I have searched many previous threads on this subject and never found a satisfactory answer
I am happy with Reg 714.411.203:
Where an earth connection to a distributor's PME network has been provided for a street electrical fixture, the earthing and bonding conductor of a street electrical fixture shall have a minimum copper equivalent cross-sectional area of 6mm2 for supply neutral conductors with copper equivalent cross-sectional areas up to 10mm2. For larger sized supply neutral conductors the main bonding shall comply with Table 54.8.
I would interpret the above to being the size of the DNOs supplying neutral conductor, so for large supplies this would require a bonding conductor up to 50mm2
However guidance note 8, 10.6.3, goes on to add
Where an earth connection to a distributor's PME network has been provided for a street electrical fixture, the earthing and bonding conductor of a street electrical fixture shall have a minimum copper equivalent cross-sectional area of 6mm2 for supply neutral conductors with copper equivalent cross-sectional areas up to 10mm2. For larger sized supply neutral conductors ('at that point') the main bonding shall comply with Table 54.8.
It is unclear to me what 'At that point' means. Is this:
- the supply cable to, for example, a local DB feeding the external lighting
- The supply cable to the lighting columns (or other street electrical fixtures) themselves
- Or still means the size of the DNO incoming supply neutral to the installation, or otherwise the origin of the installation.
Table 54.8 produces very large bonding conductor sizes for large installations, which in practise would be avoided by TT'ing the external lighting installation on the grounds of cost.
Any thoughts or further guidance appreciated