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Topic Title: EV Chargers
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Created On: 26 November 2018 08:11 am
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 26 November 2018 08:11 am
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MaintenanceTony

Posts: 14
Joined: 23 November 2018

Hello all, afraid this call for advice is for EV charging. We have a large metal framed building with metal cladding in places. Along the side of the building are our car parking spaces, no more than 0.5m from the building. Our supply is TNC-S.

We are making 2 (soon to be more) of these spaces for EV charging.
I've read and re-read the regs and the COP for the requirements for the EV chargers. They will be mounted on the metal cladding as we have nowhere else, they will be protected from the cars by a small barrier.

So to how we supply them. They are single phase and will be installed next year so under the 18th edition. Internal to each unit is an A curve RCD module with DC sensitivity so hoping that covers the DC leakage bit (although anyone know if the manufacturers supply the actual leakage of their vehicles ?)

The regs says we can't have them on a TNC-S system without satisfying certain points. We can't add a low resistance earth electrode to our supply as there is nowhere to put one, of course the building is metal and the metal framework is buried in the ground and bonded to the main earth bar so in a way we do have this but I can't isolate the building to measure it Next up is TT'ing the chargers, they are mounted to the metal of the building and with it being cladding it is a big area, the cars are also parked right next to the cladding, so does it make sense to have the chargers on the TNC-S system with an RCBO(type B curve) protecting the cable and the internal RCD of the unit providing the A curve protection ?

In addition we do have one post mount which is several metres away, I was going to TT this and fit earth electrodes along the trench dug out to the post, what earth electrode reading should I expect, should I go for 100ohms which is mentioned in the regs ?
I've got several companies coming to quote but overall responsibility is mine so I need to ensure this is done correctly.
Thanks for any advice it will be much appreciated !
 26 November 2018 09:41 am
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IronFreely

Posts: 445
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Originally posted by: Mainte

I've read and re-read the regs and the COP for the requirements for the EV chargers. They will be mounted on the metal cladding as we have nowhere else, they will be protected from the cars by a small barrier.

Are these metal parts correctly bonded?

So to how we supply them. They are single phase and will be installed next year so under the 18th edition. Internal to each unit is an A curve RCD module with DC sensitivity so hoping that covers the DC leakage bit (although anyone know if the manufacturers supply the actual leakage of their vehicles ?)

Usually these charging points come prepopulated with the correct RCD/RCBO and OCPD in accordance with 722.531 of BS7671 (I say usually, I've never seen one that wasn't prepopulated correctly) so I shouldn't worry about that.
You're extremely unlikely to meet the requirements for PME as it's only permitted for 3Phase if my interpretation of the requirements is correct.

The regs says we can't have them on a TNC-S system without satisfying certain points. We can't add a low resistance earth electrode to our supply as there is nowhere to put one, of course the building is metal and the metal framework is buried in the ground and bonded to the main earth bar so in a way we do have this but I can't isolate the building to measure it Next up is TT'ing the chargers, they are mounted to the metal of the building and with it being cladding it is a big area, the cars are also parked right next to the cladding, so does it make sense to have the chargers on the TNC-S system with an RCBO(type B curve) protecting the cable and the internal RCD of the unit providing the A curve protection ?

Two RCDs in line is usually frowned upon unless the upfront one is of the delay type to provide discrimination.

In addition we do have one post mount which is several metres away, I was going to TT this and fit earth electrodes along the trench dug out to the post, what earth electrode reading should I expect, should I go for 100ohms which is mentioned in the regs ?

I thought you said you can't add an Earth electrode? That's what TT is, it's an Earth electrode, an Ra of less than 100ohm is sufficient, others may have better advice than me but I suspect multiple earthrods is a bit of an over compensation when you're only 0.5m from the supply.

I've got several companies coming to quote but overall responsibility is mine so I need to ensure this is done correctly.

Then don't worry, so long as they provide an installation certificate in accordance with the 18th edition then you have plausible deniability and almost all the liability lies with the contractors who install, verify and certify the installation.

Thanks for any advice it will be much appreciated !


