Simultaenous Contact

where customers have lamp posts located within their driveways.

Are we talking about customer's own private lighting columns (presumably fed from their own installation) or public street lighting that happens to be within reach of the driveway?

   - Andy.

Hi Andy, in the scenario I am speaking about these sim contacts would be primarily street furniture supplied by the DNO remote from the customers supply. I will try and get some images to help here 

Hmm, the lamp posts being out of your control is tricky.  Even if you could stab the lamp post with a wander lead and verify a very low resistance to the house earth, thats only probably verifying the same substation and street main, its all a bit impractical to rely on. 

I'm intrigued by your proposed  bi‑directional earth leakage monitoring - what are you proposing to clamp around what exactly to do that, and then what would you disconnect if it was detected ? I'm envisaging problems with false alarms from diverted neutral current between CPCs and terra-firma earth.

The rest (O-PEN and RCD) are probably already there most of the time .

Mike.

I'm intrigued by your proposed  bi‑directional earth leakage monitoring - what are you proposing to clamp around what exactly to do that, and then what would you disconnect if it was detected ? I'm envisaging problems with false alarms from diverted neutral current between CPCs and terra-firma earth.

I think that a feature found in some open-PEN EVSEs - triggers when the c.p.c. is found to carry 21mA or more (possibly method M3 in IET 01) - it's a bit of a mitigation to try and fill in some of the holes in the voltage measuring approach when used on supplies derived from 3-phase distribution systems). Given the vehicle is normally on rubber tyres and hopefully hasn't been crashed into steel barriers diverted currents hopefully shouldn't be too much of an issue. (Open PEN devices aren't permitted to have bonds to extraneous-conductive-parts downstream of the device).

   - Andy.

If its that, and it operates the same all poles (include CPC) as the O-PEN then that's pretty good. If it really is looking at the current into the car in free space I'd argue you could make it a lot friskier than 21mA without ill effect, parking into the fence or very muddy tyres notwithstanding.

M.

The clamp proposal has now been put on hold due to sizing restrictions sadly. 

We will use only chargers with the 21 mA detection on them classification M3. In terms of this and the other protective layered measures do you think I have a case that I can install with no greater risk? With the switching mechanics of the earth within the charge point the car becommes an isolated metallic structure on rubber wheels with the risk from street furniture being the exact same for any person touching it, i.e the fault would go through them to earth, not via any connected earthing from the EV circuit. 

We will use only chargers with the 21 mA detection on them classification M3. In terms of this and the other protective layered measures do you think I have a case that I can install with no greater risk? With the switching mechanics of the earth within the charge point the car becommes an isolated metallic structure on rubber wheels with the risk from street furniture being the exact same for any person touching it, i.e the fault would go through them to earth, not via any connected earthing from the EV circuit. 

We will use only chargers with the 21 mA detection on them classification M3. In terms of this and the other protective layered measures do you think I have a case that I can install with no greater risk? With the switching mechanics of the earth within the charge point the car becommes an isolated metallic structure on rubber wheels with the risk from street furniture being the exact same for any person touching it, i.e the fault would go through them to earth, not via any connected earthing from the EV circuit. 

Almost, but not quite. The 21mA trip won't open the contacts until after 21+mA flows - which in this scenario is only going to happen when someone touches the car and lamp-post at the same time - so they still have a shock current passing though them - just hopefully for not quite long enough to be fatal. Only then does the car become isolated.

I do think your approach is the least worst in the circumstances through - in all probability both the lamp post and the installation will be on the same DNO's earthing system so an open-PEN approach makes a lot more sense than say TTing the EV.

Another theoretical possibility is to have the EV separated "earth free" (via an isolation transformer with PE linked to N but neither Earthed)  - as per fig A722 of BS 7671 - but I've never heard of that ever being done in practice before.

As for the letter of the regs, is the lighting column an exposed-conductive-part or an extraneous-conductive-part? I suspect these days (e.g. with internal wiring sheathed) it's probably the latter. Perhaps contrary to common sense, BS 7671's requirements for earthing system not to be within reach of each other only applies to exposed-conductive-parts, not extraneous-conductive-parts.

   - Andy.- 

Fantastic last point Andy. One that I can look into, I belive that lamp post, telecom cabs, etc would be deemed exposed conductive parts - however with the use of a lamp post fuse cut out and either split concentric or SWA there could be an argument for the latter. Reality is at the moment we are desperate for a solution.