[Jeremy Harris]

My wife's taking delivery of a new Zoe in a few weeks time, and I've just installed a new charge point adjacent to where she parks. I've read several, often confusing, stories about the Zoe being a bit fussy about connecting to some AC charge points, but haven't been able to find a definitive statement anywhere as to why this might be. Earthing seems to be a common suggestion, which suggests that the Zoe charger may check Ze, presumably with short pulses so as not to trip the supply RCD, in a similar way to an MFT would do this test. I've also read that some of the issues with the "Battery Charging Impossible" thing may relate to differences between the UK and French limits for Ze on a TT installation (no idea what the French regs say on this).

I want to ensure that my wife's first experience of EV ownership is a painless as possible, and the last thing I want is for her to experience a charging problem as soon as she gets the car. I installed the new charge point yesterday, connected to a 2 core 6mm² SWA sub-main from the incomer that already feeds the garage. The garage was already connected as a TT installation, so I just swapped out the Type A RCD for a Type B one and did all the usual tests after installation and put them on the EIC. Ra is nice and low, as the garage earth electrode is practically down to the local water table, I measured the earth loop impedance at the charge point test box, with it set to simulate a request for charge, at ~24Ω, well under the generally accepted 200Ω number in the OSG, and massively under the maximum allowed in the regs of 1667Ω.

Am I right in assuming that:

1 - The Zoe does indeed check the earth loop impedance before requesting charge?

2 - That a value of 24Ω for the earth loop impedance should easily be low enough to not cause a problem?

I'm reasonable sure things will be fine, but although I've owned three plug in cars over the past 7 or 8 (about to get my fourth) I have this lingering uncertainty about the way the Zoe does things, not least because it has a clever, if slightly unconventional, charger design.

 

[cDy]

You are correct in both assumptions:

1. The ZOE does check earth loop impedance. I believe this is because it uses the motor windings as part of the charger circuit so there may be a higher risk of shorting to the body than with separate inductors.

2. Without checking the exact numbers, I believe that the ZOE checks for an impedance less than around 200 ohms. So your value is well beneath that.

In four years and two ZOEs, I've only had the check fail a couple of times, both in camp sites, where the earthing arrangements could well be questionable - so a good safety check. I imagine the change in the regs, with earthing rods (or PEN open detection circuits) being required, might result in some installations with high impedance.

 

[Jeremy Harris]

You are correct in both assumptions:

1. The ZOE does check earth loop impedance. I believe this is because it uses the motor windings as part of the charger circuit so there may be a higher risk of shorting to the body than with separate inductors.

2. Without checking the exact numbers, I believe that the ZOE checks for an impedance less than around 200 ohms. So your value is well beneath that.

In four years and two ZOEs, I've only had the check fail a couple of times, both in camp sites, where the earthing arrangements could well be questionable - so a good safety check. I imagine the change in the regs, with earthing rods (or PEN open detection circuits) being required, might result in some installations with high impedance.

Many thanks, looks like she should be fine then. The charge point tested out OK on the simulator, plus I checked it for real by charging my Tesla with it last night, so she should be all set when her Zoe arrives in a few weeks time.

Interesting that the Zoe uses 200Ω, as although that's been the recommendation in the OSG for as long as I can remember, the regs allow up to 1667Ω in principle (30mA at not more than 50 VAC touch voltage). Not uncommon to find Ra close to 200Ω in some areas, either, in my experience, so it seems possible that a bit of dry weather together with a marginal value of Ra could be enough to cause a Zoe charger to fail the earth loop impedance check if the electrode installation was a bit marginal originally. All told it's a useful feature for the charger in the car to include, though, given that it's not at all uncommon to find earth faults in installations.

I have a suspicion that, with the prevalence of TN-C-S/PME now, there may not be many younger domestic electricians that know how to properly test an earth electrode now. I got some funny looks from one a couple of years ago when I got out my 4 terminal tester. The chap had been trading as an electrician for several years and had never seen one. When I asked how he measured Ra properly he replied that he just pushed the button on his MFT, just as he would for a PME earth loop test. Trying to explain that Ra was just the impedance of the electrode to ground, plus the conductor connecting it, and not the whole earth loop, seemed not to sink in.

 

[ElectricBeagle]

I once caught it at it - Series of bipolar pulses.

https://www.speakev.com/cdn-cgi/image/format=auto,onerror=redirect,width=1920,height=1920,fit=scale-down/https://www.speakev.com/attachments/screenshot_20181207-152242-png.108340/

 

[Jrtev]

Ah the pain that is a Zoe on TT. I've seen them unable to charge on 105 ohms. Make sure it's less than 100 in dry conditions.

