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Bonding questions

Former Community Member
Former Community Member
Hi,


I have some bonding questions which I'm hoping that the experts on here can help with.


Example - TN-C-S installation with 25mm2 tails, 16mm2 main earthing conductor, 10mm2 main bonding conductors to incoming, buried metal water and gas pipes and 4mm2 supplementary bonding in the bathrooms.


Assuming the pipes have good conductivity to earth, am I correct in assuming that (1) during normal operating conditions, a portion of the 'return' current will flow via the bonding conductors & pipes (depending on the relative resistances of that path versus the main PEN conductor path) and (2) if the PEN conductor is damaged then all of the 'return' current will flow via the bonding conductors & pipes?  If so, I'm wondering why 10mm2 bonding conductors are deemed to be sufficient, particularly in the latter case where a potentially large current is flowing and it is perhaps not obvious that a fault exists?


I also started thinking that, depending on the layout of pipework and how the supplementary bonding is done (e.g. multiple pipes coming off of the incoming water main that end up being supplementary bonded at some end point) that current could also be flowing in the supplementary bonding conductors in cases 1 and 2 above.  Which then leads me to the question of why 4mm2 supplementary bonding conductors are deemed to be sufficient...


In my mind, the only way to be sure would be to measure the relative resistances of all of the various earth paths, but the regs quote the standard bonding conductor sizes without reference to this.  Sorry if this is a silly question or if I'm missing something fundamental, but it's had me scratching my head recently.  Any guidance gratefully received.


Regards,

Graham
  • 0
    Under fault free conditions there will be current flowing in the earthing conductor and the bonding conductors. These are network circulating currents and will vary with the loading on the supply network and the load balance on the network. The currents will flow in the metallic pipe work back to the transformer by diverse routes in and out of your neighbors properties and the supply neutral and the general mass of earth. 

    In the event of a lost PEN conductor fault it could be that your 10mm bonds may glow red hot and start a fire depending on the position of the broken PEN and the current share with your neighbors bonding. 


    I have a cottage in Norfolk with an overhead PME supply no gas but a copper water pipe. My supply  enters at the front of the house and the copper pipe enters at the rear. The 10mm bond runs under the 1st floor floor boards. I have inserted a plastic joint directly after the water stop valve and the 10mm bond clamp after the plastic joint. That way I will not have diverted neutral currents flowing in the bond and setting light to my house.
  • 0
    GrahamGibson:

    Hi,


    I have some bonding questions which I'm hoping that the experts on here can help with.


    Example - TN-C-S installation with 25mm2 tails, 16mm2 main earthing conductor, 10mm2 main bonding conductors to incoming, buried metal water and gas pipes and 4mm2 supplementary bonding in the bathrooms.


    Assuming the pipes have good conductivity to earth, am I correct in assuming that (1) during normal operating conditions, a portion of the 'return' current will flow via the bonding conductors & pipes (depending on the relative resistances of that path versus the main PEN conductor path) and (2) if the PEN conductor is damaged then all of the 'return' current will flow via the bonding conductors & pipes?  If so, I'm wondering why 10mm2 bonding conductors are deemed to be sufficient, particularly in the latter case where a potentially large current is flowing and it is perhaps not obvious that a fault exists?


    I also started thinking that, depending on the layout of pipework and how the supplementary bonding is done (e.g. multiple pipes coming off of the incoming water main that end up being supplementary bonded at some end point) that current could also be flowing in the supplementary bonding conductors in cases 1 and 2 above.  Which then leads me to the question of why 4mm2 supplementary bonding conductors are deemed to be sufficient...


    In my mind, the only way to be sure would be to measure the relative resistances of all of the various earth paths, but the regs quote the standard bonding conductor sizes without reference to this.  Sorry if this is a silly question or if I'm missing something fundamental, but it's had me scratching my head recently.  Any guidance gratefully received.


    Regards,

    Graham


    I think that the magnitude of diverted neutral currents will be limited by the  resistance to true earth of the conducting underground pipes. If the ground is dry or poorly conducting like sand, the currents may be low and cause no harm.  If the roaming currents warm up and dry out the surrounding soil that will make it less conductive as well. I don't think that we have many day to day examples of diverted neutral currents causing fires or other disasters, but a year or so back we did read an account of 1.0mm2 lighting C.P.C.s having melted cables in a loft, if I remember correctly, due to diverted neutral currents.  P.M.E. supplies have multiple earthing electrodes to reduce the likelihood of a neutral Voltage rise. I can see them installed at regular points at the base of poles that support overhead lines.


