Undersized Main Earth Conductor

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.

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.