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Earth rod for EV charger

Discussion in 'Electrics UK' started by Observer, 16 Oct 2018.

  1. Observer

    Observer

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    My house has TN-C-S earth system. My electrician recently installed a new 32A power supply on external garage wall to charge an EV. He drew my attention to a section of the 17th Edition regs, that he interpreted to mean that an additional earth rod was needed for the installation because either the supply point or the vehicle being charged (in my case, both) was located outdoors.

    A separate earth rod was duly installed adjacent to the meter/cutout and connected to the MET.

    Does this interpretation and the consequent earth rod installation follow the consensus of well-informed on this board?
     
  2. flameport

    flameport

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    Perhaps.

    Exactyl what was installed, and what are the test results for the installation (on the certificate which was provided).
     
  3. Observer

    Observer

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    A 1m earth rod with a "Electical safety - do not remove" cap and 10mm2 earth cable to the MET (buried in conduit below a gravel path)

    The certificate is at home so have to check later. Do you mean the impedance value? Or something else as well?
     
  4. flameport

    flameport

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    Impedance of the earth rod. Plus test results for the new circuit and whatever else was installed.

    The purpose of such a thing is to keep the voltage on exposed metal parts at a (relatively) safe leve if the incoming PEN conductor was broken.
    Although not impossible, a single rod is rather unlikely to achieve that.
    Another option is to make the EV install TT and not connect to the TNCS earth at all, but as it's connected to the MET, that clearly isn't the case here.
     
  5. EFLImpudence

    EFLImpudence

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    Surely the purpose of an earth rod is to make the charger TT.

    Next to the meter/cut out might be far from the EV socket. What is the point of that?
     
  6. Observer

    Observer

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    It is on the other side of the house, so a long way from the EV socket.

    I was hoping for a nuanced discussion so thanks for responding. Also, what is it about an external socket for charging an EV that calls for a separate TT earth whilst an external socket for [whatever other purpose] does not?
     
  7. EFLImpudence

    EFLImpudence

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    No different really -

    I'm not well up on EV charging but I believe the car body gets earthed to the system (the house installation in your case instead of just a separated rod in the ground - TT) so that during a fault it could be at a different potential(voltage) to the ground - meaning if you touched the car and were stood on the wet ground (mud rather than a drive) you could get a shock.

    To be honest, I don't think the powers that be have thought it through completly yet.
    You have taken them by surprise.
     
  8. Observer

    Observer

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    Does seem that way, doesn't it.

    I don't get that I could charge the EV from an ordinary ring main circuit, which may or may not be RCD protected, through an external socket or an extension lead run from somewhere inside, without any questions; but if I install a separate circuit, even with RCD or RCBO protection, the regs decree that a TT earth must be added. Seems ... inconsistent.

    I don't mind the extra cost IF it is adding some reasonable degree of additional protection. But is it? Or is it addressing a risk in one possible set of circumstances while adding an additional risk in another?
     
  9. EFLImpudence

    EFLImpudence

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    Yes. Of course, they can't make up rules for extension leads - well, they could but...

    A properly installed rod may help where it is situated - but around the other side of the car, probably not and definitely not at the other side of the house.
    Either way, you would need an earthed grid under the car and extending as far as the car was reachable but they are not going to state that as a regulation. Anyway, the regulations have no authority other than requiring the installer to follow them to comply with his validation scheme.

    The car's charging system could have been made so that the body was not earthed (I don't know if any are) but the manufacturer did not do that.

    I think it is just part of something much greater that would really be needed.

    Of course, all these safety measures are only needed if/when something goes wrong. It's all a compromise.
     
  10. plugwash

    plugwash

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    The risk with having metalwork outside connected to a TN-C-S earth is if the PEN conductor fails there could be a significant voltage between that metalwork and real earth.

    The risks of using an independent TT earth are firstly that it leaves you totally reliant on RCDs for earth fault disconnection and secondly there is a risk of a significant voltage between the independent TT earth and the house earth system.
     