 

[Jeremy Harris]

Ah the pain that is a Zoe on TT. I've seen them unable to charge on 105 ohms. Make sure it's less than 100 in dry conditions.

Ra was measured at 24Ω when put I the electrode in during the very hot, dry, summer of 2013. I did an earth loop impedance test on the completed installation the day before yesterday and it was 23.4Ω at the vehicle simulator, so Ra will have been a bit lower than it was back in 2013. We're on clay here, with a stream running about 10m away from where the earth electrode is, with the top of the stream about a metre below the top of the earth electrode pit. There's just a 4ft long, 3/8" rod in there, but my guess is that the bottom of the rod is around the level of the water table.

 

[ThudnBlundr]

If it gets too dry around your earth electrode, there's always a "natural" way to add water and chemical impurities to the area (as long as it's not too public )

 

[Jeremy Harris]

If it gets too dry around your earth electrode, there's always a "natural" way to add water and chemical impurities to the area (as long as it's not too public )

I looked at a fault on a Rolec charge point that had been fitted outside a small industrial unit last year. The earth electrode pit was set into the tarmac hard standing outside, so measuring Ra was near impossible (nowhere to put the measurement electrodes). The pit was a bit loose in the tarmac, so I pulled it and it came out of the ground, complete with the electrode. A quick look down the hole showed that the thing was in what looked like a pile of rubble, so it was no wonder that the earth loop impedance was sky high. Being an industrial installation, this was the earth for the whole place. I reckon the only way the installer could possibly have got the thing to give an acceptable reading would have been to pour buckets of water over it, although I'm not convinced it had ever been properly tested.

The daft thing was that there was a field behind the industrial unit (it was on a farm), and the main board was on the rear wall, adjacent to the field. No idea why the installer chose to put the pit out the front and run a long CPC back to the board. I re-used the old pit, fitted a new 5/8" rod in the reasonably good soil around the back and got Ra down to around 50Ω, IIRC. Sadly this didn't fix the charge point fault, usual Rolec crap, this time a burned out contactor for a change.

 

[ElectricBeagle]

If it gets too dry around your earth electrode, there's always a "natural" way to add water and chemical impurities to the area (as long as it's not too public )

I used 40 litres on a rapid, it wasn't natural, more like a Growmore solution.

 

[yoh-there]

100 ohm on the old firmware, later increased to 150.

 

[Jeremy Harris]

100 ohm on the old firmware, later increased to 150.

Very many thanks, I had a feeling that you would have the definitive answer!

Either way, it looks as if, with an Ra of 24Ω (from the 2013 installation testing) and a measured Zs when I commissioned the charge point (at the vehicle connector) of 23.4Ω a couple of days ago, the new Zoe should be happy when it arrives.

I have to get things absolutely spot on for my wife, as she's been a bit nervous about the switch to an EV (despite me having owned EVs for some time now). I had to drive out to a public rapid charger in my Tesla, and walk her through the process of charging from that last week, and this afternoon I did the same thing with her at home, getting her to learn how to use the new charge point I've just installed for her, albeit with my Tesla, rather than the Zoe. The last thing I need is for her to get home with the new car and find she has a charging problem, it could undo all the effort I've put in to getting her to make the change to an EV!

 

[yoh-there]

My only advice to her would be: ram the plug in. Don't hesitate, don't treat it like a pet, just shove it.

Thanks for the stated expectation and like ;-)

 

[ElectricBeagle]

My only advice to her would be: ram the plug in. Don't hesitate, don't treat it like a pet, just shove it.

Thanks for the stated expectation and like ;-)

.. and the car?

 

[JohnMars]

Very many thanks, I had a feeling that you would have the definitive answer!

Either way, it looks as if, with an Ra of 24Ω (from the 2013 installation testing) and a measured Zs when I commissioned the charge point (at the vehicle connector) of 23.4Ω a couple of days ago, the new Zoe should be happy when it arrives.

I have to get things absolutely spot on for my wife, as she's been a bit nervous about the switch to an EV (despite me having owned EVs for some time now). I had to drive out to a public rapid charger in my Tesla, and walk her through the process of charging from that last week, and this afternoon I did the same thing with her at home, getting her to learn how to use the new charge point I've just installed for her, albeit with my Tesla, rather than the Zoe. The last thing I need is for her to get home with the new car and find she has a charging problem, it could undo all the effort I've put in to getting her to make the change to an EV!

I hear your pain, I have had to spend the last few days coaching my other half on how to charge my leaf as his ICE went wrong.

I had to turn off all the charge timers and disable the auto lock feature as I knew just one problem and it would be back to an ICE without thinking about what went wrong.

I have been more nervous that him, having to explain he’s not going to run out of battery after 20 miles and my battery % showing 80%