Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 72 replies
  • Subscribers 32 subscribers
  • Views 12097 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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
  • 0

    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?

  • 0

    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.
  • 0

    Sparkingchip:

    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 




    Tying the PME earth to all extraneous and exposed metalwork will reduce the floating neutral voltage caused by out of balance DNO three phase supplies.

    As I remember, metal windows were regarded as an anomaly (non-extraneous) and it was only necessary to supplementary bond the metal studdied supported windows. However, 'you don't have to bond that' is not an answer I would have thought was acceptable in a court of law if by some chance a peradventure were to happen 


    Legh

  • 0
    The wet and nearly naked, except for the swimming costume, bit really applies to items such as the stainless steel post lights around the hot tub in the garden, rather than equipment and fittings inside of the house.


    But it has to be remembered that TNCS earthing is not acceptable in all installations and properties and ultimately it is the installation electrician who has to decide what the earthing system will be, just because the DNO have supplied an earth terminal doesn’t mean that it should be used.
  • 0
    The one to watch for is the external cast iron soil pipe - do you want it dangerous in the back yard and safe in the bathroom, or the reverse. I am pretty sure the only time I ever bonded one was when the pipe between bath and stack was in 1.75" copper.

    I'm not sure you need 0.4 second ADS for the failing immersion, you certainly did not under previous regs, 5secs would have been considered enough. Hopefully the CPC of the immersion supply serves to pop the fuse in a sensible time., I suspect that  adding the lighting CPC in parallel with it, and bonds to a selection of metal parts, does not appreciably alter the breaking time, unless those parts have an additional and  lower resistance path back to earth. We do ourselves no favours with our tendency to use reduced CPCs in twin and earth, as the on-fault voltage is more like 2/3 than 1/2 of the supply.

  • 0
    Agreed, by bonding both extraneous and exposed metalwork with the cpcs of the various circuits you would maintain the voltage at an equillibrium across simultaneous touchable metal items and reduce the size of the fault voltage at the same time through the increased cpcs....... and then they introduced PME and told the world it may not be necessary to supply supplementary bonding.....lol


    Legh
  • 0

    Sparkingchip:

    The wet and nearly naked, except for the swimming costume, bit really applies to items such as the stainless steel post lights around the hot tub in the garden, rather than equipment and fittings inside of the house.


    But it has to be remembered that TNCS earthing is not acceptable in all installations and properties and ultimately it is the installation electrician who has to decide what the earthing system will be, just because the DNO have supplied an earth terminal doesn’t mean that it should be used.




    I really don't think its a recommended practice to export PME beyond the natural bonding zone of the house installation. Usually (my method) would be to provides a PME protected 2-core SWA into an insulated terminal box and a new TT earth to the hot tub. But then there are always exceptions to the rule.


    Legh

  • 0
    One of the basic requirements for establishing a equipotential zone within a house is that there is a sheet of polythene underneath solid concrete floors and the like or a nice dry suspended timber floor and so on, along with a decent damp course.


    Screwing a bracket to a wall to hold a metal bath in place should not make it extraneous, bonding though will introduce an earth potential when there wasn’t one in the first place.


    I have seen the metal legs supporting plastic and fibreglass baths bonded, sometimes it’s not a bad idea to stop and think about what you are doing and why.


    Andy Betteridge
  • 0

    AJJewsbury:

    But what about situations like my old Grandmother's house. ... If you consider only the protective devices of the bathroom sockets ...




    Slip of the pen there, or a big bathroom?


    Andy's point is well-made, but the supply was probably not PME when the installation was made (mine certainly wasn't) and as I have said before, 30 years ago sparks seemed to get carried away with bonding. Only one hot to cold pipe bond is required for a soldered copper water installation. However, under every wash hand basin, bath and sink in our house they are bonded. Crikey, there is even bonding between the pipes beneath both of our double kitchen sinks as well as to the sink itself.

  • 0
    "Sparks seemed to get carried away with bonding"

    I know, hundreds of thousands of sparks, wiring millions of properties, what ever we're they thinking of? 

    Oh yes, wiring to the "IEE regulations"!!! 

    Regards, UKPN.

Community Rules & Guidelines