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Undersized Main Earth Conductor

Former Community Member
Former Community Member
Hi Folks,

I was carrying out an EICR on a domestic property with a PME supply.

The Main earth conductor is 10mm, the water and gas bonding is also 10mm connected to the earth terminal in the consumer unit.

Would you give this a C3 or C2?


I know there's an equation knocking about for checking main earth conductors however, I'm under the impression that this only works if the bonding conductors are terminated at the MET and not the consumer unit.


Any suggestions will be welcome.


Thanks
  • 0
    Hi stakes electrical:

    Hi Folks,

    I was carrying out an EICR on a domestic property with a PME supply.

    The Main earth conductor is 10mm, the water and gas bonding is also 10mm connected to the earth terminal in the consumer unit.

    Would you give this a C3 or C2?


    I know there's an equation knocking about for checking main earth conductors however, I'm under the impression that this only works if the bonding conductors are terminated at the MET and not the consumer unit.


    Any suggestions will be welcome.


    Thanks


     I like Table 54.7. No maths involved.


    Z.


  • 0



    I know there's an equation knocking about for checking main earth conductors however, I'm under the impression that this only works if the bonding conductors are terminated at the MET and not the consumer unit.


     


    I do not think that that is correct. Most new consumer units have terminals marked for bonding conductors and normally the main earthing terminal supplied at the cut out is not far away, so the consumer unit earthing bar can be considered as the main earthing terminal.


    Z.


  • 0
    Does it look cooked, are there signs of high diverted current in the bonding conductors?.Assuming it has been there for a while and looks in good shape,  connections are tight and not corroded etc, then at most it may be a  C3. 

    To compare, there are plenty of DNO heads with 6mm tail (not 16!!) coming out the side, and most of them are not dripping molten plastic, so we have to assume that in most cases the guidance for using 16mm if in doubt is erring on the side of belt and braces.


    After all a chunk of 16mm won't even twitch if you use it to blow a 100 A fuse, in fact it will take 100A or more all day,  -there are plety of 16mm meter tails as well.


    Nothing to say the MET cannot be in the CU or any other box if there is room, or indeed that there may be more than one marshalling blcok as well, so long as it is all solidly connected and does not look corroded or loose, then that is a tick in the box.


    (The NICIEC guide) for example has in the section at the back
    Departures from the requirements of the current edition of BS 7671 that do not give rise to danger or need improvement
    Inadequate cross-sectional area of a mainprotective bonding conductor provided that theconductor is at least 6 mm2 and that there is no evidence of thermal damage


    Now NICIEC are not the last word on this, but are a good steer for the level to aim at.


    Mike.


  • 0
    Using Table 54.7 is fine but if the earthing conductor is a smaller CSA than 54.7 requires then you need to do the calculation in accordance with Regulation 543.1.3 which will highly likely come up with a smaller CSA.


    However if the installation is PME Regulation 542.3.1 requires the earthing conductor to have a minimum CSA in accordance with Table 54.8.



    So a 10mm Earthing Conductor in this case is probably going to meet the requirements of BS 7671 but you must do the adiabatic equation to verify it.
  • 0
    Hi stakes electrical:

    I know there's an equation knocking about for checking main earth conductors ...


    Before DZ says anything, I shall say that the turn of phrase above does not give me much confidence in the OP's credentials for inspection and testing.


    Any road, it's the adiabatic which JP mentions above.


  • 0
    I know there's an equation knocking about for checking main earth conductors however, I'm under the impression that this only works if the bonding conductors are terminated at the MET and not the consumer unit.

    Half right - where the conductor is serving as both a circuit protective conductor and a bonding conductor, it'll need to simultaneously satisfy the requirements for both. So without calculation it's min 16mm² c.p.c. for 16mm² or 25mm² tails as per table 58.7 and min 10mm² (usually for a domestic) for PME bonding as per table 54.8 - hence the default 16mm².


    By calculation you are likely to reduce the size needed as c.p.c. - but you can't apply that result alone, as whatever that says you still can't go below 10mm² for the bonding function.   But as the existing is already 10mm² you only need to check that the c.p.c. requirements aren't for more, in this particular case.


    So you'll need Zdb at the CU and the characteristics of the upstream (probably DNO) fuse and plug them into S = √( I² t ) / k


      - Andy.

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