I'd just let the installers get on with it Mate, just go for a reputable company with expirience in installing EV points and it's up to them to asses the individual requirements and certify it

Edited: 26 November 2018 at 09:52 am by IronFreely
 26 November 2018 10:10 am
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chrispearson

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TT-ing the EV points would leave two simultaneously accessible earthing systems, so that's out.

If the supply to the building is three phase, and if the extra load of a car would be a small proportion of the total such that the phases remain reasonably balanced, option (i) of 722.411.4.1 may be achievable - see Annex A722.
 26 November 2018 10:10 am
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mapj1

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This sort of thing is going to become a common question I think, and although there is a COP it is not clear that the advice it gives is ideal for such situations.
The reasoning behind the need for TT is that someone touching the car will presumably have their feet on the ground beside it, and we wish their hands and feet to be at the same potential, or as near as we can manage.

The risk is felt to be higher than that of a socket supplying garden mower, which is occasional use, and not that likely to be class one anyway, and higher risk than a PME earthed bus stop or lamp post , because apparently, folk touch cars more often, (though I'm not sure how true that really is), and (this is certainly true) the currents involved in charging a car are higher, and there is potential for a larger N-E offset voltage than in the case of a few hundred watts of lighting.

BUT, in your case, you already have a PME wired building, presumably already with a large load current inside, with its outer PME bonded metal body exposed.

However, if there is a path or paved carpark beside it, then anyone leaning on the building is at the PME earth voltage anyway, as the floor is not that conductive.

Even if you were to drive a row of electrodes in along side the building, if the foundation steels are buried, then the spikes will end up more or less connected to it, via a lower resistance than to the general terra-firma mass of earth hundreds of yards away.

What is your local soil like? - if you are not sure, maybe drive a short test rod a good few rod lengths from the building and any lamp posts or metal fencing that may extend the PME earth and distort the results, and measure it as you would Zs. (no bare feet while you do this please) Any contractor who is taking this seriously will either want to do this or ask you the same questions, so its a useful exercise when the weather is agreeable.

Although you think you can't, it may actually be easier to add a number of electrodes to the existing PME system, than to create TT islands or even transformer isolated TN-S islands, that cannot be made out of the reach of the PME metalwork. (out of reach is really a couple of metres in this context.)
Now a question from left field - how is lightning protection of your metal building organised ?, as assumming that involves electrodes, they should have break-points that allow the impedance to be verified..

I am afraid this is not an answer, but may help frame the questions that eventually lead to an answer.
Understanding that hopefully means you can distinguish quotes from 'speedy bodgitt and leggit' from a company that propose a proper solution.

-------------------------
regards Mike


Edited: 26 November 2018 at 10:17 am by mapj1
 26 November 2018 11:17 am
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AJJewsbury

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I suppose the "on paper" answer is to TT the charge points (as an east way to comply with the requirements of 18th 722) and then bond the TT earthing system system to any simultaneously accessible extraneous-conductive-parts (e.g. the building cladding) in accordance with the general requirements of section 411.

Naturally that defeats the sprit of not using the PME earthing facility - but isn't really an unusual situation wth TT installations in general (Zs dropping like a brick when bonding is connected due to shared metallic pipework to next door which is on PME is relatively common) and no-one protests at that.

As far as I can see that allows a common sense solution while still meeting the letter of the regs of not using the PME facility to as a means of earthing.

Radical?

(The other solution would be to TT the entire installation...)

- Andy.
 26 November 2018 12:08 pm
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gkenyon

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Originally posted by: AJJewsbury

I suppose the "on paper" answer is to TT the charge points (as an east way to comply with the requirements of 18th 722) and then bond the TT earthing system system to any simultaneously accessible extraneous-conductive-parts (e.g. the building cladding) in accordance with the general requirements of section 411.
Noooo ... If that is done, then you will still be required to comply with 722.411.4.1, as the bonding will connect the PE of the charge-point to the PME earth.



Naturally that defeats the sprit of not using the PME earthing facility - but isn't really an unusual situation wth TT installations in general (Zs dropping like a brick when bonding is connected due to shared metallic pipework to next door which is on PME is relatively common) and no-one protests at that.



