AJJewsbury:

In the spirit of debate still, why does a TT system (or voltage monitoring electrode for a 'unicorn' device) have to be separate from the influence of a PME system? Someone touching the car isn't going to be standing on some theoretically perfect 0V plane, they're going to be stood on the same ground the car is standing on (or at least within a 1m or so of it). So if there are a few tens of volts from a PME system (or from any other source) under the car, surely it's better for the EVSE c.p.c. to be at that same voltage or at least as close as we can make - so we're minimising the voltage difference between the car and the ground it's stood on - rather than some perfect 0V which is only represetantive of the ground potential half a mile away. Isn't that the principle of equipotentiality?


So bang in a rod as close as you can to the (centre of?) the parking space (not hitting buried services notwithstanding) and be done.


  

• If the TT electrode is too close (say within 1 m), it's effectively touching PME, so you've not got separation and therefore no point in the electrode. You also need to allow for ground subsidance.

• You might well then want to argue about the person not standing at the "general mass of earth", but sometimes the voltage drops off quite quickly, even a couple of metres, and there's no guarantee the potential at the feet of a person will be that at the TT earth electrode.

• Conversely, and because of the previous point, in small curtilage properties, you might well be standing over some metalwork (say incoming gas or water pipe) connected to the PME earth, and in certain cases, you simply return the  PME touch voltage ... this of course means that going to the trouble of TT in many dwellings is pointless !

 Measurement earth electrode

• If it's too close, you're not measuring any voltage difference so the device won't ever operate.

• The minimum separation recommended is 2 m because if you are any closer, with ground subsidance etc., you still need to maintain a reasonable distance.

• At that distance, the device should trip at 40 V, not 70 V

AJJewsbury:

In the spirit of debate still, why does a TT system (or voltage monitoring electrode for a 'unicorn' device) have to be separate from the influence of a PME system? Someone touching the car isn't going to be standing on some theoretically perfect 0V plane, they're going to be stood on the same ground the car is standing on (or at least within a 1m or so of it). So if there are a few tens of volts from a PME system (or from any other source) under the car, surely it's better for the EVSE c.p.c. to be at that same voltage or at least as close as we can make - so we're minimising the voltage difference between the car and the ground it's stood on - rather than some perfect 0V which is only represetantive of the ground potential half a mile away. Isn't that the principle of equipotentiality?


So bang in a rod as close as you can to the (centre of?) the parking space (not hitting buried services notwithstanding) and be done.


  

• If the TT electrode is too close (say within 1 m), it's effectively touching PME, so you've not got separation and therefore no point in the electrode. You also need to allow for ground subsidance.

• You might well then want to argue about the person not standing at the "general mass of earth", but sometimes the voltage drops off quite quickly, even a couple of metres, and there's no guarantee the potential at the feet of a person will be that at the TT earth electrode.

• Conversely, and because of the previous point, in small curtilage properties, you might well be standing over some metalwork (say incoming gas or water pipe) connected to the PME earth, and in certain cases, you simply return the  PME touch voltage ... this of course means that going to the trouble of TT in many dwellings is pointless !

 Measurement earth electrode

• If it's too close, you're not measuring any voltage difference so the device won't ever operate.

• The minimum separation recommended is 2 m because if you are any closer, with ground subsidance etc., you still need to maintain a reasonable distance.

• At that distance, the device should trip at 40 V, not 70 V