PME, Earth Faults & EV Charger Safety: WCED Explained
Around 80% of UK homes are on PME earthing — and it creates a specific hazard for outdoor EV chargers. Here's what the risk is, what the law requires, and how WCED protection solves it.
What Is PME Earthing?
PME (Protective Multiple Earthing), also known as TN-C-S, is the earthing arrangement used on most UK properties. In a PME system, the neutral conductor and the protective earth (PE) share a single combined conductor — the PEN conductor — between the distribution transformer and your property's meter. They then separate inside your property.
Under normal conditions, this works fine. The problem arises if the PEN conductor develops a fault — a break or high-resistance connection between the transformer and your property.
If the PEN conductor breaks while current is flowing, the neutral voltage rises. Because the earth conductor is connected to the same PEN conductor, the earth in your property rises to the same dangerous voltage. Any conductive object connected to that earth — including an EV being charged — becomes live. The vehicle body can reach mains voltage. This is potentially fatal.
Why EV Charging Makes This More Dangerous
Under BS 7671:2018 Amendment 2, Regulation 722.411.4 specifically requires additional protection for EV supply equipment (EVSE) on PME supplies installed outdoors or in garages accessible from outside.
The reason EV charging is singled out: a person connecting or disconnecting a charging cable while standing outdoors is at significant risk if the vehicle body becomes live. Indoor equipment or equipment not accessible from outside carries lower risk — hence the specific outdoor requirement.
What Is a WCED?
A WCED (Whole-Current Earth Disconnect) is a protective device that continuously monitors the PME earth conductor. If it detects a PEN conductor fault — evidenced by an imbalance in the measured currents — it disconnects the EVSE from the PME earth before dangerous voltages can appear on the vehicle body.
EcoHarmony supplies WCED devices and pre-assembled PME protection distribution boards from WCED specialists, in both single-phase and three-phase configurations.
| Product | For | Link |
|---|---|---|
| Single-Phase WCED Device | Single-phase EV charger on PME supply | View → |
| Three-Phase WCED Device | Three-phase EV charger on PME supply | View → |
| PME Protection Distribution Board (1-phase) | Complete board with WCED, RCD, MCB included | View → |
| PME Protection Distribution Board (3-phase) | Three-phase complete board | View → |
Frequently Asked Questions
How do I know if my home has PME earthing?
Check your meter tails or ask your DNO. PME (TN-C-S) supplies typically have a combined neutral and earth conductor on the incoming supply side. Most urban properties built or rewired after the 1970s are on PME. If unsure, an electrician can confirm during the pre-installation survey.
What is the alternative to a WCED?
The alternative is to install a TT earth electrode — driving a copper earth rod into the ground and using it as an independent earth, separate from the PME supply. This is equally compliant but requires suitable soil conditions and additional installation work. Most electricians prefer WCED devices for their simplicity.
Does every EV charger installation need PME protection?
Only if: (1) the property has a PME supply (TN-C-S), and (2) the charger is installed outdoors or in a garage accessible from outside. An indoor garage not accessible from outside, or properties with TN-S or TT supplies, may not require a WCED. Always confirm with the installer.