EV Charging Earthing Considerations (Open-PEN)

Earthing is one of the most important safety considerations for EV charging. On the PME (TN-C-S) supplies common in the UK, a specific hazard — the open-PEN fault — must be addressed.

The open-PEN hazard

On a PME supply the earth is derived from the supply neutral. If the combined protective-earth-and-neutral (PEN) conductor breaks (an "open-PEN"), earthed metalwork — including the bodywork of a vehicle being charged — could rise to a dangerous voltage. Because a person may be touching the vehicle and standing on the ground outside, the risk is significant.

How BS 7671 addresses it

Section 722 of BS 7671 requires protective measures against this scenario. Accepted approaches include:

• devices that disconnect the charger if the voltage rises dangerously (many modern chargers include open-PEN protection); or
• providing a separate TT earth electrode for the charging installation.

Why it matters

This is not optional detail — it is a core life-safety requirement unique to EV charging. A competent installer will select the correct approach for the site's earthing arrangement and charger type. It is a key check in any EV site survey.