on replacing consumer units and lighting cct without cpc.

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Hi not really much to do with your original post but in similar situations, no cpc on lights, I have found people will upgrade consumer unit and bonding but dont want lights rewired, esp if old customers. So I have been using this website to change existing lights with the proviso that all tests are ok on the relevant circuit.

http://www.double-insulated-lighting.co.uk/

Just thought it may help when estimating and getting the job

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Hi not really much to do with your original post but in similar situations, no cpc on lights, I have found people will upgrade consumer unit and bonding but dont want lights rewired, esp if old customers. So I have been using this website to change existing lights with the proviso that all tests are ok on the relevant circuit.

http://www.double-insulated-lighting.co.uk/

Just thought it may help when estimating and getting the job

You are the professional and your advice should be taken on board as you have the persons safety to consider . I was asked to change some light fittings which were metal and the existing wiring had no cpc as the fittings in question were not class II I advised to customer of the potential dangers of a fitting becoming live and he was prepared to turn a blind eye to this !! However I wasn't prepared to do the same so I explained the options open to him and I left him to think about it . 2 yrs on I'm still waiting ?:|

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Yse Megaohm its frusrating when you know someone else will do it and not care. I lost a job today because I wouldnt leave the boiler spur downstairs after boiler was moved upstairs! and I wanted to fit rcbo's or a new c/u spark that got it said 100ma rcd was fine.

on a plus side the customer actually rang to tell me I didnt get it, first for everything

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you could have fitted a lockable spur, labelled it and left it where it is?

l love the box the the right of the home page on that site, 'why spend a fortune on a rewire' hmmmmmmm maybe in an attempt to make your home safer?????

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Seem to remember 4Mohms being quoted as an average body resistance during a discussion with inspection and testing tutor.

The 30mA (0.003A) and disconnection time values of our RCDs is based on an average body resistance and the time it would take tae kill ye! :z:z:_|O)O)O)

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Yes Wozz its a thought but I like the isolation a bit nearer than a different floor, not sure why lockable makes it better either but each to his own.

Well double insulated is safer if no cpc present although I agree with your point about a rewire being much better, sadly not a perfect world though is it?

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