PME fault protection is a safety function used in EV charging installations on TN-C-S (PME) earthing systems to protect against a broken or disconnected PEN conductor (combined Protective Earth + Neutral). If the PEN is lost upstream, dangerous touch voltages can appear on exposed metal parts and the vehicle body. PME fault protection is designed to prevent that risk by ensuring the charge point is automatically made safe.
PME fault protection is often referred to as open-PEN protection or PEN fault protection.
Why PME Fault Protection Matters for EV Charging
EV charging connects a vehicle to the electrical installation, often outdoors and accessible to the public. In a PEN fault scenario, the EV and charger can become a shock hazard. PME fault protection helps:
– Reduce risk of electric shock from elevated touch voltage on the vehicle/chassis
– Ensure compliance with common UK EV charging earthing requirements (Section 722 context)
– Improve public and workplace charging safety on PME supplies
– Avoid reliance on user behavior (the protection is automatic)
How a PME (Open-PEN) Fault Can Occur
A PME system uses a combined conductor upstream. If that conductor becomes open circuit due to damage or a network fault, the installation’s “earth reference” can rise significantly under load. This can create hazardous voltages between:
– The vehicle/charger metalwork and true earth
– Different conductive parts within the installation
– The ground and nearby bonded metal services
How PME Fault Protection Works
PME fault protection is typically implemented in one of these ways:
– Open-PEN detection device (often built into the charge point or installed externally) that monitors supply conditions and disconnects the charger when a fault is detected
– Alternative earthing arrangement for the charging circuit (e.g., a TT arrangement with a local earth electrode and appropriate protective devices), so a PEN fault on the supply does not create dangerous touch voltages at the charge point
– Automatic disconnection logic coordinated with the charger’s contactors so the vehicle is isolated quickly when unsafe conditions are detected
Typical Installation Components and Requirements
Depending on site design and local rules, PME fault protection may involve:
– A charge point with built-in open-PEN protection
– An external PEN fault detection unit upstream of the charge point(s)
– Correct earthing and bonding design (including equipotential bonding)
– Correct protective devices (RCD/RCBO selection must match the charger and site earthing design)
– Clear commissioning tests and recorded results for compliance and handover
Common Use Cases
PME fault protection is especially relevant for:
– Residential and workplace AC charging on TN-C-S (PME) supplies
– Public destination charging where users can touch the vehicle and charger simultaneously
– Sites without practical options for a dedicated earthing arrangement per charger
– Multi-charger car parks where a single earthing decision affects many charge points
Benefits
– Higher safety for users in the event of an upstream PEN failure
– Enables compliant deployment of EV charge points on PME supplies
– Can reduce the need for additional earthing hardware when built-in protection is used (site-dependent)
– Improves reliability of enforcement and safety policies by using automatic detection and disconnection
Limitations and Practical Considerations
– Protection method must be matched to the site earthing design and local regulations
– Detection-based solutions require correct thresholds, stability against nuisance trips, and reliable disconnection hardware
– Multi-charger sites may need coordinated design so one device properly protects the intended circuits
– Documentation and commissioning are critical for proving the safety function works as intended
Related Glossary Terms
TN-C-S (PME)
PEN conductor
Open-PEN protection
Earthing arrangement (TT / TN)
Equipotential bonding
RCD / RCBO
Fault detection
Commissioning documentation
Electrical safety compliance