From Electrical Installation Guide
The main equipotential conductor
This conductor must, in general, have a c.s.a. at least equal to half of that of the largest PE conductor, but in no case need exceed 25 mm2 (copper) or 35 mm2 (aluminium) while its minimum c.s.a. is 6 mm2 (copper) or 10 mm2 (aluminium).
Supplementary equipotential conductor
This conductor allows an exposed conductive part which is remote from the nearest main equipotential conductor (PE conductor) to be connected to a local protective conductor. Its c.s.a. must be at least half of that of the protective conductor to which it is connected.
If it connects two exposed conductive parts (M1 and M2 in Figure G63) its c.s.a. must be at least equal to that of the smaller of the two PE conductors (for M1 and M2). Equipotential conductors which are not incorporated in a cable, should be protected mechanically by conduits, ducting, etc. wherever possible.
Other important uses for supplementary equipotential conductors concern the reduction of the earth-fault loop impedance, particularly for fault protection (indirect contact protection) schemes in TN- or IT-earthed installations, and in special locations with increased electrical risk (refer to IEC 60364-4-41).