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Bonding a metal bath

David-Adams
Good evening


I am having one of those moments where I can’t sleep because I am worrying about something I probably don’t need to. That’s the rational part of me talking, unfortunately the anxiety monster won’t b***** off. 


We have just had our old cast iron bath replaced with a nice new shiny steel one. The old bath was bonded back to the terminal block by main consumer unit over 6mm earth cable via the airing cupboard. We also have an electric shower. 


The new bath has no taps on it (they are wall-mounted and fed by copper pipes). The waste is all plastic. The electric shower has been replaced with a new electric shower. The copper pipes in the airing cupboard have been connected via a new 4mm earth cable. My electrician says that according to the 18th edition, there is no requirement for the metal bath to be bonded. 


I have no reason to doubt him, except for the anxiety monster eating away at my brain I mentioned earlier. What limited literature I have found seems to suggest this is correct, but in some circles it is a hotly debated topic with contradicting views. I was just wondering if someone could confirm this for me please. I would also be interested in understanding why this is the case to satisfy my own natural curiosity of all things!


Many thanks in advance
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    For older installations you'd have to look at the worst case which would depend on the size of the protective device protecting the bathroom circuits.



    But what about situations like my old Grandmother's house. Hot water cylinder in alcove cupboard in 2nd bedroom - immersion heater on 15A BS 3036 and plastic water cistern in the loft.

    Lets say the bathroom is being refurbished and all its circuits (but not the rest of the house) will be on 30mA RCDs. Without supplementary bonding the hot water pipework is in contact with the immersion's c.p.c. but isolated from the main bonding on the cold water by the plastic cistern. A fault on the immersion would likely hold the hot pipework at a hazardous voltage for possibly several seconds - generating a serious shock hazard between the hot and cold taps in the bathroom.


    If you consider only the protective devices of the bathroom sockets you'd think that an acceptable resistance would be 50V/30mA = 1667 Ohms - which certainly be met between the hot and cold taps (cold via main bonding to MET, hot via immersion c.p.c. to MET) and so conclude no extra supplementary bonding is required - yet the situation is clearly unsatisfactory. So in short, I argue that you need to consider all circuits that could impose a hazardous voltage on extraneous-conductive-parts that are accessible within the bathroom - not just circuits within the bathroom. Without very detailed knowledge that probably means considering every circuit in the entire installation - including any submains - which likely makes the whole thing much more onerous.

     

    from what I remember it could be as high as 500A. Guidance suggests 0.05 Ohms between extraneous metalwork



    I've not got the latest guidance, but I thought it has been revised along the lines of expect up to 0.05 Ohms for the connection between bonding conductor and extraneous-conductive-part (i.e. the resistance of the clamp) - plus the expected resistance of the bonding conductors themselves. After all you don't need that many metres of 4mm² to get to 50mΩ - and BS 7671's actual requirement is 50V/Ia for supplementary bonding - which could be considerably higher. For sure if you happen to measure <0.05Ohms then you can pass it without having to bother with any calculations, but that's not quite the same as an acceptable limit.


      - Andy.
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    UKPN:

    It may be screwed to the wall, extreme condensation, running water, as I mentioned, area of special risk.

    Regards, UKPN 




    As in, screwed through the bath and fixed into the wall with metal screws? By which you would have to cut screw holes into the bath in order to fix it that way? If so, then no, I have absolutely not done that. Who would and why?

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    UKPN:

    It may be screwed to the wall, extreme condensation, running water, as I mentioned, area of special risk.

    Regards, UKPN 



    Do I take it that you think extreme condensation and conditions are more likely than every-day occurrences?

     

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    AJJewsbury:




    To prove the necessity of supplementary bonding within each indivual case would it not be the most obvious solution to take measurements for both continuity and insulation resistance and then compare them against recognised acceptable safe values?



    Ah, but what are the "recognised acceptable safe values? I'm thinking especially of R≤50V/Ia when not all the circuits in the premises are 30mA RCD protected...

      - Andy.


