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

Question

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 &ldquo;simultaneously accessible conductive parts shall be connected to the same earthing system individually, in groups or collectively.&rdquo; 
 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

GTB · Accepted Answer

Dean, 
 Obviously as you have indicated and good advice from Graham, and maybe its a remote option that could be considered. 
 And that would be to install or erect some kind of "Wall/partition" between the two EV charging bays, to prevent similtaneous contact? Might not look pretty and obviously something a car could hit if not being carefull, but if done correctly could perhaps give an option if UKPN confirm the worst, just need to consider the fire rating and combustible nature of any "Wall/partition". 
 
 Cheers GTB

gkenyon · Answer

Dean Williams said: What happens in that case? 
 Some insurers have terms that require a certain distance from combustible materials [some include shrubs, trees etc in that definition] from which EVs must be when on charge. 
 The IET Code of Practice refers to an industry publication RC59 Fire safety when charging electric vehicles , which can be freely downloaded here: https://www.thefpa.co.uk/advice-and-guidance/free-documents?=%20Medium-sized%20Businesses,%20MAINTENANCE%20CHECKLISTS%20-%20APPENDIX%20TEMPLATE%20FORMS,%20CONTROL%20OF%20DUST%20EXPLOSIONS&q=RC59%20FIRE%20SAFETY%20WHEN%20CHARGING%20ELECTRIC%20VEHICLES 
 The difficulty for designers is that insurance company terms vary, and they don't all align fully to RC59 ... so this is something that should be discussed with the Client(s) prior to the design being finalised.

Alan B · Answer

Just looking at this discussion from a practical point of view with a view to ADS. 
 The supply to the EV charge points will have RCD protection on their supply, probably also a built in RCD. They will also have open pen detection, which should mean that if there was a significant issue with earthing due to an open pen conductor they should shut down. The RCD protection means that there only needs to be 30mA flowing to earth for the RCD to trip, I am struggle to think of a scenario where there would be a situation where the earthing of the two properties is separated to such an extent that 30mA will not flow under fault conditions, especially with open pen detection in place. I can see that where RCD protection is not included and much higher currents are required for ADS having a guaranteed common earth is sensibly required. Another approach that could be considered is looking at Indra chargers, they have an operating mode where they measure the voltage difference between their main earth and a reference electrode, if this increases beyond 30V the charger shuts down.

gkenyon · Answer

Dean Williams said: 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. 
 Dean Williams said: 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?

Dean Williams said: 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.

gkenyon · Answer

Dean Williams said: 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. 
 Dean Williams said: 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)?

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

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