TN-S Earthing for Electric Vehicle Charging

Question

Hi 
 If a building has a TN-S earthing arrangement (and the utility company have confirmed it won't be amended to a TN-C-S in the future), does this remove the requirement for O-PEN devices or an earth electrode for an electric vehicle charger. 
 Thanks

mapj1 · Accepted Answer

Yes - there is no longer a credible supply-side condition that could cause the body of the car to float to full mains voltage relative to true earth without some kind of ADS operation so those requirements do not apply. It must be said that neither electrode not OPEN detector would do any harm if fitted however. 
 It must be said that round here to secure such an assurance from a DNO would be unusual, as they are very much in the habit of performing a kind of PME conversion, when neutral and earth get combined, at repairs when TN-S street cables fail. If nothing else it saves them keeping spare reels of both 5 core and 4 core, and makes it easier for them to ensure lower earth impedances/ voltage drops without significant cable replacement. So the advice at least round here for normal domestic situations (Hants) is that it is better to treat TN-S as if it might one day become TNC-s anyway.(Or in reality, to push the nomenclature, perhaps more of a TNS-C-S-s - where the small letters are consumer side and the capitals are what is in the street beneath your feet as you walk along.) 
 However, if you have a private transformer, then you are in change of the what happens to it, so a TNS system can be guaranteed, then go for it. 
 Mike.

gkenyon · Answer

mapj1 said: Yes - there is no longer a credible supply-side condition that could cause the body of the car to float to full mains voltage relative to true earth without some kind of ADS operation so those requirements do not apply. 
 Except that, just as PEN conductors can "break" over time, so can the protective conductor in a similarly constructed cable for TN-S systems ... and then "leakage" (action of noise filters) - or diverted neutral currents through shared extraneous-conductive-parts with PME installations - could also lead to hazardous touch-voltages and touch-currents. In this case, there would probably not be lights dimming to alert the installation owner, or users ... and the risk is probably less if there are extraneous-conductive-parts and/or fortuitous connection of exposed-conductive-parts with Earth, because hopefully we are talking currents of the order of mA, or a few A, not lots A as with the broken PEN situation. However, I would agree that Regulation 722.411.4.1 would not apply to a TN-S installation from a private transformer, or (as per the IET Code of Practice) where the DNO has stated it will never be converted to PME.

gkenyon · Answer

Electron666 said: I was trying to steer any responses away from the reliability of a TN-S arrangement remaining so, which is a conversation of its own. 
 Agreed ... but a criteria to be addressed according to the IET Code of Practice for EV Charging Equipment Installation, which provides what might be considered good practice on the subject.

mapj1 · Answer

True - but a TN-s with a broken cpc tends to become an accidental TT, which then requires additional faults between L and E to become really dangerous - and as car charger installations and more modern wiring generally tend to include RCDs, these second faults are also likely to be detected. The level of adventitious earthing needed to keep the touch voltage to a safe level is really very weak, and likely to be provided by incidental earthed objects, though I do agree there will be corner cases where it is not. 
 A broken PEN requires no further fault, just some loads connected L-N to become immediately dangerous and an improbably good earth to keep the touch voltage down to a safe level against a large single phase load pulling up. 
 M.

gkenyon · Answer

mapj1 said: TN-s with a broken cpc tends to become an accidental TT 
 Usually ... but only if there are extraneous-conductive-parts. A modern domestic installation may have no extraneous-conductive-parts. 
 mapj1 said: which then requires additional faults between L and E to become really dangerous - and as car charger installations and more modern wiring generally tend to include RCDs, these second faults are also likely to be detected. 
 Some "faults" are built into many items of equipment in Class Y capacitors ... and the "leakage" is low enough on each circuit not to operate the RCDs, but will sum in the interconnected protective conductors, waiting for the the conductive element (person) eventually providing the connection with Earth. Most of the time, leakage in domestic installations isn't a worry ... but if more than one installation is downstream of the break in the protective conductor, who knows? 
 mapj1 said: The level of adventitious earthing needed to keep the touch voltage to a safe level is really very weak, and likely to be provided by incidental earthed objects, though I do agree there will be corner cases where it is not. 
 Agreed. 
 mapj1 said: A broken PEN requires no further fault, just some loads connected L-N to become immediately dangerous and an improbably good earth to keep the touch voltage down to a safe level against a large single phase load pulling up. 
 I think I've illustrated a mechanism above by which a broken PE in the distribution may give rise to immediate hazardous touch currents in some domestic installations. I think it goes to show that there are compromises and issues with the three common earthing arrangements in the public distribution networks.

Electron666 · Answer

Thanks

TN-S Earthing for Electric Vehicle Charging

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