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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

    There has been no death. The wall will not be steam stripped of wall paper as it has recently been painted with new emulsion paint.



    I'm not sure that you can legislate your own BS7671 or H&S codes on the basis of an individual case. There has to be common ground for all electrical systems within each location (domestic is just one type). It is safe or it is not safe. A metal plate switch is either class 2 or not, if not then it is a class 1 accessory and must be earthed via a cpc. The regs do not allow us to use electrical separation in domestic environments as a sole method of basic protection. it also requires another layer of insulation to provide protection. 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. 


    Legh
  • 0
    The retrospectoscope is a very fine instrument which sees things in very fine detail.


    I have never been cross-examined as an expert witness, but I would like to think that it was because either I advised the solicitors that the case was weak (I never charged for an initial opinion), or that my report was sufficiently robust. My last ever case was particularly sad because the Claimant's widow died before the Defendant conceded. (It was nothing to do with electrickery which came into my life subsequently.)


    In any event, I think that there is a danger that we are being very harsh on Zoomup. There must be a range of reasonable opinion and I don't think that Zoomup has gone outside of it.
  • 0

    Zoomup:

    . . . I went out and bought a twin switch of the type installed in the flat out of curiosity. . . The switch has an earth terminal to the plate. . . 




    That tells me that the switch is not class 2 construction, and the manufacturer is expecting the terminal to be earthed. 


    Regards,


    Alan. 

  • 0

    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.


  • 0

    perspicacious:
     In my case the lighting circuit is protected by a 6A R.C.B.O.


    And the failure rate of RCDs compared to circuit-breakers is?


    Regards


    BOD




    It is brand new and has been tested. It works perfectly.


    Z.

  • 0

    Alan Capon:




    Zoomup:

    . . . I went out and bought a twin switch of the type installed in the flat out of curiosity. . . The switch has an earth terminal to the plate. . . 




    That tells me that the switch is not class 2 construction, and the manufacturer is expecting the terminal to be earthed. 


    Regards,


    Alan. 


     




    Could that earth terminal be purely to terminate a single CPC or to join two (or more) CPCs together? ie Similar to an earth terminal in a plastic patress box.


    Clive

  • 0
    With the greatest respect to anyone doing EICs


    There are better electricians out there than you or I will ever be (There always are - better than I, at the very least at areas such as hospitals, mines, factories, HV, Commercial, Domestic, shops, ships, Automation, pumps and motors, Controls, etc)


    There are better, more learned people out there than you or I will ever be (I don't know everything and never will, even the best experts will argue with other experts about the minuite of details.)


    If you're registered with the NICEIC or similar, seek their input too, and just do as they recommend. Their advice is always with covering your back in mind.  It is always thought out and reasoned; it is not always the best advice for a specific circumstance, but the best advice that can be given over the phone.  I don't always agree with the NICEIC but in order to be "in the club" I sometimes just have to do what they say I have to do. 


    This forum brings in a whole range of people who have had much more experience than I in electrical areas that I have learned less of than others - EICRs are one example - I think I'm competent, and the NICEIC would agree from 13 years of assessments as an AC, but I don't do the dam things day in and day out - I actively dislike the dam things. Someone else who does these EICRs every day will always be better than I in deciding how to code a fault. 


    Some of those sorts of people have probably contributed to this thread with the best advice they can. (free of charge and with no Ulterior motives I believe)


    I know the ones that I hold in highest esteem and who's opinions I value the most. When a collection of valued opinions all say I'm wrong - with any code at all - Id give serious thought to considering if I am in fact, wrong. (Please forgive me if I am wrong here - I believe Zoomup probably posted this thread to get validation for his decision, but must have had some doubts about the decision in order to seek validation)


    I think the consensus in this case case would seem to be: Under the described circumstances, under no circumstances is an unearthed class 1 accessory OK for continued use or to be deemed satisfactory on an EICR. (Unless its an earth free installation or something very unusual like an isolating transformer has been fitted on the lighting circuit - even then - this is me trying to be overly clever and think outside the box - I'm not sure it would make a difference to the EICR coding - does anyone know if that would be a solution?)


    I know there are probably millions of unearthed class one accessories up and down the country, without even an RCD, its mostly never been a problem. Statistically - probably - an extremely small chance of there being an issue, made even smaller by the use of an RCD that would cut the fault duration to a very short time. I know the chances are, that he'll be fine for ever - I think everyone kind of knows that.


    But why take the risk if you don't need to? Against all of the professional advice, from every governing body and expert you're likely to come up against.............Code 2 and unsatisfactory is the only answer here I think........


    Kind Regards

    Tatty
  • Former Community Member
    0 Former Community Member
    An RCD won't limit the current - the fault level would be the same with or without the RCD.


    It can only limit the time before disconnection based on a graduation of exactly what the fault current magnitude is.


    As the old skool used to mutter "it's the volts that jolt, it's the mills that kill"


    Regards


    OMS


  • 0
    Tatty, I think you pretty much hit the nail on the head.

    I vote for code 2 unsatisfactory.

  • 0

    tattyinengland:

    I know there are probably millions of unearthed class one accessories up and down the country, without even an RCD, its mostly never been a problem. 




    Very well argued post, Tatty. To add a bit of perspective, I grew up in a Victorian house with unearthed metal light switches in some locations, the electrics probably having been installed between the wars. Back then (that is to say, at the time I was growing up) it was not a requirement to have either earthing on the lighting circuit or RCDs fitted - in fact the protection was by fuses rather than CBs. I can remember on a number of occasions getting shocks when switching lights on or off, which would be reported to the appropriate authorities (my parents). While I am sure this particular house is no longer in this position (I believe the new owner did a conversion to allow him to rent rooms to students, so would need the electrical installation certified) there are bound to be some residences still like this. Looking back I think "how could my parents let their children be at risk like that?" but at the time it was not considered a major problem either by ourselves or, presumably, the IET Regs.  If I was living there now, I like to think changes would be made, but when you live with something like that there is an all too human tendency to ignore the risk (think of people living next to a volcano) so who knows.....

    Alasdair

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