Recently sat the 18th Edition Course and of the understanding EV Charging points shall be fed from a TT Supply. PME is acceptable only when feeding any other equipment ie Lamp posts Ticket Machines for example and providing Type A or Type B RCD is fitted and the Charger point is then to be separately spiked. Just looking on the attached and a company called Pod Point suggest TN-S is acceptable with no need for an earth spike. Wondered if anyone else has come across this diagram and can confirm TN-S is acceptable without the spike (see - page 2)
The prohibition/limitation on using the supplier's earth is only for PME systems - so TN-S (even perhaps PNB) is fine according to the regs.
The trouble is that unless you've got your own private transformer (or generator) you can't be sure that you've really got a true TN-S supply - the DNOs have been happily combining N and PE whenever they modify existing networks or replace cables so what appears to be TN-S at the cutout (e.g. earthing conductor connected just to the cable sheath) might really be PME once under the pavement - and so is subject to the same risks from broken CNE conductors and so on. So most people make the assumption that a DNO provided LV supply, unless specifically guaranteed, the earth terminal should be treated as PME.
Andy is correct, in practical terms, the IET Code of Practice for EV Charging Equipment Installation recommends all TN-C-S, systems and TN-S systems connected by the DNO (i.e. not private generator or transformer), are considered to be TN-C-S (i.e. PME rules apply) for the purpose of EV charging equipment installation - because either the are already, or might at a future point, become at least partly TN-C-S at some point in the distribution.
Nice write up by EV company, some good advice there. always best for supplies outside the equipotential zone to have an independent earth, but if we offer a separate earth return, and an example is a cable attached to an incoming supply sheath via a clip, this is ideal. Temporary builders supplies, which because an equipotential zone cannot be guaranteed are another example of using a separate earth return. So go ahead and feel free to use the facility.
Note that the leaflet was written in 2015, so pre-dates the 18th edition and the latest advice on EV charging. However, the advice within is still correct under the new regs, although there have been updated guidelines published by the IET, which in effect rather reduces the options if the supply is PME. But TNS is not an issue.
The problem as others have stated, is that TNS and TNC-s may be mixed on the same substation nowadays, so you need to know quite a lot about the system to be sure it is pure TNS.
The EV Charge points are planned to be installed in the service yard of an office block. The mains supply is quoted as TNS and is fed via an independant Transformer located in the service yard off the DNO network. The Transformer also has full generator back up with UPS back up covering all critical equipment on site like comms rooms. Sounds to me from the advice that we should spike it and assume TNS as a PME even though it will never be converted to a PME in future but cannot offer any guarantees.
There are, however, a number of serious safety concerns with making a separate TT earthing system for the charge points, not least:
there must be no possibilityof simultaneous contact between the vehicles on charge, and exposed- or extraneous-conductive-parts in the original installation (this is a basic requirement of BS 7671).
there must also be separation underground, between the earth electrode(s) for the charge points, and any buried metalwork and exposed conductive parts connected to the PME earthing terminal. Some DNOs have minimum separation requirements of as much as 3.6 m. I would also be wary about the earthing arrangements for the HV transfromer.
An alternative is to use an isolating transformer for each charge point, but this comes with a cost.
To complete this installation safely, sounds like it's not an east one. It's definitely worth getting a handle on the earthing arrangements ... as has been said, working to the Code of Practice strongly recommended.
It is a tricky one. We will take a good look at the COP and also the earthing arrangements before we make a final decision. Many thanks for the information.
Actually reading your description of the transformer and gensets, it sounds to me like one of the few cases where it is always going to be TNS, and you can proceed to use the system earth.
Nothing to stop you leaving a note at the origin saying that the charging points use the TNS earth, and if this is ever changed the earthing of the charge points must be re-visited. In all systems you can bond an additional earth rod to the supply earth at any point, much as you can add a water pipe bond at any point, so you can be TT ready but linked to the TNS if you wish.