    For older installations you'd have to look at the worst case which would depend on the size of the protective device protecting the bathroom circuits. from what I remember it could be as high as 500A. Guidance suggests 0.05 Ohms between extraneous metalwork giving a maximum voltage difference of 25V. for the 5secs the fault is active.

    I can't see how you could anticipate this accurately without some form of measurement.


    Legh

  • 0
    It may be screwed to the wall, extreme condensation, running water, as I mentioned, area of special risk.

    Regards, UKPN
  • 0
    What are you getting at? Who is 'arguing the toss'?


    We were told that the bath is an isolated metal item - no pipes connected and plastic waste.

    Taking that as true, it is not an extraneous-c-p, what tests do you consider necessary?


    Nothing has changed in the 18th from the 17th so the electrician is not correct in using that as a reason, although he is correct in his view not to earth (bond incorrectly) the bath.
  • 0


    bob-tahoma:

    Good evening


    I am having one of those moments where I can’t sleep because I am worrying about something I probably don’t need to. That’s the rational part of me talking, unfortunately the anxiety monster won’t b***** off. 


    We have just had our old cast iron bath replaced with a nice new shiny steel one. The old bath was bonded back to the terminal block by main consumer unit over 6mm earth cable via the airing cupboard. We also have an electric shower. 


    The new bath has no taps on it (they are wall-mounted and fed by copper pipes). The waste is all plastic. The electric shower has been replaced with a new electric shower. The copper pipes in the airing cupboard have been connected via a new 4mm earth cable. My electrician says that according to the 18th edition, there is no requirement for the metal bath to be bonded. 


    I have no reason to doubt him, except for the anxiety monster eating away at my brain I mentioned earlier. What limited literature I have found seems to suggest this is correct, but in some circles it is a hotly debated topic with contradicting views. I was just wondering if someone could confirm this for me please. I would also be interested in understanding why this is the case to satisfy my own natural curiosity of all things!


    Many thanks in advance




    and

               "That was not necessary for the original question. It was answered very quickly.



                 'Arguing the toss' only became necessary because of subsequent incorrect statements."




    and so


    geoffsd    :

    Assuming your bath is upstairs and not in the garden.


    The old bath was not bonded - it was earthed ensuring you would get the maximum shock had you touched it at the same time as a live wire or appliance.


    As your new bath is not connected to anything electrical or to any metal which is connected to anything electrical you must not earth it (bond it unnecessarily).

    Think of it as a metal door handle or a metal knife and fork. It cannot become live nor provide a path to earth.


    Nothing to do with the 18th or any other edition.




    Not to be controversial, well, I have read the OP and the immediate answer and it appears to be untested against the 18th ed. regs, GN3, GN5, GN8. where the values of minimum allowable resistance for continuity and the minimum allowable insulation resistance to provide the necessary protection against electric shock when a wet body makes simultaneous contact with extraneous metalwork which may be at different electrical potentials.

    The guidance given in Reg 701.415.2 sums it up.


    Legh

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    "Crittal window frames" I`d never heard that term till about a week ago. Now I keep hearing it. Every week I learn something now
  • 0
    In some instances the regs accept 70 volts rather than 50 volts as the upper limit for touch voltage, so that isn’t set in stone.


    However, when naked, barefoot and wet, you also have to consider the “phantom voltage” that there may be on metal pipework and taps due to the ten or so volts that there may be due to the earth being taken from the DNO neutral which is after all a live conductor.


    One of the people most likely to notice a tingling feeling due to phantom voltage is the window cleaner in soggy training shoes washing the window panes in the steel Crittal window frames that the electrician has carefully connected supplementary equipotential potential bonding conductors to, with the window cleaner being outside of the equipotential zone standing in the garden.


    Andy Betteridge
  • 0

    To prove the necessity of supplementary bonding within each indivual case would it not be the most obvious solution to take measurements for both continuity and insulation resistance and then compare them against recognised acceptable safe values?



    Ah, but what are the "recognised acceptable safe values? I'm thinking especially of R≤50V/Ia when not all the circuits in the premises are 30mA RCD protected...

      - Andy.

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