Earthing Systems (TN, TT) and EV Chargers: What Installers Must Check

Short answer: the earthing system of the supply decides the charger’s protection. TN-S is the simplest. TN-C-S (PME) needs open-PEN protection or an earth electrode, because a broken neutral can make the car’s body live. TT needs a tested earth electrode and an RCD. Find out the system first, and test the earth on site.

The common systems

System	How the earth reaches you	Notes for EV chargers
TN-S	Separate earth and neutral from the utility transformer	Simplest; standard protection
TN-C-S (PME)	Utility combines earth and neutral (PEN) up to your intake, then splits	Risk if the PEN breaks upstream; open-PEN protection or electrode needed
TT	No utility earth; you drive an electrode	Electrode resistance and RCD must match; test it
IT	Isolated or high-impedance earth (rare)	Needs specialist design

Country practice varies. The UK has many PME supplies, much of continental Europe uses TT or TN-S, and the US uses a grounded neutral with a separate grounding electrode system.

The PME open-PEN problem

On a PME supply, the neutral is also the protective earth. If the combined conductor breaks upstream, the neutral and earth float to a higher voltage. A car plugged into a charger is connected to the earth. If the car body is touched by someone standing on wet ground, they receive a shock.

For this reason wiring rules in several countries require that, on PME supplies, an outdoor EV charger has:

• open-PEN detection inside the charger or in a protective device, which disconnects when it senses a dangerous voltage, or
• a separate earth electrode that meets a resistance limit and keeps touch voltage low, or
• isolation of the car from the supply earth in other ways, or an island arrangement.

Many chargers offer PEN protection as an option, such as the PEN protection (UK) option on our wall-mounted chargers. Check the exact requirements in your country.

TT supplies

On TT the installation has its own earth electrode. The RCD must trip when the fault voltage on exposed parts would exceed a safe limit (commonly 50 V). The simple rule: electrode resistance × RCD trip current ≤ 50 V. For a 30 mA RCD the theoretical limit is high, but real installations aim far below it, because soil dries and rods corrode. Test, record and re-test periodically.

How to test

Test	Instrument	What it shows
Earth electrode resistance	Earth tester (fall-of-potential or clamp)	Resistance to ground
Earth loop impedance	Loop tester	Whether the breaker or fuse will trip fast enough in TN systems
RCD trip time and current	RCD tester	Whether the RCD works at its rating
Continuity of protective conductors	Low-resistance ohmmeter	The earth path is complete
Polarity and phase	Multifunction tester	Correct wiring

Results go on the installation certificate. See the commissioning checklist.

Installation tips

1. Use a dedicated circuit back to the board, with its own protective devices.
2. Do not rely on the building’s metal frame for the earth unless it is designed for it.
3. Bond metal parts near the charger: canopy, poles, bollards — as the code requires.
4. Check soil when driving a rod: dry sand or rock gives high resistance; use longer or multiple rods or chemical electrodes.
5. Protect the connection against corrosion, especially near the coast.
6. Combine with surge protection; surge devices need a good earth — see surge and lightning protection.

What to ask the supplier

Which earthing systems the charger supports, whether PEN protection is built in or optional, whether the charger checks the earth connection before charging, and what the manual requires for TT sites. Also see the installation requirements guide and RCD requirements.