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Good question. Because it would trip the RCD everytime the PEN floated fractionally.
Why?

I have a stupid & naive question. Why can't the earth rod be connected to the PME earth?
Because PME is earthed away from the property. If that earth fails then everyone downstream of the break will have a PEN that floats toward 230V, which is the problem this car charge earthing malarkey is trying to address.
If you have a local earth rod that is connected to the PEN then in the event of that fault, and possibly/probably in normal operation, large currents will (try) and use your earth rod as their way home. That can get quite interesting - in the Chinese way. And nobody wants that.
 

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I have a stupid & naive question. Why can't the earth rod be connected to the PME earth? We have an attached garage with the meter cupboard inside it. Therefore the garage is within the equipotential zone. This is fine for charging the car inside the garage but if charging just outside on the gravel drive, either fails to comply with the 18th Edition or worse poses a risk of shock. The gravel drive is not insulated from the garage slab so why not treat as an extraneous conductive part?. In this case I can't see that hybrid TT system is a solution.
Dry gravel is not very conductive, but your variation would work if you have a conductive mat (say some sheets of concrete reinforcing mesh) under the drive gravel, connected to PME earth so as to extend the equipotential zone to include the area of the drive (possibly would need to extend up to 2 metres outside where your car may be parked).

Such earthing mats would need bonding conductors of a size equivalent to those specified for incoming metallic service pipes as on a wet day if the incoming combined Neutral/Earth went open circuit the wires to the earthing mats could be providing the return path for all the electricity you are using!

Good question. Because it would trip the RCD everytime the PEN floated fractionally.
The RCD would not trip because it is only measuring the difference between the currents flowing in the Phase and Neutral wires in the circuit(s) it is protecting. The connection between the Neutral and Earth in a house with a PME supply is on the supply side of the RCD, so any currents flowing from the PME earthing terminal to the above mentioned earthing mats, or to any incoming metal service pipes (gas, water) will not be measured by the RCD.
 

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Well thanks for clearing that up. Complicated isn’t it? Does that mean that without either the Matte or the PEN relay, you have to decide whether you charge the EV inside the equipotential zone or outside? You can’t do both with the same earthing arrangement. I though the big problem with EV cars was range anxiety and charging away from home.
 

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Does that mean that without either the Matte or the PEN relay, you have to decide whether you charge the EV inside the equipotential zone or outside? You can’t do both with the same earthing arrangement.
It's risk management, it's not black and white. Likely usage is a factor.
For a garage charge point, if the point is installed in a location where it could be reasonably expected to be used to charge a car outside (e.g. it's fitted by the main door), then the special earthing arrangements will usually apply.
If it is located where it would not be expected to charge a car outside (eg. right at the back of the garage) then it would be earthed the same as the garage electrics.
If it's partway along the garage ... judgement call :unsure: :)
 

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The device by MATT:E is called O:pEN (phase, earth, neutral) I'm guessing.
Spark saying it's safer is looking for an easy life.
It can also cause issues if the supply voltage rises above 253v. It shuts down the system until such time as it drops back down. Some may think that's a good thing but if it keeps your car from charging when you want it to then not so good.
So its no good for me, being located near to a rural substation I have observed the supply voltage rise overnight to over 253.0v - the substation is probably still set to work to the old 240v +/- 6% voltage limits (225.6v to 254.4v).
 

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So its no good for me, being located near to a rural substation I have observed the supply voltage rise overnight to over 253.0v - the substation is probably still set to work to the old 240v +/- 6% voltage limits (225.6v to 254.4v).
Almost certainly, probably nothing has been changed since CENELEC. Even pre CENELEC the DNO played fast and loose with the limits.
 

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It is this device:



To give you an idea of cost it is available from CEF amongst others for £150 ish.

I still contend that this type of advice is not safe in all circumstances - particularly failure of the PEN between your property and the sub-station. It does however seem to meet the proposed amendments to the regulations.
 

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Renault ZOE ZE40 i Dynamic Nav June 2017
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We have a charger installed under the 17th edition regulations, with no earth spike, so I wanted to understand the risk of not having one. It's never a bad idea to be an informed consumer. My understanding is that if the supply neutral is broken and our EV is connected then the bodywork could rise to mains voltage presenting an electrocution risk. It's that risk that section 722 of the 18th regs set out to address.

On the other hand, with no neutral nothing would work in the house and the lights on my EVSE would not be illuminated so the failure is quite obvious. So I think you avoid the risk by checking that the lights on the EVSE are on before plugging or unplugging the car, or touching the car whilst plugged in. But I'd think the risk of just the neutral supply failing by itself must be pretty low. Happy to be corrected by an electrician if I'm wrong (I'm only an electronics engineer:)).

