Under BS 7671:2018 Section 722, electric vehicle charging equipment represents a unique installation challenge. Continuous high-current loads, outdoor conductive vehicle bodies in contact with the general mass of earth, PME open-PEN hazards, and the emergence of bidirectional vehicle-to-grid (V2G/V2H) generation demand rigorous compliance.
The PME Open-PEN Shock Hazard Outdoors
On a standard Protective Multiple Earthing (PME / TN-C-S) supply, the neutral and protective earth conductors are combined in the distributor's supply cable (the PEN conductor).
If the PEN conductor breaks upstream in the street network while customer single-phase loads are operating:
- Return currents from loads inside the property pull the installation's Main Earthing Terminal (MET) up to line potential (230 V).
- Because an electric vehicle's metallic chassis is connected to the protective conductor (PE) via the charging cable, the entire car body becomes an exposed-conductive-part energised at 230 V.
- A person standing outside on wet ground (true earth) touching the car door handle provides the only return path to earth, resulting in a potentially fatal electric shock.
PME Earthing Rules: The Four Legitimate Methods (Regulation 722.411.4.1)
Regulation 722.411.4.1 explicitly states: "A PME facility shall not be used as the means of earthing for the protective-conductor contact of a charging point located outdoors, or that might reasonably be expected to charge a vehicle outdoors, unless one of the following methods is used:"
| Method | Statutory Mechanism (Regulation 722.411.4.1) | Status / Field Reality |
|---|---|---|
| (i) / (a) | Three-phase load balanced such that MET-to-Earth voltage ≤ 70 V rms | DELETED by Amendment 2:2022. No longer permitted in BS 7671. |
| (ii) / (b) | MET connected to an installation earth electrode (Regulation 544.1.1) with resistance (Ra) sufficiently low that MET-to-Earth voltage cannot exceed 70 V rms on an open PEN. | Retains PME. (Requires an extremely low electrode resistance, typically Ra ≤ 1 to 2 Ω, which is almost impossible to achieve domestically). |
| (iii) / (c) | Protection device that disconnects the vehicle from both the live conductors and the protective conductor (PE) per Regulation 543.3.3.101 within 5 seconds if the CPC-to-Earth potential exceeds 70 V rms. | Earth reference probe method (incorporating auxiliary electrode to measure true ground potential). Need not operate for transient excursions < 4 s. |
| (iv) / (d) | Single-phase voltage-monitoring device: disconnects live conductors and PE within 5 seconds if the utilisation voltage between line and neutral rises above 253 V rms or drops below 207 V rms. | Most common UK solution. Operates without an external earth rod. Restricted strictly to single-phase supplies. |
| (v) / (e) | Alternative device to (iii) or (iv) providing no lesser degree of safety, typically incorporated directly into EVSE charging units. | Widely deployed in modern smart chargers with integral open-PEN detection. |
Note 3 to Regulation 722.411.4.1 (Converting to TT):
Converting an outdoor EV charger to a standalone TT earthing arrangement by isolating the incoming PME earth and sinking an earth rod may not be appropriate. If the vehicle is parked within touch distance of other metallic items connected to the PME supply (e.g. metal clad garage doors, outdoor metallic taps, or structural steelwork), a touch voltage hazard remains between the TT car chassis and the PME metalwork.
Circuit Diversity Rules vs Dynamic Load Balancing: Regulation 722.311
Sizing cables and distribution equipment for EV charging differs fundamentally from standard domestic sub-circuits:
- The Zero-Diversity Rule (Regulation 722.311):Under Regulation 722.311, the design current (Ib) of the circuit supplying a charging point must be taken as its full rated current. Installers are strictly prohibited from applying standard domestic diversity factors (such as On-Site Guide Appendix A) to an EV circuit. A 7.4 kW single-phase unit is a 32 A continuous load for 6 to 10 hours.
- Dynamic Load Curtailment (Regulation 722.311.201):Load curtailment (automatic current reduction or temporary disconnection) may be taken into account when sizing the supply only where an automated Dynamic Load Balancing (DLB) system is installed. The DLB system monitors the main cut-out fuse via a current transformer (CT clamp) and automatically throttles EV charging current to prevent exceeding the DNO service head rating (e.g. 60 A or 80 A).
Bidirectional EV Systems (V2G / V2H) & Generating Sets
Bidirectional charging allows an electric vehicle to act as an energy storage battery, exporting power back into the home (Vehicle-to-Home, V2H) or the grid (Vehicle-to-Grid, V2G). Under BS 7671:
- Dual Classification: An exporting EV charger is legally classified as both a current-using item of equipment AND a low-voltage generating set. It must comply simultaneously with Section 722, Section 551, and Chapter 82 (Prosumer's Electrical Installations).
- Bidirectional Protective Devices (Regulation 530.3.201):Circuit breakers, RCBOs, and RCDs installed in bidirectional circuits where power flows in both directions must be certified for bidirectional power flow. Unidirectional devices may fail to quench arcs or experience trip coil damage during reverse current flow.
- Grid-Parallel vs Islanding Operation:Grid-parallel export must incorporate loss-of-mains protection certified to ENA EREC G98 (≤ 16 A/phase) or EREC G99 with formal DNO approval. For V2H emergency islanding mode during grid blackouts, an automatic, mechanically interlocked changeover switch must physically disconnect the PME supply to prevent hazardous back-feeding into the distributor's de-energized street network.
RCD Selection for EV Circuits: Regulation 722.531.2.101
Every EV charging point must be individually protected by a 30 mA RCD:
- Type A RCD + 6 mA RDC-DD:A standard 30 mA Type A RCD / RCBO is compliant only where the EVSE incorporates an integral Residual Direct Current Detecting Device (RDC-DD) certified to BS EN IEC 62955 that disconnects the supply when smooth DC leakage reaches 6 mA.
- Type B RCD:If the charging point does not contain a certified BS EN IEC 62955 RDC-DD, an upstream Type B RCD is mandatory to protect against smooth DC fault currents blinding the protection.