Why is an earth rod alone unviable for outdoor PME EV charging?

Under BS 7671 Regulation 722.411.4.1 (ii), an earth electrode must keep touch voltage below 70V rms during an open-PEN fault. For a 40A unbalanced neutral load, this requires an earth resistance of <= 1.75 ohms (or <= 0.7 ohms at 100A). In typical UK soil (50-200 ohm.m), a standard 1.2m rod achieves ~71.6 ohms, making reliance on an earth rod alone physically impossible without an extensive earthing mat.

What is an Open-PEN detection device (O-PEN) under Regulation 722.411.4.1?

An O-PEN device monitors supply voltage (typically disconnecting if phase voltage rises above 253V or drops below 207V, or by sensing neutral-to-earth potential). Upon fault detection, it automatically disconnects all live conductors and the protective earth within 5 seconds to isolate the vehicle chassis from shock risk.

Can diversity be applied to EV charging circuits under BS 7671?

No. Under Regulation 722.311, dedicated EV circuits must be sized for 100% of the rated continuous current because vehicles draw maximum load for hours. Diversity is only permitted under Regulation 722.311.201 when an automated Dynamic Load Balancing (DLB) or Active Load Management (ALM) system is installed.

What RCD type is required for an EV charging circuit?

Under Regulation 722.531.2.101, each charging point must be protected by either a Type B RCD or a Type A RCD combined with a 6mA DC residual direct current detecting device (RDC-DD) conforming to BS IEC 62955. Standard Type AC RCDs are strictly prohibited as DC leakage can blind their magnetic cores.

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EV Charging & Open-PEN Earthing Sizer

Verify circuit capacity, voltage drop, and statutory earthing compliance for electric vehicle supply equipment (EVSE). Evaluates 100% continuous duty cable sizing (Reg 722.311), broken supply PEN touch voltages (Reg 722.411.4.1), earth rod physical resistance limits, RCD selection (Reg 722.531.2.101), and mandatory surge protection (Section 443/534).

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⚠️ BS 7671 Reg 722.411.4.1 Open-PEN Warning:An upstream Type A or Type B 30 mA RCD cannot protect against a broken supply PEN conductor because line and neutral currents remain balanced! On PME (TN-C-S) supplies outdoors, protection requires an approved device that isolates all live conductors and the protective earth conductor, a dedicated TT earthing island, or an earth electrode conforming to 722.411.4.1(ii).

EVSE & Supply Electrical ParametersCharger Rating / ModeRated output of the EVSE. Under Reg 722.311, no diversity applies without ALM.Supply Voltage & PhasesStandard UK low-voltage distribution supply.Circuit Run Length (m)One-way cable length from consumer unit/distribution board to EVSE.Installation Method & Cable TypeGoverns current-carrying capacity ($I_t$) and mV/A/m factors (BS 7671 App 4).Active Load Management (ALM / DLB)Automated curtailment systems allow supply capacity reduction.

Earthing & Outdoor Location (Reg 722.411.4.1)Supply Earthing SystemPME supplies outdoors carry open-PEN shock risks under broken neutral conditions.Charging Location EnvironmentOutdoor EVSE touches true Earth; strict Reg 722.411.4.1 conditions apply.Upstream Neutral Unbalance Current (A)Unbalanced single-phase load returning via PEN conductor (typically 30 A to 60 A).Soil Resistivity (ρ in Ω·m)Typical UK soil: 50 Ω·m (loam/clay) to 200 Ω·m (gravel/sandstone).

Recommended Cable CSA & Thermal Check

-- mm²

Design Current $I_b$: -- A | Capacity $I_t$: -- A

Circuit Voltage Drop

-- V

-- % (Max 5.0% = 11.5 V)

Terminal Voltage at EVSE

-- V

Within 230 V ± 10% Window

Section 722.411.4.1 Earthing Verdict

Analysing...

Checking earthing compliance...

Required $R_A$ (≤ 70V)

-- Ω

1.2m Earth Rod Resistance

-- Ω

Mandatory Protection Specification

RCD Protection (Reg 722.531.2.101): Type A RCD + 6 mA DC RDC-DD (BS IEC 62955) or Type B RCD. Type AC strictly prohibited.
Surge Protection (Section 443/534): Type 2 Surge Protection Device (SPD) mandated for outdoor equipment to protect electronic vehicle battery chargers.
Duty Factor (Reg 722.311): Sized for 100% continuous unmanaged load. Active Load Management (ALM) allows load curtailment under Reg 722.311.201.

Show Full Mathematical Derivation (AM2 Working Standard)

BS 7671:2018+A4:2026

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

First-Principles Mathematical Derivation

1. Design Current without Diversity

Under BS 7671 Regulation 722.311, dedicated EV charging circuits draw maximum rated current continuously for hours. Unless an automated Dynamic Load Balancing (DLB) system is installed under Regulation 722.311.201, diversity factor is strictly 1.0:

1-Phase: I_b = P / (U_0 × cos φ)
3-Phase: I_b = P / (√3 × U_L × cos φ)

2. Open-PEN Touch Voltage & Rod Limit

If the supply neutral breaks on a PME (TN-C-S) service, unbalanced load current flows through the consumer's earth electrode. Under Regulation 722.411.4.1 (ii), touch voltage must not exceed 70 V rms:

R_A ≤ 70 V / I_unbalanced
For 40 A unbalance: R_A ≤ 1.75 Ω

Using Dwight's formula R = (ρ / (2πL)) × (ln(8L/d) - 1), a standard 1.2m rod in 100 Ω·m soil achieves ~71.6 Ω. Achieving < 2 Ω with earth rods alone is practically impossible in typical UK soils, proving why Method (iv) O-PEN devices or TT islands are standard practice.

3. RCD & DC Blinding Protection

Under Regulation 722.531.2.101, vehicle rectifiers can inject smooth DC fault currents into the AC supply. Standard Type AC RCDs saturate (“blind”) at > 6 mA DC, disabling their trip mechanism:

Protection = Type A RCD + 6mA DC RDC-DD (BS IEC 62955)
OR: Type B RCD (detects pure DC, high-frequency & AC faults)