Street Lighting - PME Earthing Guidance Note 8

I think, historically, most of these regs have been thought of in the context of public street lighting tapped directly unmetered off the DNO's mains running along the street using a length of 10mm² or 16mm² concentric. The 6mm² rule was really just recognising that councils had carried on using the pre-PME bonding size even though the mains had been converted to PME for years without encountering any problems. PME bonding sizes are often based on assumptions about diverted N currents (hence they're often bigger than for a TN-S system with equal fault currents) - but as Mike says, the resistance to Earth of a lone column planted in the soil will be considerable (unlike a gas or water pipe in the house) so diverted N currents aren't really a consideration in practice.

There is a bit of a flaw in BS 7671 in that it considers all extraneous-conductive-parts to be equal - and thus works on the assumption that they may have a very low resistance to Earth (either directly to true Earth or to the supply N via bonding in neighbouring installation) - there's no obvious scope for realizing that some (like lighting columns) must by necessity have a large impedance, so normal bonding sizes are completely OTT.

In some situations lighting columns fed from large supplies (especially small decorative domestic ones in gardens) are almost treated as if they're not extraneous-conductive-parts at all and so the protective conductor is sized purely as if it were a c.p.c.. Given the definitions, it's almost plausible to claim that a lighting column is part of the electrical installation and so an exposed-conductive-part rather than an extraneous-conductive-part (or even neither if there's double/reinforced insulation (e.g. sheathed cables) between all live parts and the column).

   - Andy.

Not sure how it could be an extraneous conductive part as it’s hardly introducing a potential that is not already present by virtue of being stuck in the ground. As you say, usually pvc/pvc cables within so not likely an exposed conductive part either. In any event, with the proliferation of EVSEs on the street, best if it’s not on a PME terminal.

I think in some situations it could be an extreanous conductive part. If there are multiple columns planted in the ground, which are normally connected to the supply earth (whether or not we consider them exposed conductive parts) could form a impedance to earth low enough to be an extreanous conductive part. Stuck in the ground is exteanous, as it is not always the same potential as the earth supplied by the DNO.

Like Andy says it will depend on the installation, but one that matches the recommended earth electrodes of BS7430 would have a low impedance to true earth.

In such a case the entire 'external lighting' circuit is extreanous, and so would require bonding to the MET as per Table 54.8, but only where they entered the equipotential zone, I.e. the building supplying said external lighting circuit. 

Having a bonding conductor of said size for the entire circuit is only necessary for diverted Neutral currents, or keeping the touch potential at a safe level <60V during broken PEN faults, which would require a very low impedance to true earth via electrodes (and a very low impedance earth mat at the Tx but this is beyond our control)

All this is good theory, but still no idea what GN8 means by 'at that point'???

Side Note: EV charging kind of irks me, as the risk of broken PEN conductors is similar for all exterior earthed metalwork (imagine a couple of 2kw radiant heaters at a pub smoking area, fed in earthed metal conduit, next to an unbonded metal railing or fence, on the PME supply).

The risks are the same but only one requires TT earthing or an Open PEN detection device (other methods of compliance are available )

Why not have the open PEN detection device at the origin of the installation, thus all earthed metalwork is prevented from being raised to up to 230V in the case of a broken PEN conductors?

I think in some situations it could be an extreanous conductive part. If there are multiple columns planted in the ground, which are normally connected to the supply earth (whether or not we consider them exposed conductive parts) could form a impedance to earth low enough to be an extreanous conductive part. Stuck in the ground is exteanous, as it is not always the same potential as the earth supplied by the DNO.

Like Andy says it will depend on the installation, but one that matches the recommended earth electrodes of BS7430 would have a low impedance to true earth.

In such a case the entire 'external lighting' circuit is extreanous, and so would require bonding to the MET as per Table 54.8, but only where they entered the equipotential zone, I.e. the building supplying said external lighting circuit. 

Having a bonding conductor of said size for the entire circuit is only necessary for diverted Neutral currents, or keeping the touch potential at a safe level <60V during broken PEN faults, which would require a very low impedance to true earth via electrodes (and a very low impedance earth mat at the Tx but this is beyond our control)

All this is good theory, but still no idea what GN8 means by 'at that point'???

Side Note: EV charging kind of irks me, as the risk of broken PEN conductors is similar for all exterior earthed metalwork (imagine a couple of 2kw radiant heaters at a pub smoking area, fed in earthed metal conduit, next to an unbonded metal railing or fence, on the PME supply).

The risks are the same but only one requires TT earthing or an Open PEN detection device (other methods of compliance are available )

Why not have the open PEN detection device at the origin of the installation, thus all earthed metalwork is prevented from being raised to up to 230V in the case of a broken PEN conductors?