EV Charging Earthing & Open-PEN Fault Protection Explained

Supplying an electric vehicle charger from a standard UK domestic PME (TN-C-S) supply introduces a unique safety hazard: the broken PEN conductor. Understanding the physics behind open-PEN fault protection is vital for designing compliant EV installations.

The PME Shock Hazard Outdoors

In a TN-C-S (PME) supply, the Protective Neutral and Earth are combined in a single conductor (PEN) up to the consumer's cut-out. Inside the property, the main earthing terminal connects to the incoming neutral. Under normal conditions, this is safe and effective.

However, if the network supply PEN conductor breaks upstream in the street:

• Load current from connected domestic appliances attempts to return to the transformer via any parallel earth path.
• The property's protective earthing system (including the vehicle chassis plugged in outside) rises toward full line voltage (230 V).
• A person touching the car while standing on true earth (a damp driveway or wet pavement) bridges this potential difference, creating a lethal electric shock hazard.

The Three Approved Solutions

BS 7671 Section 722 prohibits connecting an EV charge point to a PME earthing terminal unless one of the approved protective measures is satisfied:

1. Separation into a TT System (Earth Electrode)

The EV circuit earth is completely isolated from the PME earth, and connected to an independent earth rod (TT system). While simple in theory, achieving a low, stable earth resistance in stony or dry ground can be challenging, and risk assessments must ensure no simultaneous contact is possible between the TT vehicle and PME metallic parts (e.g. garage doors or metallic water pipes).

2. Electronic Open-PEN Detection Devices (O-PEN)

Modern UK EV chargers increasingly incorporate built-in electronic Open-PEN monitoring. These devices continuously monitor the single-phase supply voltage:

• Operating Principle: If the phase-to-neutral voltage drops below 207 V or exceeds 253 V (indicating an open neutral shift), the unit disconnects all poles - Live, Neutral, and Earth - within seconds.
• Advantage: Eliminates the need to drive an earth rod into driveways or risk hitting underground gas and water pipes.

3. Voltage Threshold Monitoring (70 V CPC Reference)

Alternative commercial devices monitor the potential between the installation CPC and an auxiliary reference electrode. If the potential exceeds 70 V, the device trips all active and protective conductors to the charge point.

RCD and DC Leakage Requirements

In addition to open-PEN protection, every AC EV charging point must be protected against smooth DC residual currents exceeding 6 mA, which can blind standard Type A RCDs:

• Standard Domestic Setup: A charge point with built-in 6 mA RDC-DD (BS IEC 62955) paired with an upstream 30 mA Type A RCBO.
• Without Integral RDC-DD: A dedicated Type B RCD must be installed upstream.