PEN faults: causes, detection, and how to protect your EV charging installation
PEN faults are rare but serious. When they occur on a PME electrical supply, they can energise the metalwork of connected equipment — including electric vehicles — to dangerous voltages without any obvious warning. This guide explains how PEN faults occur, how detection devices work, and what protection options are available for EV charging installations.
What is a PEN fault?
A PEN fault is an open-circuit failure of the combined Protective Earth and Neutral (PEN) conductor in a TN-C-S (PME) electrical supply system. In a PME system, the distribution network uses a single conductor to perform both the neutral and earth functions. If this conductor loses continuity — whether through physical damage, corrosion, or a loose connection — the installation loses its earth reference.
The immediate consequence is that the earth terminal in the affected installation rises in voltage. Because it is no longer connected to true earth through the PEN conductor, it seeks an alternative path. In practice, this means any metalwork bonded to the installation earth — including the chassis of an EV being charged — can reach a dangerous voltage relative to true earth.
How do PEN faults occur?
The PEN conductor runs from the distribution substation through the underground or overhead network to your property's supply cut-out. Failures can occur at any point in this path. The most common causes are:
- Physical damage: Excavation work is the most frequent cause — contractors striking underground cables during groundworks or road maintenance. A single cable strike can break the PEN conductor while leaving the live conductors intact, meaning the supply continues to work normally while the earth is lost.
- Corrosion: Older cable infrastructure — particularly in coastal or industrially polluted areas — is susceptible to corrosion of conductors and connections over time. Gradual degradation can increase resistance before a complete break occurs.
- Loose connections: Poor terminations in distribution cabinets, street pillars, meter boxes, or at the supply cut-out can work loose over time due to thermal cycling (conductors expanding and contracting with temperature changes) or vibration. A loose PEN connection may be intermittent — presenting no fault under light load but failing under the sustained current draw of EV charging.
- Surge or lightning damage: High-energy transients from lightning strikes or switching operations on the distribution network can damage conductors or blow fuses, creating a break in the PEN conductor.
Why is EV charging a particular risk during a PEN fault?
EV charging creates conditions that make PEN faults more hazardous than they would be for other loads:
- The vehicle is large, conductive, and accessible to people standing on true earth — completing the shock circuit through the person's body.
- Charging sessions are typically long (several hours) and often occur overnight, increasing exposure time.
- The sustained current draw of EV charging (typically 16–32A) can expose latent connection problems that wouldn't manifest under lighter domestic loads.
- Vehicles are often parked outdoors, where the person approaching the car is in direct contact with true earth (the ground).
This combination of factors — accessible metalwork, long duration, high current, and outdoor location — is precisely why BS 7671:2018+A2:2022 introduced specific PEN fault protection requirements for EV installations.
How does a PEN fault detection device work?
A PEN fault detection device continuously monitors the supply voltage at the point of installation. It works on the principle that a PEN fault will cause the measured voltage to deviate from normal operating range — either rising above the upper threshold or falling below the lower threshold — because the earth reference has shifted.
WCED PEN fault detection devices monitor both the 230V single-phase supply (for single-phase units) or the 400V three-phase supply (for three-phase units). When the voltage moves outside the normal operating range:
- Under-voltage condition (below 207V): The device isolates the live, neutral, and earth within 5 seconds. It automatically resets once normal voltage is restored.
- Over-voltage condition (above 253V): The device isolates within 5 seconds, but requires a manual reset. This is a deliberate safety measure — automatic reconnection into a potentially damaged supply is prevented.
The device disconnects all three conductors — live, neutral, and earth — simultaneously, ensuring the connected equipment is fully isolated from the supply.
Standalone detection devices vs. integrated charger protection
There are two approaches to meeting the BS 7671 requirement for PEN fault protection:
External detection devices are installed between the consumer unit and the charger. They work with any compatible EV charger and are the standard solution for most installations. They can be retrofitted to existing charger setups without replacing the charger itself. WCED units are available as standalone modules or pre-assembled in IP30 and IP65 enclosures with RCBO and Type 2 surge protection already fitted.
Integrated charger protection means the charger itself contains the PEN fault detection circuitry. The Simpson & Partners Home Series uses this approach — the charger monitors the supply continuously and disconnects automatically if a fault is detected. Where integrated protection is present and verified, no external detection device is required.
For new installations with chargers that lack built-in protection, a pre-fitted enclosure combining the RCBO, PEN fault detection, and surge protection in a single unit is the most efficient solution — it reduces installation time and ensures all components are correctly rated and coordinated.
What about earth rods?
An earth rod is an alternative to a detection device for achieving compliance. Installing a rod converts the local installation from TN-C-S (PME) to TT earthing, providing an independent earth path that is not affected by a PEN conductor failure.
The rod must achieve a resistance to earth of 15Ω or lower (as recommended by BS 7671 for EV installations), and this must be measured and recorded. In urban areas with paved surfaces, achieving this resistance can be difficult and may require multiple rods or specialist installation.
Earth rods are passive and have no electronics to maintain or test — but they do require the resistance measurement to be recorded and periodically verified. Ground conditions can change over time, particularly in dry weather.
Can a single device protect multiple chargers?
Yes. A correctly rated three-phase PEN fault detection device installed upstream of multiple chargers will protect all of them simultaneously. This is the standard approach for commercial sites, car parks, and multi-unit developments. A single WCED three-phase unit rated at 100A can protect up to nine 7kW single-phase chargers or three 22kW three-phase chargers from a single installation point.
Testing and maintenance
PEN fault detection devices should be functionally tested at commissioning. Ongoing testing should be carried out annually or after any significant electrical work affecting the supply. Some devices include fault logging, which records disconnection events and can help identify intermittent supply problems. If a device trips repeatedly, the supply should be investigated — repeated operation indicates a real fault condition, not a device malfunction.
Summary
PEN faults are caused by failures of the combined earth and neutral conductor in PME supply systems. They are rare but dangerous, and EV charging creates specific conditions that increase both the likelihood of triggering a latent fault and the severity of the consequence. PEN fault detection devices work by continuously monitoring supply voltage and disconnecting automatically when a fault condition is detected. For most EV installations without built-in charger protection, a pre-fitted unit combining RCBO, PEN fault detection, and surge protection in a single enclosure is the most practical solution.