Local Earthing cable size

I suspect the answer is "it depends"...

Firstly it depends on the purpose of this additional conductor - is it a duplicate c.p.c. to provide enhanced reliability in the case of  high protective conductor currents in normal service for instance (in which case it's normally sized in the same way as the main c.p.c.) or is it for supplementary bonding - in which case it'll likely be minimum half the size of the c.p.c. or if it's effectively connecting two exposed-conductive-parts together, no smaller than the smaller c.p.c..  Then there are mechanical considerations - which likely put a minimum of 2.5mm² or 4mm² depending on quite what level of protection it's given.

Also there may not be a simple single IEC answer - as this sort of thing often varies between national standards (the above is from the UK's BS 7671).

   - Andy.

I suspect the answer is "it depends"...

Firstly it depends on the purpose of this additional conductor - is it a duplicate c.p.c. to provide enhanced reliability in the case of  high protective conductor currents in normal service for instance (in which case it's normally sized in the same way as the main c.p.c.) or is it for supplementary bonding - in which case it'll likely be minimum half the size of the c.p.c. or if it's effectively connecting two exposed-conductive-parts together, no smaller than the smaller c.p.c..  Then there are mechanical considerations - which likely put a minimum of 2.5mm² or 4mm² depending on quite what level of protection it's given.

Also there may not be a simple single IEC answer - as this sort of thing often varies between national standards (the above is from the UK's BS 7671).

   - Andy.