    Z.


  • 0
    Oh ek!

    IET Forums - Diverted neutral "explosion" (theiet.org)


    Z.
  • 0
    In principle your concern is well founded, but it all hinges on what really is a 'good' connection to what I shall call the  terra-firma earth, not to be confused with the MET earth or the CPC earth.....


    This is an area that BS7671 does not handle very clearly, indeed it is fair to say that the best approach to earthing and bonding or not has rumbled on as being a bit contentious since the first edition of the regs in the 1880s, getting worse ~ 100 years later in the 1980s as PME and bonding mania co-incided, and has eased a bit with the RCD


    The problem for the PEN fault is there is no sensible totally  safe size, unless you go totally impractical and use the substation fuses as your guide, as the diverted current via your pipes and NE bond may be much larger than the rating of your service cable, in the case of a fault where your house now carries the neutral current of the rest of the street.

    However it is almost never that bad - there is not normally a case where only one properly has functioning main bonds and no-one else does, so the current,  which on a typical 3 phase supply is smaller than the phase currents anyway, is divided by many paths.


    That's not to say there are not odd things that happen ( like in this US video) but the cable sizes are a compromise as the alternative is  too unwieldy. 

    One could consider fuses in the earth path to protect the cables, but there are other very good reasons why we do not do this.


    At the same time the standards to not distinguish things that are really loosely earthed, like short lengths of pipe between say an oil tank and a boiler house, or planted 'street furniture'  where the resistance as an electrode cannot be less than several tens of ohms, and a really thin bit of wire would be more than adequate.

    Insulated joints (" I J "s to the gas people ) are a thing and slowly becoming more common, but can make other situations worse, as there may be different voltages on either side of that join.

    Mike



  • 0
    Then there is this type of thing to worry about or to lose sleep over.

    IET Forums - Alarming News Article (theiet.org)


    Z.
  • 0
    Good question - basically the main bonds are sized only for normal operating conductions - they're a decent fraction of the PEN conductor and experience shows that a fairly large proportion of N currents will follow the PEN. Broken PENs can cause a lot of trouble - if you really wanted to size bonds for that you'd have to consider some of your neighbour's N currents not just your own, and could end up with a size similar to what's running down the street - which probably wouldn't be practical.


    I'd argue that the resistance of buried metallic service pipework to true earth isn't an overriding consideration though - since it's more than likely connected back to the supply N point by a (or more likely many) continuous metallic path(s) - via neighbours' installations.


    The limit for supplementary bonding is actually 2.5mm² or half the size of the corresponding c.p.c. if it's connecting an exposed-conductive-part, or the smaller c.p.c. if interconnecting two exposed-conductive-parts - the 4mm² limit is purely for mechanical robustness considerations where it doesn't have other protection. But in a similar way that main bonding should be 'shunted' by the supply PEN (so the PEN always takes a significant proportion of the current), if supplementary bonding is exposed to diverted N currents, it should similarly be 'shunted' by the main bonding (you shouldn't be getting diverted N currents from within the installation).


    For sure you can construct theoretical arrangements where the bonding conductors look to be undersized, but in the experience of millions of homes across many decades suggests it's not a practical risk. (A bit like the unbalanced ring circuit problem.)


       - Andy.
  • 0
    John Peckham:

    My supply  enters at the front of the house and the copper pipe enters at the rear. The 10mm bond runs under the 1st floor floor boards. I have inserted a plastic joint directly after the water stop valve and the 10mm bond clamp after the plastic joint. That way I will not have diverted neutral currents flowing in the bond and setting light to my house. 




    If I have understood correctly, haven't you just disconnected the 'bonding' conductor from the extraneous-conductive-part thus negating the conductor's true purpose and making its presence futile?


  • 0
    Bear in mind there's a lot of slack in normal cable CCC ratings - they're chosen on the basis that if you run them continuously at max rating, the insulation won't more rapidly degrade than the expected 50 year (or whatever) lifetime. If you change your definition to "won't reach 250 degrees (the ignition temperature of wood/paper) for a day or so until the DNO fix their PEN", then you can get away with a lot smaller CSA.
  • Former Community Member
    0 Former Community Member
    Many thanks to you all for your replies.


    Regards,

    Graham

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