EVs, Street furniture, PME and TT configurations

and highlights the problems in the regs we do not really distinguish that  cycle rack from a cast iron water main, and the both the Ze, and the chances of being bonded to the local PME CPC at an adjacent property are very different, and therefore so is the associated shock risk.

That makes a lot of sense. There's a lot of difference between:

• A "true" extraneous-conductive-part - i.e. something that can only pick up a true earth potential (like a post stuck in the ground, or even, generally, the surface of the ground itself), and

• A "shared" extraneous-conductive-part - (like a water or gas pipe) that's liable to pick up a voltage due to earth faults in other installations or broken CNEs. In effect these are extensions of exposed-conductive-parts of other installations and so could be maintained at a hazardous voltage for a significant duration.

Likewise exposed-conductive-parts could be subdivided:

• "Hazardous" exposed-conductive-parts - where the hazardous voltage might persist for longer than acceptable from a shock protection point of view (e.g. on systems where one or more circuits has a disconnection time exceeding that in table 41.1 - e.g. distribution circuits, larger final circuits or where the earthing facility is shared with other installations or distribution network)

• "Non-hazardous" exposed-conductive-parts where should a hazardous voltage ever appear, it won't persist for long enough to pose a shock hazard (i.e. the earthing system serves only circuits whose disconnection times comply with 41.1)

I can see an argument that no bonding or separation is needed between any two "true" extraneous-conductive-parts, since no hazardous voltage can exists between them.

Likewise no bonding or separation is needed between a "non-hazardous" exposed-conductive-part and an "true" extraneous-conductive-part.


I suppose that the voltage difference between two separate "non-hazardous" exposed-conductive-parts might need a bit more thought - e.g. if each was limited (for long duration situations) to 50V (by R_A x I_Δn) above true earth then in the worst possible conditions (e.g. two systems fed from different supplies 180 degrees out of phase and carrying maximum leakage currents) then a hazardous 100V could exist between the two (although 50V √ 3 = 87V is a more likely limit when the supply is common from a typical 3-phase system). Limiting the definition of "non-hazardous" exposed-conductive-parts to TT systems where R_A x I_Δn ≤ 25V (as hinted at earlier) as well as disconnection times in accordance with table 41.1 should I think mean that no hazardous voltage can persist for a hazardous length of time between any two "non-hazardous" exposed-conductive-parts as well as between those parts and true earth ("true" extraneous-conductive-parts) - so no bonding (or separation) between such parts would be needed either.


(I've ignored the possibility of two simultaneous earth faults on the two different TT systems putting 400V (or possibly 460V) between them for up to 0.2s - as I think the chances of a second, independent, fault within that timescale is so remote as not to be plausible - but if anyone thinks it's worth entertaining (due to faulty RCDs for instance) we could think about ways of mitigating that risk too - I notice that the 40ms disconnection time for 30mA RCDs would seem to satisfy even the worst case column of table 41.1 - so maybe caravan-style duplicate RCDs might solve that.)

I think any form of words  that really says 'don't bother if its just an isolated metal thing planted in or the ground' is reasonable.

Absolutely - copy & pasting 714.411.3.1.2 into 721 might be a good (and consistent) start. Although I think I'd like to see it extended to include "non-hazardous" exposed-conductive-parts of other installations - so we have have one charge point next to another charge point down the street without having to bond them all together. If we could then include other street furniture within the "non-hazardous" group (or at least an 'acceptable hazard' group) - e.g. by deeming that small load PME'd lamp posts with an additional electrode to be such (justified by long experience, even if in theory it's flawed) them we might have a practical, and reasonably safe, way forward.


  - Andy.

and highlights the problems in the regs we do not really distinguish that  cycle rack from a cast iron water main, and the both the Ze, and the chances of being bonded to the local PME CPC at an adjacent property are very different, and therefore so is the associated shock risk.

That makes a lot of sense. There's a lot of difference between:

• A "true" extraneous-conductive-part - i.e. something that can only pick up a true earth potential (like a post stuck in the ground, or even, generally, the surface of the ground itself), and

• A "shared" extraneous-conductive-part - (like a water or gas pipe) that's liable to pick up a voltage due to earth faults in other installations or broken CNEs. In effect these are extensions of exposed-conductive-parts of other installations and so could be maintained at a hazardous voltage for a significant duration.

Likewise exposed-conductive-parts could be subdivided:

• "Hazardous" exposed-conductive-parts - where the hazardous voltage might persist for longer than acceptable from a shock protection point of view (e.g. on systems where one or more circuits has a disconnection time exceeding that in table 41.1 - e.g. distribution circuits, larger final circuits or where the earthing facility is shared with other installations or distribution network)

• "Non-hazardous" exposed-conductive-parts where should a hazardous voltage ever appear, it won't persist for long enough to pose a shock hazard (i.e. the earthing system serves only circuits whose disconnection times comply with 41.1)

I can see an argument that no bonding or separation is needed between any two "true" extraneous-conductive-parts, since no hazardous voltage can exists between them.

Likewise no bonding or separation is needed between a "non-hazardous" exposed-conductive-part and an "true" extraneous-conductive-part.


I suppose that the voltage difference between two separate "non-hazardous" exposed-conductive-parts might need a bit more thought - e.g. if each was limited (for long duration situations) to 50V (by R_A x I_Δn) above true earth then in the worst possible conditions (e.g. two systems fed from different supplies 180 degrees out of phase and carrying maximum leakage currents) then a hazardous 100V could exist between the two (although 50V √ 3 = 87V is a more likely limit when the supply is common from a typical 3-phase system). Limiting the definition of "non-hazardous" exposed-conductive-parts to TT systems where R_A x I_Δn ≤ 25V (as hinted at earlier) as well as disconnection times in accordance with table 41.1 should I think mean that no hazardous voltage can persist for a hazardous length of time between any two "non-hazardous" exposed-conductive-parts as well as between those parts and true earth ("true" extraneous-conductive-parts) - so no bonding (or separation) between such parts would be needed either.


(I've ignored the possibility of two simultaneous earth faults on the two different TT systems putting 400V (or possibly 460V) between them for up to 0.2s - as I think the chances of a second, independent, fault within that timescale is so remote as not to be plausible - but if anyone thinks it's worth entertaining (due to faulty RCDs for instance) we could think about ways of mitigating that risk too - I notice that the 40ms disconnection time for 30mA RCDs would seem to satisfy even the worst case column of table 41.1 - so maybe caravan-style duplicate RCDs might solve that.)

I think any form of words  that really says 'don't bother if its just an isolated metal thing planted in or the ground' is reasonable.

Absolutely - copy & pasting 714.411.3.1.2 into 721 might be a good (and consistent) start. Although I think I'd like to see it extended to include "non-hazardous" exposed-conductive-parts of other installations - so we have have one charge point next to another charge point down the street without having to bond them all together. If we could then include other street furniture within the "non-hazardous" group (or at least an 'acceptable hazard' group) - e.g. by deeming that small load PME'd lamp posts with an additional electrode to be such (justified by long experience, even if in theory it's flawed) them we might have a practical, and reasonably safe, way forward.


  - Andy.