As far as I can see that allows a common sense solution while still meeting the letter of the regs of not using the PME facility to as a means of earthing.



Radical?
It's not an acceptable method of complying with BS 7671. This is discussed in the CoP.

As Mike has said, it's possible the existing building complies with condition 722.411.4.1 (ii) - provided of course the building structural elements satisfy the requirements of the general rules for an earth electrode, AND the resistance is low enough.



(The other solution would be to TT the entire installation...)
That might bring about more problems than it solves ... for example, revisit all protective devices providing ADS and isolation ... and might depend on what other PME installations are in close proximity (a good example here, although on smaller installations, is that it's really not a good idea to TT a semi-detached house if next door is still PME).

Options that have not been discussed:

- Isolation transformer for each charging point, as discussed in the CoP (adds ££££).

- Charging point equipment with the device discussed in 722.411.4.1(iii) built-in. I am led to believe that these are starting to become available ... but I have not had the opportunity to see them, or assess any details that might enable me to pass further comment.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 26 November 2018 12:30 pm
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AJJewsbury

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Noooo ... If that is done, then you will still be required to comply with 722.411.4.1, as the bonding will connect the PE of the charge-point to the PME earth.

I'm still playing devil's advocate!

Which words of 722.411.4.1 aren't being complied with? The PME system isn't being used as the means of earthing - that's being provided by a local electrode as normal for a TT system (In BS 7671 terms bonding conductors don't become earthing conductors just because their overall impedance is lower). I can see nothing that precludes bonding of extraneous-conductve-parts regardless of how they attain their earthy potential.

I'm not sure how an isolating transformer would help - if you built a local TN-S system you'd still need an earth reference (back to the same problem); if you had a separated (or IT) system them some EVs wouldn't charge as the earth loop impedance would be too high. That only leaves a sort of hybrid system with the EVSE's c.p.c. connected to the isolated N but not earth - which AFAIK isn't an approach that BS 7671 recognises.

- Andy.
 26 November 2018 01:00 pm
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MaintenanceTony

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Thanks for the replies guys. To answer a few we don't have any soil local the whole site is tarmac or paving which I guess does negate a bit of the risks involved.
I don't think TT'ing the whole installation is practical either so...

I've read about the devices mentioned in BS7671, am I right in thinking they would connect to the earth of the EV and the PME earth and look for a potential difference and if that difference is too high they would drop out the supply to the EV point ? Would they be as simple as a 4 pole contactor inside the charge point ?

Our on site contractors have discussed this with their NICEIC engineer and they would leave the charger on the TNC-S system because it is fitted on the metal cladding.
 26 November 2018 01:29 pm
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Zoomup

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Annex A722.3 shows a formula to ascertain Raev. It appears that this figure will have to be below one Ohm. Raev is the sum of the resistances of the earth electrode and the protective conductor connecting it to the main earthing terminal of the installation in Ohms. This may be difficult to reliably achieve in practice I feel, where soil conditions can very in freezing or very dry times.

Z.
 26 November 2018 01:30 pm
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gkenyon

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Originally posted by: MaintenanceTony
Our on site contractors have discussed this with their NICEIC engineer and they would leave the charger on the TNC-S system because it is fitted on the metal cladding.
That's very interesting - I can see the point for the charger, but the vehicle is then also connected to the outlet, and that's not connected to the building cladding.

I am left wondering whether they would advise the same for a mobile/transportable unit to Section 717 (711.411.4) or a caravan (see 708.411.4) - both of which the same argument could be applied to, and are very similar situations with respect to touch voltage (except that potentially, the car charger might have more power delivery and therefore higher touch voltage under these circumstances)?

However, as discussed earlier, 722.411.4.1 (ii) may well be complied with, and in this particular circumstance, that would make it compliant and there would be no issue.

And further, if there are any exposed-conductive-parts of the charging equipment, to go on a separate earthing system, there would have to be some means provided to prevent simultaneous contact with the building cladding - and in the case of isolation transformer, insulation provided to prevent physical contact between the two earthing systems.