Here are some relevant videos on the subject and the recent amendment that I found useful. Your electrician should understand all of this. It's not clear to me that there is a really good solution at present - hence the recent amendment to allow some options. It does look as though running the EVSE connected only to its own earth spike (a TT connection) doesn't work (needs about 2 ohms to ground!). The videos show some of the problems and potential solutions:

TN-C-S Danger - Broken PEN Conductor (Combined Earth & Neutral)

BS7671 Section 722 Electric Vehicle Charging Earthing Arrangements

18th Edition Breakdown - Q16 - Earth Electrodes and Electric Vehicle Charging Points

Q 16a 18th Breakdown - EV and Earth Electrodes Part 2

BS7671 18th Edition Amendment 1- Section 722 Electric Vehicle Charging
 

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Hi, I too am an electronics engineer, not a qualified electrician. I installed my Ohme EVSE unit ("charger") myself, with the help of an electrician friend of mine (to perform the final connection to my meter box). He's not trained in EVSE installations and didn't understand the issues around the earth rod. I had my installation verified by the "official" installer, who measured everything and declared the installation well within regs (my earth spike was 60 ohms to the house earth). My understanding of the reason for the earth spike in rev 18 of the regs is so that the EVSE and RCBO feeding it, can detect the "loss of PEN" condition that you mention. Without the earth spike, the whole installation will just jump to 240v (with respect to ground) but no current will flow in any of the earth leakage detectors so no isolation switches will trip. With a separate, definate earth reference, any fault in the supply earth or neutral will be detected, as the unit has a 60ohm (regs say must be less than 200ohms) path to true ground (the same ground you're standing on when touching the car) which is enough to conduct trip current for the safety isolation switches. The loss of PEN is very rare, but according to youtube electrician channels, it does happen occasionally. Now my EVSE has two earth references - the neutral from supply (which is connected to earth at the meter box - TN-C-S) and the separate "true" earth from the spike, so can detect all anticipated fault conditions. It cost about a tenner in parts and took about an hour to put in, so no hardship for the extra safety it affords, IMHO.
 

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BTW, it's obvious that, notwithstanding earth spikes and the like, one should NEVER wash the car while plugged in!! (I avoid this potential danger by simply never washing the car ;-))
 

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My understanding of the reason for the earth spike in rev 18 of the regs is so that the EVSE and RCBO feeding it, can detect the "loss of PEN" condition that you mention. Without the earth spike, the whole installation will just jump to 240v (with respect to ground) but no current will flow in any of the earth leakage detectors so no isolation switches will trip.
While the above is possible, in most cases that's not why the earth rod is fitted.

Where the house/building has a PME supply (with a PEN) the earth rod is typically installed to provide the EVSE with an earth connection entirely separate to and independent of the house/building earth taken from the PEN. It provides safety because if there were a PEN fault where the voltage on the PEN rose to a dangerous level, it would not affect the EVSE earth which would remain at true earth potential via its earth rod.

The 18th regs do allow for designs of EVSE installations which have an earth rod and use it only as a way of monitoring the voltage between the true earth and the earth from the PEN, disconnecting the live conductors and the protective earth to the EVSE if the voltage rises to an unsafe level. I wouldn't assume that your EV charger does this though unless it's clearly stated in the instructions/installation manual that this is what it is doing because it may just be using the rod as a separate earth as described above. The earthing connections at the EVSE are quite important.
 

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Unless of course your charger is in an attached garage and you charge your car in their, which is what I do. Daft actually because I am standing on the same piece of concrete as the driveway. There is no bonded metalwork in my garage so no difference really.
So, electricians all over the country have been banging in earth rods at quite significant time and cost, meanwhile at least one manufacturer has come up with a design that doesn’t need earth rods. Couldn’t make it up really.
 

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The uncertainties around earth rod installations do make me think that solutions like MATT-E boxes are perhaps a better answer. The downside is their continuous power consumption.
 

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The uncertainties around earth rod installations do make me think that solutions like MATT-E boxes are perhaps a better answer. The downside is their continuous power consumption.
We've just had our charger installed with the Matt-E box by a local electrician. They had just been on a course for installing chargers and were told that where there was a chance of hitting services with an earth rod, it was the better option. Really pleased with it. My friend is getting his installation done by Octopus and the installer quoted with the Matt-E by default. Clearly some installers see it as an easy job.
 

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  1. We've just had our charger installed with the Matt-E box by a local electrician. They had just been on a course for installing chargers and were told that where there was a chance of hitting services with an earth rod, it was the better option. Really pleased with it. My friend is getting his installation done by Octopus and the installer quoted with the Matt-E by default. Clearly some installers see it as an easy job.
The courses that most ordinary electricians attend are free and generally are promoted by equipment manufacturers such as Rolec: so of course they promote the installation of the same manufacturers stuff. Suspect same with Matt-e
 

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Hi all, Matt-e make a big deal of the problems installing an EVSE as TT. This is simply not the case for most domestic installations.

Now consider that a properly constructed TT EVSE is an inherently safe installation, against the hazard of a broken neutral. That is because the local TT earth is NOT in any way connected to the neutral. It doesn't rely on electronics which may drift away from calibration, may fail, generally require maintenance.

I suspect that the Matt-e device is being promoted as a nice little earner, that saves the installer the inconvenience of having to do a little site investigation.
 

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I keep thinking that an auto testing RCD with no earthing of the car may have been a much better option then the mess with earning systems. How do we even know a TT earth will remain connected for the next 30 years, as in real life only rental properties get safety checks.
 
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