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Street Lighting - PME Earthing Guidance Note 8

GlasgowMatt

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

Parents

  • There are 2 parts to this - the 6mm figure is a minimum, below which a single length of grellow is deemed too flimsy so of the supply cable were for example 4mm SWA -, the earth would still need to be 6mm.

    But you are going the other way. If the supply to the lamp post is coming into the pole base fuse  head in say 35mm, then there is an assumption about the maximum likely fault current, not being enough to melt a bit of 16mm2 , or at least the supply cable goes first ;-) and 50mm supply may fry 16mm but not 25.etc.

    Of course this idea that the single wire will survive the cable is a most  'courageous'  assumption. But really the current that can flow in the earthing and bonding is limited  - we just need the supply cable insulation to melt and remove supply with a bang, before the copper core in the earth  fuses to open circuit... , really the current may be a lot more limited if the fault path includes the terra-firme earth in which the street lamp or whatever has been "planted" as that path is tens of ohms.

    Mike.

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  • There are 2 parts to this - the 6mm figure is a minimum, below which a single length of grellow is deemed too flimsy so of the supply cable were for example 4mm SWA -, the earth would still need to be 6mm.

    But you are going the other way. If the supply to the lamp post is coming into the pole base fuse  head in say 35mm, then there is an assumption about the maximum likely fault current, not being enough to melt a bit of 16mm2 , or at least the supply cable goes first ;-) and 50mm supply may fry 16mm but not 25.etc.

    Of course this idea that the single wire will survive the cable is a most  'courageous'  assumption. But really the current that can flow in the earthing and bonding is limited  - we just need the supply cable insulation to melt and remove supply with a bang, before the copper core in the earth  fuses to open circuit... , really the current may be a lot more limited if the fault path includes the terra-firme earth in which the street lamp or whatever has been "planted" as that path is tens of ohms.

    Mike.

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