Earth Resistance Issues - Renault Z.E. Forum
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post #1 of 14 (permalink) Old 12-31-2012, 06:15 AM Thread Starter
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Earth Resistance Issues

Hi all,

I've been told that earth resistance could be a problem when installing an EV charger, and that it wouldn't become clear until the actual charge point installation took place. The advice is to try and get this checked before buying an EV!

British Gas, after two cancelled appointments, finally sent round a very nice engineer to quote on installing a charge point. In case anyone's interested, it was for 1,200 as my fuse box is on the opposite side of the house to the wall where the charger would be affixed.

The engineer and I chatted for quite a while, and he was very helpful in explaining a few things. He mentioned that the biggest uncertainty that is faced in installing a charge point is the resistance in the ground.

Apparently as well as a separate fuse board, he'll need to install an "earth spike" for that circuit too. The maximum resistance that the car/charger will function with is 150 ohms; any higher and the charge point will cease to function as a safety measure.

He said that in 3 out of 10 installations he's done (which is the extent of all installations in the South East since March 2012) it turned out that the earth resistance was too high to install a functioning charge point. This can be caused by very dry ground, the composition of the soil, or most likely of all a lot of building detritus (harcore, bricks, concrete) in the soil. He said this is particularly a problem in new-builds, which is unfortunate for me an my new-build house!

He was quite unhappy that he can't perform the test until actually doing the installation. He said it would be much better to do it as part of the quoting process, but because it requires certain equipment and more time.

He told me that he's done installations where an EV has been purchased and parked on the drive, but it's been impossible to then install a home charge point. Naturally this is an undesirable state of affairs, so his advice was to try and talk British Gas into scheduling an earth resistance test before purchasing the car.

I might try and talk to Renault to explain this problem with the process. If they do go ahead with having the finance package cover the charger installation, then surely the deposit would already have been paid on the car by the time it becomes clear that the charger won't be viable?

The engineer said that a maximum resistance of 150 ohms is actually quite low, and normally 600-700 ohms (may have misrecalled that figure) might be considered acceptable. His opinion was that the charger and car manufacturers had agreed to set a very low threshold for safety reasons, for fear of anyone having an accident and the resulting litigation and bad publicity killing the EV market.
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post #2 of 14 (permalink) Old 12-31-2012, 04:29 PM
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RE: Earth Resistance Issues

Quote:
Originally Posted by Deejay
I've been told that earth resistance could be a problem when installing an EV charger
Interesting. Can't you find a good ground in your lightning protection or will that kill the charger when a lightning strike happens.

How about the grounding of your regular electrical system? Wouldn't your safety been affected by a too-high ground resistance as well?

Regards
Umbi
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post #3 of 14 (permalink) Old 12-31-2012, 04:57 PM Thread Starter
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RE: Earth Resistance Issues

Quote:
Originally Posted by umberto
Interesting. Can't you find a good ground in your lightning protection or will that kill the charger when a lightning strike happens.
I'm guessing that would kill the charger, although I'm not aware of any lightening protection on the house.

Quote:
Originally Posted by umberto
How about the grounding of your regular electrical system? Wouldn't your safety been affected by a too-high ground resistance as well?
I'm honestly not sure - I don't know much about electricity. I gather the charging circuit needs a separate earth electrode, and it seemed that the engineer was suggesting that an earth resistance of 700 ohms would normally be considered safe, but the EV industry was erring too far on the side of caution with a maximum of 150 ohms.

I'd be interested to see Andrew*Debbie's thoughts, as they have a background in electrical engineering I believe?
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post #4 of 14 (permalink) Old 01-02-2013, 06:26 AM
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RE: Earth Resistance Issues

I read the installation manual for the Rolec wall pod and had been wondering how to make the ground work. The manual quotes requirements but doesn't say how to do it.

More later when I have time to post and I'm not so jet lagged.


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post #5 of 14 (permalink) Old 01-06-2013, 11:54 AM
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RE: Earth Resistance Issues

Finally have some time to write a reply.

I am not an electrician and have limited experience with UK wiring. This post is not installation advice. I welcome any corrections or suggestions.


The recommended earthing system for an outdoor charge pod is TT. I hadn't seen this grounding system in the US.


