Earthing Rods for EV chargers

They are to provide protection if there’s a PEN conductor fault in the distribution network.

There are two variations, an earth rod with a very low resistance to hold down touch voltage on earthed metalwork, being the vehicle being charged and a reference rod to measure voltage to true earth.

Providing correctly installed earth rods to hold down the touch voltage if there is a broken PEN conductor is not generally achievable, the means of protection that now being used is Open-PEN Protection using voltage measurement within the EVSE which is also preferred to voltage measurement using a reference earth rod.

If an Open-PEN is detected all lives, neutral and earth are disconnected at the EVSE, so there isn’t a high touch voltage of the vehicle body.

johnwicklover said:

Why is earthing rod needed in a TN-C-S System but not required in a TNS system?

Use of an earth electrode for TN-C-S is not mandatory.

There are restrictions for installation of EV charging equipment in installations with TN-C-S ("PME") systems in the UK, to protect people from a shock risk from the body of a car, due to certain faults in the LV supply network in the UK.

The options available to the installer of EV charging equipment, if there is a PME supply, according to BS 7671:2018+A2:2022 are:

• Use the option in BS 7671 Regulation 722.411.4.1 (ii) ... which permits an earth electrode to be used to limit touch voltage in a fault. Sadly, this requires an earth electrode resistance so low that it is only possible to achieve in certain installations (for example, where there is significant structural steelwork that can be used as the electrode).
• Use the option in BS 7671 Regulation 722.411.4.1 (iii), (iv) or (v) ... which permits fitting of a device to help detect and disconnect the vehicle from the supply (and supply earthing) if specific faults occur in the distribution network. Some of those devices may use a functional earth electrode a sufficient distance from all buried metalwork and exposed conductors connected to the PME earthing arrangement
• Provide a separate TT earthing arrangement for the EV charging equipment, which requires a consumer's earth electrode, and separation of earthing systems between TT and PME.
• Use an isolating transformer to supply the charging point as described in Annex A722 of BS 7671.

If the installation has a TN-S earthing arrangement, this problem is unlikely to exist, BUT if the supply is received at LV, then many distribution network operators in the UK may have converted part of the TN-S network to have combined neutral and earth cable, and there is no way for the installer to know.

If the earthing arrangement is TN-S, supplied from a transformer where it is guaranteed PME will not be supplied (e.g. a private HV to LV transformer), then the above precautions are not necessary.

johnwicklover said:

Could you have a earthing rod in a TNS system ?

Yes, supplementary earth electrodes can be used in TN-S systems - they can also be used in TN-C-S (PME) systems. Supplementary earth electrodes are recommended for TN systems by BS 7671, and would usually be necessary if the installation was a prosumer's electrical installation that included island mode operation.

EVSE installers seem to think that Open-PEN Protection built into the EVSE is the means of dealing with all the issues, allowing them to get in and out of a job in the shortest possible time.

But voltage measurement as an alternative means of protection is not as straightforward as it may seem.

https://youtu.be/p562IZ1nOSM

Second question first, as it is easier. You can add an earth spike in a parallel with any existing earthing system that you would be happy to bond a water main or a lamp post to - as far as the electrons are concerned it is the same.

On an EV charger there are two possible ways an electrode may be used either

1) in parallel with the supply earth to pull a local carrot of earth to the same potential as suppliers earth and the car body so it all moves together. 

2) As a TT island to pull the car body to the same potential as the local earth and the supply live and neutral can bounce about as much as it likes.

None of this matters a hoot, until the wheels come off with the supply cable or its joints  underground and the supply earth wiring is not at the same voltage as the terra-firma ground under your feet.  On a TNC-s system this is quite possible to raise the neutral and earth to a near-live voltage, while  on a TNS it is unlikely that the earth voltage will rise, but far more likely the fault loop impedance will rise.

The EV charge point may include some logic that disconnects the car (all poles including earth) if the electrode earth voltage and the supply one are more than say 20V apart. Not all do.

Mike.

 After watching that my head hurts.

Z.