If I were the contractor, I'd certainly be looking at the availability of the device in 711.411.4.1 (iii), or checking the building structural metalwork can be used to demonstrate 711.411.4.1 (ii).

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com

Edited: 26 November 2018 at 01:44 pm by gkenyon
 26 November 2018 01:40 pm
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gkenyon

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Originally posted by: AJJewsbury

Noooo ... If that is done, then you will still be required to comply with 722.411.4.1, as the bonding will connect the PE of the charge-point to the PME earth.


I'm still playing devil's advocate!



Which words of 722.411.4.1 aren't being complied with? The PME system isn't being used as the means of earthing - that's being provided by a local electrode as normal for a TT system (In BS 7671 terms bonding conductors don't become earthing conductors just because their overall impedance is lower). I can see nothing that precludes bonding of extraneous-conductve-parts regardless of how they attain their earthy potential.
I see where you are coming from, but have to disagree with the logic.

(a) You have an earth electrode as the means of earthing. This is directly connected to the PME earth, therefore the PME earth also forms part of the means of earthing.

(b) when you form a TT system for part of an installation, that is a separate electrical system (perhaps even separate "installation") as it has a separate means of earthing. Interconnecting the two earthing systems (even if you deem it "protective bonding") should comply with other parts of the general rules, and effectively bonding the two makes them part of the "same earthing system".

I'm sure there can be arguments until the cows come home on this, but I don't think it's that hard.



I'm not sure how an isolating transformer would help - if you built a local TN-S system you'd still need an earth reference (back to the same problem); if you had a separated (or IT) system them some EVs wouldn't charge as the earth loop impedance would be too high. That only leaves a sort of hybrid system with the EVSE's c.p.c. connected to the isolated N but not earth - which AFAIK isn't an approach that BS 7671 recognises.



- Andy.
See IET CoP for details ... this has been in there since the 1st Edition. BS 7430 does appear to permit this arrangement.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 26 November 2018 01:59 pm
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mapj1

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I'm not sure how an isolating transformer would help
I agree. It is exactly no more help than having a private genset and either TT-islanding, not allowed as within reach of the PME building, or having a private genset and cross linking the neutral of it to the PME earth, whcih effectvely puts the car body back at the PME earth potential again.

I'm with the NIC man in this case, to have the exposed bits of the cars and the exposed metal bits of the building at the same voltage, even if it is the PME earth, is the safest option. I'm however well aware that is not really what the COP says.
The fact the ground is tarmac re-inforces this - in effect there is no way even a barefoot person can easily be more in touch with terra-firma than with the building and drilling a half inch hole in the tarmac to put an electrode in may be quite instructive, but is arguably introducing a small risk where there was not so much of one before.


As an aside asphalt/ tarmac and bitmac (your car park will be one of these) are all fairly good insulators compared to soils, more comparable to dry sand and gravel mixes.
A typical range of text book asphalt resistivity values might be
a- Dry Conditions: (0.2 to3.0)x1E7 Ohm-meter
b- Wet Conditions: (1.0 to 600)x 1E4 Ohm-meter.

The lowest wet weather value of 10,000 Ohm-m is sometimes used to determine step & touch potentials. (10 k ohms between the faces of a metre cube of the stuff, or the same for about a foot square 4 inches thick).
Realisitically, this gives more than 23k to terra- firma for a pair of (even quite large) bare feet on a car park of a few inches thick, so shock current quite limited, and really, less than 10mA.)


It is intersting to note the assumptions of
Shoe resistance = 4000 ohm/shoe
High resistivity chippings resistance = 2000 ohm/shoe
Tarmac = 30,000 ohm/shoe
Dry concrete = 6,000 ohm/shoe.
in this
western power document about workers and exposed voltages at substations.

-------------------------
regards Mike


Edited: 26 November 2018 at 02:08 pm by mapj1
 26 November 2018 02:13 pm
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gkenyon

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I don't disagree, but I'm bemused that one of the options (ii) or (iii) was not included as a caveat in the advice.