In US houses, the ground is connected to the neutral wire coming from the utility service connection. Many American houses do not have a ground rod at all (yikes!). If there is a ground rod, it is directly connected to the neutral line. Optionally there is a lightning suppressor connected between the hot lines and your house ground. Damage from lightning strikes is common as most houses don't have a ground rod or suppressor. Go figure.

TT stands for Twin Earth. There are two separate ground connections. One ground is back in the utility system somewhere. It is probably at the transformer back at the street level. This first ground is tied to neutral. The second ground is at the Charge Pod.

Here is a diagram google found:





The silver shaded box is the charge pod and your car. Normally power flows from the Line (L) through the load (your Zoe) and back out the Neutral.

The ground connection is for protection only. Normally there isn't any current flowing the ground. Under normal circumstances the ground doesn't do anything.

If there is a catastrophic failure, like running over the charger lead with a lawn mower, the line could be connected to the protective ground conductor. In this case, current does flow in the ground wire. A protective device, called an RCD in the UK, monitors the ground wire. If it sees over 30mA of current, the RCD disconnects the circuit. UK regulations require this to happen with in 0.2 seconds.

If the impedance between the local ground connection and the one back at the utility pole is very high, not much current will flow in the ground (dashed line in the diagram) In this case, the RCD may not disconnect the load. If true, there could be dangerous voltage present somewhere.

I'm not exactly sure what the standard for a good ground impedance would be. 200 ohms or less seems reasonable to me. But don't take my guess as correct. In theory an impedance as high as 7000 ohms would still be enough to trip the RCD, but a connection that poor won't be reliable. And you want this to be reliable.




One suggestion: Don't cover the ground rod with cement or anything waterproof. To stay under 150, the soil will likely need to be damp. If it drys out and there is a cement covering, it will be difficult to get it wet.
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post #6 of 14 (permalink) Old 01-06-2013, 05:47 PM
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RE: Earth Resistance Issues

Quote:
Originally Posted by Andrew*Debbie

I am not an electrician
Me neither. Thanks for your post.

In documents I found there is quoted that TT-systems require a measurement of the ground resistance. The recommendation here is RA*IA<50 V where RA is the resistance I'm not sure, what IA means here because the higher the ra the lower the ia will be (and lower currents should be good).

Regards
Umbi
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post #7 of 14 (permalink) Old 01-11-2013, 03:59 AM Thread Starter
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RE: Earth Resistance Issues

As mentioned on another thread on this board, the nice people behind the @RenaultZE twitter feed spotted me griping and got someone from Renault UK to get in touch with me.

After telling Renault UK about the poor experience I'd had with British Gas, the next day I got a call from the Senior Operations Manager at what I was told was the new Electric Vehicles department. He's arranged to come and get the earth resistance checked, without any charge, and without committing to the quote.

I fed back to Renault how it was a bit of a daft process to only do the earth check upon the installation visit, so hopefully this will get resolved by the time they start marketing the Zoe.
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post #8 of 14 (permalink) Old 01-15-2013, 08:21 AM Thread Starter
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RE: Earth Resistance Issues

British Gas visited today to check the earth resistance, and I'm pleased to say the reading they got was 59.5 ohms, which is way below the maximum allowable.

Glad to see them do this test, and I hope it's a default part of the process for all subsequent potential EV purchasers.

Now all I need is for Renault to hurry up and release the Zoe...
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post #9 of 14 (permalink) Old 01-15-2013, 10:34 AM
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RE: Earth Resistance Issues

Quote:
Originally Posted by umberto

I'm not sure, what IA means here because the higher the ra the lower the ia will be (and lower currents should be good).
IA is perhaps 30mA as that is What will trip an RCD.

In that case RA*.030A < 50V or

RA < 1667 ohms. In theory, this limits the ground fault voltage to < 50V which is fairly safe. I say in theory because 1667 ohms may be an intermittent connection.




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post #10 of 14 (permalink) Old 01-16-2013, 05:39 PM
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RE: Earth Resistance Issues

Quote:
Originally Posted by Andrew*Debbie
What will trip an RCD.
Thank you! That must be the solution. Then it makes sense, if the earth resistance is to high, there might be dangerous voltages and no rcd action.

Regards
Umbi

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