SIMULTANEOUS CONTACT QUESTION - 2 x EV vehicles parked next to each other

Scenario

A remote block of 3 garages, 2 will be fed from separate properties via 40m of 10mm EV SWA connected to 2 individual new CU's

( no cars will be parked inside the garages but charged on the drive in front of the garage door)

1 property is TNC-S - Ze 0.30 Ohms and the other is visually a TNS but with Ze 0.32 Ohms and similar L-N reading.

The 2 chargers will have Pen fault protection and the relevant RCD protection as usual.

There are underground services within 1 -10m of a potential TT rod so could be problematic and a maximum 2.5m between separate garage earth rods is possible.


The concern is simultaneous contact between the 2 vehicles.

Regulation 411.3.1.1 states that “simultaneously accessible conductive parts shall be connected to the same earthing system individually, in groups or collectively.”

I am currently waiting for UK Power networks to confirm the 2 properties are connected to the same earthing system.


The questions are:

Is Pen fault and RCD protection enough when connecting both cars to TNS/TNCS or is there anything else we can do?

Is separate TT earth rods along with simultaneous risk assessment the safest option in this case?

Thank you


I've attached a sketch of the current proposal to help explain.
PDF

Parents
  • Is Pen fault and RCD protection enough when connecting both cars to TNS/TNCS or is there anything else we can do?

    No, Regulation 411.3.1.1 stipulates clearly that simultaneously-accessible exposed-conductive-parts must be connected for the same earthing system. This is required in order for automatic disconnection of supply (ADS) to work safely (regardless of whether the RCD or an OCPD is providing that automatic disconnection).

    The OPDD only protects against open-PEN faults, and has no impact on ADS.

    Is separate TT earth rods along with simultaneous risk assessment the safest option in this case?

    No ... separate TT systems alone won't help either ... both chargers would have to be on the same earthing system.

    The TT systems could work, if they were bonded together ... but if they are from separate installations, what will make sure the bonding is effective in the future, and who is responsible for maintaining the connection in the years to come?

    I am currently waiting for UK Power networks to confirm the 2 properties are connected to the same earthing system.

    That is the best option ... if they are not, then unless there's any way to prevent simultaneous contact, there doesn't appear to be a simple way of making the arrangement conformant to BS 7671.

  • Hi Graham, thank you for your help with this it is very much appreciated.

    Currently under regs am i right with the following:

    The best & safest option for the 2 clients is:

    1. DNO confirmation of both supplies being connected to the same earthing systems

    2. Pen fault/RCD  protection to both chargers?

    Failing a response from the DNO

    If we were to divorce the PME for both garages and bond both new CU's to one earth rod or condudisc flex( which has up to 4 ways available) complete with relevant labeling/EIC notes, would that satisfy BS7671 and make the installation compliant?

    Kind regards

    DeanW

  • Hi Graham, thank you for your help with this it is very much appreciated.

    Currently under regs am i right with the following:

    The best & safest option for the 2 clients is:

    1. DNO confirmation of both supplies being connected to the same earthing systems

    2. Pen fault/RCD  protection to both chargers?

    Yes, I believe that would be classed as conformant.

    If we were to divorce the PME for both garages and bond both new CU's to one earth rod or condudisc flex( which has up to 4 ways available) complete with relevant labeling/EIC notes, would that satisfy BS7671 and make the installation compliant?

    Yes, BUT ... unless there's some legal agreement from both Clients that this will be maintained, and perhaps who by (may have to go into the property deeds as a Covenant??) I'm not sure it could be relied upon ??

    I'm not a legal professional, but this arrangement of "shared earth" probably needs legal advice (what if one owner removes their charger, and the means of earthing ... leaving the other without a means of earthing, for example)?

  • Thank you once again Graham

    This project has been on my mind forever. I feel reassured that the safety of the client(s) has been optimized and our installation will be compliant to BS7671.

    Fingers crossed with UKPN!

