EVs, Street furniture, PME and TT configurations

Thinking a bit further - the hazard from a PME'd lamppost suffering from a broken CNE is often mitigated by the supplementary electrodes that DNOs (if not BS 7671) have been insisting be installed -  and given the modest load can be reasonably successful at keeping touch voltages below hazardous levels.

Yes, it's possible, to an extent, to use earth electrodes to transfer any EPR on the CNE to the ground around the earth electrode.

If the total load downstream of the PEN that breaks is only something small like a lamp post and the load is hundreds of watts or less, I agree, the light will keep working dimly as a sort of single wire earth return, assuming a respectable electrode resistance,  and the touch and step voltages will be significant, but probably safe enough to persist for days or weeks until the fault is noticed.. For higher current single load loads, like the car charger,  or just a break further back down the chain, it is not clear that this works well enough. 


This is where ADS in the form of the magic all-pole breaking (including the CPC ) lost neutral detector is needed, either with phase/ neutral variation detector, or using a low current, high Z pilot earth electrode to allow some sensing.


In the mean time free draining ballast spread on the ground around he charger and vehicle will reduce the danger from step voltage, in two ways, once because it raises the contact resistance, and secondly because the stones are so darn uncomfortable, no-one in bare feet will stand on them if they can possibly avoid it.


I sense that the advice will need to change as it is probably becoming unworkable right now.

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.

Working on the streets of London surely DNO PME earth stakes must be assumed to be like rats, never more than a couple of metres away.


 Andy

I see this is an old thread but it has been interesting reading. 7.3 of the IET guide states that EVSE should not be installed in a location where the equipment itself or the vehicle on charge is less than 2.5m from any exposed metalwork connected to true Earth. In London and any other urban centre this gets pretty difficult - street signs, cycle hoops, bollards are all planted in the ground. However these will generally be encased in concrete. How should it be determined whether these are truely connected to Earth?

However these will generally be encased in concrete. How should it be determined whether these are truely connected to Earth?

The text book method is to insulation test (500V) between the part in question and a known good earth (e.g. a supplier's earth terminal or a previously tested TT electrode) - if the resistance is high enough to prevent a shock current then it can be considered not to be an extraneous-conductive-part. Commonly the value of 10mA is used - hence looking for a resistance of over 23kΩ, although in some situations 0.5mA might be preferred (the threshold of perception) in which case the limit would be closer to half a meg.


  - Andy.

Example here. The bollard is within 2.5m of the charging bay, should it be bonded to the TT earthing arrangement of the EVSE?

Well if you measure its resistance to earth and it's over 23kOhms then probably not.


If it's under then a bit more thinking is needed. If you considered it the equivalent of a highway environment and the EVSE an item of street furniture then maybe you can omit bonding under regulation 714.411.3.1.2. Otherwise it seems that bonding would be required. The trouble then is that if the bay next to that one has a similar item, with not quite 2.5m clear between them you might feel the need to bond that as well, and so on right across the car park... which might start to feel a little silly.


  - Andy.

Why are we taking dangerous Voltages out to electric vehicles to charge them? Can't a safer lower Voltage be used? Perhaps having the vehicles' batteries divided into separate smaller banks for charging. 

https://www.ecobatindustrial.tech/forklift-battery-chargers/?gclid=EAIaIQobChMI7emhu92Z5gIVVeDtCh37ggzAEAAYAiAAEgLM8PD_BwE


Z.

Here is the no wires and plugs to trip over solution, and very reduced shock risk.......

https://www.youtube.com/watch?v=a9_mpd1AIfk



Z.