Understanding PME protection is essential for any electrician installing EV chargers in the UK. This guide explains what PEN faults are, why BS 7671 requires automatic disconnection, and how to choose and install the right PME protection device — including for multi-charger and three-phase setups.
The PME Problem for EV Chargers
Most UK properties are supplied via a TN-C-S (PME) earthing system. Under normal conditions, this works reliably. But the combined PEN conductor that delivers both neutral and earth from the substation introduces a specific risk: if the conductor is broken, the earth potential at the property can rise to line voltage.
For an EV charger, that's especially dangerous. The charging cable makes a direct conductive connection to the car's metal chassis. If the earthing system becomes live during a PEN fault, the risk of electric shock from touching the vehicle is real and potentially fatal.
The Regulatory Requirement
Regulation 722.411.4.1 of BS 7671 18th Edition (Amendment 2:2022) requires EV charge points to disconnect automatically if a PEN fault is detected on a PME supply. The two compliant options are to use TT earthing, or to fit a dedicated PEN fault detection device.
In practice, the vast majority of installers use a PEN fault detection unit. It's faster, more cost-effective, and avoids the need for earth rods or significant groundworks.
How PEN Fault Detection Works
A PEN fault detection unit monitors voltage continuously across the line, neutral, and earth conductors. If the voltage diverges beyond the defined safe threshold — typically ±10% of nominal — the device trips and disconnects the charger from the supply. The whole process happens automatically, within milliseconds, before any dangerous voltage can reach the vehicle or user.
The better units on the market combine this monitoring function with additional protection in a single housing: Type 2 surge protection to guard against transient overvoltages, and RCBO overcurrent protection for each charger circuit. This all-in-one approach is both space-efficient and straightforward to certify.
Choosing the Right Device
When selecting a PME protection device for an EV charger install, consider the following:
- Single-phase or three-phase: Match the device to the supply. Three-phase supplies require a unit that monitors all phases.
- Number of charger circuits: Some units support two, three, or more chargers from a single enclosure — useful for commercial or fleet installs.
- RCBO vs MCB protection: RCBOs provide both residual current and overcurrent protection per circuit. If the charger already includes its own RCBO, an MCB-only unit may suffice.
- IP rating: IP40 is suitable for indoor locations such as garages. For external mounting, IP65 or above is required.
- Surge protection: BS 7671 Amendment 2 requires surge protection unless a risk assessment determines otherwise. Most installers include it as standard.
PME protection for your install
Eco Harmony's WCED Range
The Eco Harmony WCED range is designed specifically for EV charger installations in the UK, with units for single-phase, three-phase, and multi-charger setups. Key options include:
- 3-Phase PME 40A RCBO + Surge Protection Unit — suitable for Tesla Wall Connectors and other three-phase chargers, available in 2-, 3-, and 6-charger configurations.
- Single-Phase PME Protection Boards — for domestic installs and garages, with or without surge protection.
- TPN Distribution Boards with SPD — for multi-charger commercial sites where centralised distribution and protection are needed.
All units are pre-assembled, ready to install, and compliant with BS 7671 Amendment 2:2022. Purchasing a distribution board and MCBs together qualifies for 25% off the MCBs.
Installation Guidance
A few practical points worth keeping in mind on site:
- Always confirm the earthing arrangement before starting. If the supply is PME (TN-C-S), PME protection is required.
- Mount the protection unit in a dry, accessible location. Use IP40 as a minimum for indoor locations; IP65 for outdoor or exposed positions.
- After installation, test the PEN fault detection function using a suitable simulator or the manufacturer's test procedure. Record the test result on your certification.
- Ensure surge protection modules are clearly labelled and that replacement modules are available — some types need replacing after absorbing a significant surge.
A Typical Three-Phase Example
Consider a residential customer with a three-phase supply who wants a Tesla Gen 3 Wall Connector. The charger includes DC leakage detection, but no PEN fault monitoring. The installer fits an Eco Harmony 3-Phase PME 40A RCBO + SPD unit upstream of the charger. The device provides PEN fault detection across all three phases, overcurrent protection via the RCBO, and transient overvoltage protection via the integrated SPD. The installation passes NICEIC inspection, the customer's insurer is satisfied, and the install is documented as fully compliant with BS 7671 A2:2022.
Summary
PEN faults are uncommon — but when they occur on an unprotected install, the consequences can be severe. Fitting the right PME protection device is a straightforward way to meet your regulatory obligations, protect your customers, and future-proof your work. With Eco Harmony's WCED range, it doesn't need to be complicated or expensive.
Browse the full PME Protection range →
Frequently Asked Questions
What does a PEN fault detection unit do?
A PEN fault detection unit continuously monitors the voltage between line, neutral, and earth conductors. If the voltage diverges beyond a defined safe threshold — typically ±10% of nominal — the device trips and disconnects the EV charger from the supply automatically. This prevents dangerous voltages from reaching the vehicle or user in the event of a lost neutral on a PME supply.
Do I need RCBO or MCB protection with my PME device?
An RCBO provides both residual current protection and overcurrent protection in a single device. If the EV charger itself does not include an RCBO, you should fit one in the PME protection unit. If the charger already includes its own RCBO, a standalone MCB in the PME enclosure may be sufficient — though many installers standardise on RCBO throughout for consistency and future-proofing.
What IP rating does a PME protection unit need?
For indoor locations such as garages, utility rooms, or plant rooms, IP40 is the minimum required rating. For outdoor or exposed installations, IP65 or above is needed. Eco Harmony offers both IP40 metal enclosures and IP65 weatherproof enclosures across the WCED range.
Is surge protection required for EV charger installations under BS 7671?
BS 7671 Amendment 2:2022 requires surge protection (SPD) for EV charger circuits unless a risk assessment concludes it is not necessary. In practice, most installers include Type 2 SPD as standard. Eco Harmony's WCED PME units include integrated Type 2 surge protection, meeting this requirement without additional components.
Can one PME protection unit serve multiple EV chargers?
Yes. Eco Harmony's 3-Phase PME units are available in configurations supporting 2, 3, or 6 charger circuits from a single enclosure. For larger commercial or fleet installations, a TPN Distribution Board with SPD can be paired with the PME unit to provide centralised protection and distribution across multiple bays.
How do I test PEN fault detection after installation?
After installation, the PEN fault detection function should be tested using a PEN fault simulator or the manufacturer's specified test procedure. The test result should be recorded on the electrical installation certificate. Always follow the manufacturer's guidance — test methods vary between devices.