Otherwise, the installer would have to declare a departure and provide justification for "no less safe" - Reg 120.3

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 26 November 2018 03:01 pm
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Zoomup

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Whilst walking down to a local shop to buy some frozen peas and other food, I walked past a small local substation. Its ground is covered in stones. I was about to suggest an insulating surface around the E.V. car charging stations but Mike beat me to it. Also why not have insulated enclosures around the E.V.s charging points so that it is impossible to touch the E.V. and the building's conductive cladding at the same time. The conductive cladding that is immediately touchable could have insulating panels attached to prevent it applying a Voltage to a person in that location. It would not cost much.

Z.
 26 November 2018 03:28 pm
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mapj1

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well,

Regulation 722.411.4.1(ii) requires a very low resistance earth electrode to mitigate the effects of an open circuit PEN conductor fault on the supply.
If the building supply is good for a few hundred amps this will be 'hard', even using the permitted rise of 70V rather than 25V or 50V we normally would like.
This is in effect re-creating the reg 542.1.201 that (perhaps thankfully) did not survive past the BS7671 draft for consultation document, that required additional earth electrodes, supplementing any earthing facility provided by the electricity supplier, to prevent the risk of dangerous touch voltage in the event of the loss of the main connection to earth. For the same reasons, it is not practical on much more than a very small supply

Even if the desired sub-ohm electrode resistance could be achieved at the premises, it is not clear that the electrode resistance at the substation end would be good enough to make the loop work as intended.

Regulation 722.411.4.1(iii) refers to protection by an earth voltage operated device that opens the CPC as well as L and N.
This device will also require an earth electrode, alebeit a small one, and would be quite practical, but most importantly, has anyone ever seen one ?

-------------------------
regards Mike
 26 November 2018 03:37 pm
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Zoomup

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Couldn't we use an old design V.O.E.L.C.B. with a contactor to disconnect all poles and the C.P.C? They are still available on certain auction sites, although they are a bit dated, but sometimes unused ones surface.

Z.
 26 November 2018 03:47 pm
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chrispearson

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What about option (i), folks?
 26 November 2018 04:35 pm
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AJJewsbury

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What about option (i), folks?

Wouldn't you need a pretty large (compared to the EVSE), and always-on, 3-phase load (or balanced group of single phase loads) to achieve that?

(a) You have an earth electrode as the means of earthing. This is directly connected to the PME earth, therefore the PME earth also forms part of the means of earthing.

I think there are (administrative) dragons in that way thinking - by that logic a bonded metallic gas pipe could contribute to the means of earthing - yet it's prohibited to use gas pipes for such a purpose. Yes the physics doesn't care, but BS 7671 has clear distinctions between earthing and bonding. If nothing else it's an interesting thought experiment that an arrangement that obviously doesn't provide 722's intended results does seem to meet the required wording.

Couldn't we use an old design V.O.E.L.C.B. with a contactor to disconnect all poles and the C.P.C? They are still available on certain auction sites, although they are a bit dated, but sometimes unused ones surface.

In principle yes, but in practice you'd probably want an arrangement that unfailingly made PE first and broke it last - including in situations where one of the contacts was welded. Old VOELCBs intended to switch only live conductors wouldn't have that feature.

- Andy.
 26 November 2018 04:45 pm
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chrispearson

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Originally posted by: AJJewsbury

What about option (i), folks?


Wouldn't you need a pretty large (compared to the EVSE), and always-on, 3-phase load (or balanced group of single phase loads) to achieve that?


As I suggested above. At the moment all we know is that there is a large metal-framed building with several car parking spaces. We haven't been told about its use or consumption.
 26 November 2018 04:51 pm
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MaintenanceTony

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Again thanks guys, not sure if I am more confused or not than when I started. This must be a quite common thing and something that needs raising with the powers that be....?

Trying to come up with a solution if we move the charge units away from the building and give them their own TT supply and install some earth rods what reading for the earth rods are we aiming for ?, I guess this is without the issue of what type of RCBO, some of the units we have looked at have DC sensitive ones installed
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