    Kind regards

    DeanW

  • There are streets of pre-war  TT earthed houses near me and I'm sure its not the only  example in the UK, A few have car chargers, and I imagine its only a matter of time before some pair of neighbours do if not already.

    Now they all have rods near the front door, and very much not reliably overlapping electrode zones arise from those. The big shared  unofficial electrode is the lead  service pipes and cast iron water mains that link them, but slowly the lead pipes in the front gardens are changing to blue plastic, which long term may be better or worse for health, but we can be sure its a lot worse electrically as it puts more reliance on the individual household electrodes some of which are in a poor state.

    Don't think anyone installing a charger even thinks about a possible voltage offset between the CPCs of adjacent properties and I'm pretty sure not all the installers check the electrode resistances either.

    The risk is similar.

    Mike

Reply
  • There are streets of pre-war  TT earthed houses near me and I'm sure its not the only  example in the UK, A few have car chargers, and I imagine its only a matter of time before some pair of neighbours do if not already.

    Now they all have rods near the front door, and very much not reliably overlapping electrode zones arise from those. The big shared  unofficial electrode is the lead  service pipes and cast iron water mains that link them, but slowly the lead pipes in the front gardens are changing to blue plastic, which long term may be better or worse for health, but we can be sure its a lot worse electrically as it puts more reliance on the individual household electrodes some of which are in a poor state.

    Don't think anyone installing a charger even thinks about a possible voltage offset between the CPCs of adjacent properties and I'm pretty sure not all the installers check the electrode resistances either.

    The risk is similar.

    Mike

Children
  • Hi Mike

    Agreed, as installers we need discussions like this, more and more people are taking up EV's and the overlaps are inevitable.

    DeanW

  • Hi   

    Really intersting point regards the issue of possible voltage offsets. How do you view some of the present issues with cars being charged near street furniture but being supplied by a domestic property. The potential here is the fact the house is on a PME and the lampost a dedicated TN-S. The car is within the 2.5 metre distance so simultaneous contact is possible. 

    Just for my own clarity as well - I see a lot of comments around the 2 earthing systems and the possible issues with ADS, what are the other concerns here if you do not mind me asking? In respects to how you view the faults, what are the risks in simple terms? The more I look at this issue the more I find it impossible to offer a solution. 

  • Really intersting point regards the issue of possible voltage offsets. How do you view some of the present issues with cars being charged near street furniture but being supplied by a domestic property.

    If the street furniture has simultaneously-accessible exposed-conductive-parts, the IET CoP states they should be on the same earthing system, or mechanical protection/barriers etc put in place to prevent simultaneous contact.

    On-street, this assessment is absolutely necessary, regardless of whether it's a private or public supply - otherwise it doesn't conform to BS 7671 Regulation 411.3.1.1.


    The potential here is the fact the house is on a PME and the lampost a dedicated TN-S. The car is within the 2.5 metre distance so simultaneous contact is possible. 

    Wouldn't matter whether it was two separate PME systems, two TN-S, or two TT (or any combination) ... if they are not classed as the same earthing system, doesn't conform to BS 7671. This is, as stated previously, spelled out in the IET CoP for EV Charging Equipment Installation (Section 3.5):

    Just for my own clarity as well - I see a lot of comments around the 2 earthing systems and the possible issues with ADS, what are the other concerns here if you do not mind me asking? In respects to how you view the faults, what are the risks in simple terms? The more I look at this issue the more I find it impossible to offer a solution. 

    The issue is a fundamental one relating to touch potential during a fault, and disconnection times ... and hand-to-hand vs hand-to-feet touch potential. Basically, the disconnection times in Chapter 41 apply only where Regulation 411.3.3 is conformed to.

    Remember also, that it's not JUST the disconnection time of the EV circuit ... but could be a 1 s (TT) or 5 s (TN) for a distribution circuit in the same installation, which will transfer a potential through the cpc to the exposed-conductive-part.

  • Excellent as always Graham, thank you very much.