  11. flameport

    flameport

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    The regulation is this one - mainly (ii) as (i) will never apply in any domestic installation and (iii) which is not shown requires a device to disconnect the supply if voltage between earth and the protective conductor exceeds 70V.

    There is an exception where all of this can be ignored as seen in the text image, with the useless' reasonably practical' part. However that has been deleted in the 18th edition, so very shortly one of the three items will be required if using a TN-C-S supply.

    Installing EV systems as TT is the most common option, with their own electrode just for the EV installation and not connected to the TN-C-S earth at all. That avoids all of the below, and the electrode just needs to comply with the normal requirements for any other TT installation.

    722.411.4.1.jpg

    The problem with (ii) is that the electrode has to be of very low impedance to be a viable option.
    This is the calculation, Ra being the maximum sum of the electrode and protective conductor resistance:


    elec_res.jpg

    with the other items being Uo supply voltage (230V) and the current Iinst being the full load of the installation including the EV charger. 50 amps would be a conservative estimate, given that the charger is 30 and another 20 for whatever might be in use elsewhere. In many cases it could be far higher.

    However with a 50 amps load, the required resistance is 2 ohms, something which will be near impossible with any electrode.
    Even with a totally implausible and useless 10 amps, the resistance is a maximum of 10 ohms - still rather difficult and not going to happen with a single rod.

    Therefore for the original query, they have either got an electrode attempting to comply with 722.411.4.1(ii) which almost certainly does not, as it's impedance will be far too high.
    Or it was intended to be a TT installation (as is most commonly done), and it's all gone wrong because it's connected to the TN-C-S earth as well.
     
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  12. Observer

    Observer

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    Thanks for all that. This is a really interesting discussion. That section you quoted is the one that my electrician applied and discussed with me.

    The installation certificate for my job gives measured resistance of the rod as 74.1 ohms (I can't see anthing else on the installation certificate of much relevance). As you pointed out, that's way higher than the 2 ohms that would be calculated using the formula in A722.3 but, as I understand (from another electrician who specialises in EV chargers), is actually a pretty good reading for a earth rod and quite a bit less than the 100 ohms that (he says) is the accepted (or stipulated) maximum impedance for additional earth for EV chargers. Do you agree?

    His view also was that any earth (and certainly the one that has been achieved in my case) is better than none and that you can't have too much eartb protection - the alternative earth that it provides in the event of a PEN fault may not be perfect, or even very good, but it does offer some reasonable level of protection.

    I'd be interested to read your further thoughts.
     
  13. ban-all-sheds

    ban-all-sheds

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    He is, scarily, missing the point and being unable to read and/or understand the Wiring Regulations.

    The resistance he has got is not bad, and it is within the guidelines for what's considered reliable. For a TT earth.

    It clearly does not comply with the requirements for the local earth electrode you need with a PME earth if you want to use it for an EV charging point.

    If you look at 722.411.4.1, you do not have (i) or (ii) or (iii), so your charging point may not use your supply's PME earth.
     
  14. Observer

    Observer

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    What about "The ... regulation need not be applied if none of (i), (ii) or (iii) is reasonably practicable"?

    As I understand, it's not "reasonably practicable" to get very low resistance on a single rod, whether TT or part of PME. The second (specialist EV) electrician was quite positive in saying that what 'my' electrician actually did was a fair interpretation. He did say that 18th edition from next year will require a locally sited additional rod but that the earth from the MET would also be used.

    [edit]He laso said that there are plenty of TT installations with earth electrode resistance of 00s ohms.
     
  15. ban-all-sheds

    ban-all-sheds

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    Now it is I who can't read - I was thinking in terms of the 18th.

    But I was understanding, I think..


    It doesn't have to be very low resistance for TT.



    I wonder why it won't be allowed in 11 weeks time - are the laws of physics due to change?


    I believe that he is wrong. The "reasonably practicable" get-out clause will have been removed, so a PME earth may not be used, no matter how many local rods you have unless with the local rods you end up in compliance with (ii)


    There are. But (hopefully) there are no PME ones with a Ze that high.
     
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