PME Earthing and EV Charging: PEN Fault Risk, TT Earthing Solutions and Approved Protection Devices

Quick Answer: PME (Protective Multiple Earthing, also called TN-C-S) presents a specific risk for EV charging: if the PEN conductor fails, the vehicle chassis can become energised to dangerous voltage. BS 7671:2018+A2:2022 Regulation 722.411.4.1 requires that where PME earthing is used, additional protection is provided — either by earthing the EV circuit via a TT (separate earth electrode) arrangement, or by a OZEV/BS 7671-approved PEN fault detection device.

Summary

PME earthing is used by most UK Distribution Network Operators (DNOs) for residential supplies. It is generally safe and reliable. However, it has a specific vulnerability: if the combined neutral/earth (PEN) conductor in the distribution network develops a fault (break, corrosion, connection failure), the earth terminal at the consumer's premises can be raised to a dangerous potential — potentially approaching the full supply voltage.

For most household equipment, this PME fault scenario is a low-probability, short-duration event that causes nuisance rather than serious harm. For EV charging, it is different: the EV's conductive chassis is connected to the chargepoint earth, and therefore to the PME earth terminal. In a PME fault, the EV would be energised. A person simultaneously touching the EV (or the chargepoint) while standing on wet ground or a metal structure connected to true earth could receive a potentially fatal electric shock.

This risk was identified in the EV charging context during the development of Chapter 722 and is addressed specifically by Regulation 722.411.4.1. Every domestic EV installation must either isolate the EV circuit from the PME earth or include an approved protection device.

Key Facts

• PME (TN-C-S) — most UK residential supplies use PME earthing; combined neutral and earth (PEN) conductor in distribution network
• TN-S — alternative system where separate neutral and earth conductors exist throughout; PME concerns do not apply
• PEN conductor fault — failure of the combined neutral/earth conductor; can raise earth potential at premises
• Risk to EV — EV chassis energised to supply voltage during PEN fault; lethal contact risk if person simultaneously contacts EV and true earth
• Regulation 722.411.4.1 — BS 7671:2018+A2:2022; specific requirement for PME protection in EV installations
• TT earthing — separate earth rod for the EV circuit, disconnected from PME; the simplest and most common solution
• PEN fault detection device — electronic device continuously monitoring PEN conductor integrity; disconnects EV if PEN fault detected
• Approved PEN detection devices — must meet BS 7671 requirements; Garo EV Protection, EVPD devices, and similar
• DNO confirmation — always confirm the earthing system with the DNO or by inspection before designing the EV installation
• Earth rod depth — typically 1.2–2.4m driven earth rod; resistance to earth must meet BS 7671 requirements (typically <200Ω for RCD to operate; ideally <100Ω)
• Check before assuming — TNS installations (properties where neutral and earth are separate all the way from the transformer) do not have the PME risk; confirm the system type first

Quick Reference Table: Earthing System Identification and EV Solution

Detailed Guidance

Identifying the Earthing System

Before designing an EV installation, identify the earthing system. Methods:

1. Inspect the meter and cut-out:

• Remove the meter cupboard cover (if accessible without specialist tool)
• Examine the incoming service conductors
• TN-S: Two separate coloured conductors entering the cut-out (neutral grey/blue, separate earth green/yellow from the DNO)
• PME (TN-C-S): A single combined neutral/earth entry, with the earth connection made at the consumer unit via a neutral-to-earth link
• The presence of a MET (Main Earth Terminal) bonded to the neutral at the service head indicates PME

2. Contact the DNO: Ring the DNO's technical helpline with the address. They can confirm the earthing arrangement. This is the most reliable method and creates a documented record.

3. Check for the earth terminal type: PME supplies have a combined neutral/earth that is separated at the service fuse assembly. If the earth terminal is a clip or bar connected directly to the neutral bar, this indicates PME.

Solution 1: TT Earthing for EV Circuit

The TT solution involves providing a separate earth electrode (typically a copper-bonded steel earth rod) for the EV chargepoint circuit. The EV circuit is connected to this electrode, not to the premises' PME earth terminal.

Implementation:

1. Drive a 1.2m or 2.4m earth rod at the chargepoint location (or in the garden, connected by earth cable)
2. Connect the protective conductor (earth wire) of the EV circuit to the earth rod, NOT to the main earth terminal
3. Do not connect the EV circuit earth to any other part of the premises earthing system
4. Install an RCD on the EV circuit (the RCD must operate on a TT earthed circuit — earth fault loop impedance on TT is typically higher than TN-S)

Earth fault loop impedance on TT: For a TT circuit, earth fault loop impedance (Ze) depends on the earth rod resistance. A good earth rod in typical soil achieves 50–100Ω. With a 30mA RCD, this is sufficient to cause RCD operation (voltage = 0.03 × 50 = 1.5V → below shock threshold for RCD operation). Check that Ze × I_a (the RCD trip current × 2, per BS 7671) ≤ Uo/2 (115V).

Earth rod installation considerations:

• Drive the rod vertically; horizontal is acceptable but vertical is more effective
• Rod should be minimum 600mm from foundations and service trenches
• In very dry conditions, soil resistance can be high; water the area before testing
• Test earth rod resistance using a dedicated earth loop tester or earth rod tester after installation
• Use green/yellow sleeved earth wire from rod to chargepoint; minimum 6mm² for protective earth conductors

Solution 2: PEN Fault Detection Device

A PEN fault detection device monitors the PME PEN conductor and disconnects the EV chargepoint if it detects a fault condition that would raise the earth potential.

How it works:

• Continuously measures the voltage difference between the PEN conductor and a reference (often the line voltage)
• If this difference exceeds a threshold indicating PEN conductor failure, it disconnects the chargepoint supply within a specified time (typically <200ms)
• Devices monitor neutral conductor current and voltage as proxies for PEN conductor integrity

Approved devices (check current approvals with OZEV and manufacturer):

• Garo EV Protection (GAR PEN/PME)
• Sim Gard (SMGD EV protection)
• EVPD modules integrated into some chargepoints

Installation: PEN detection devices are typically installed at or near the consumer unit. They require a connection to the incoming supply in addition to the circuit supply. Follow the manufacturer's specific installation instructions precisely.

Limitations:

• PEN detection is a monitoring approach, not a physical separation — if the detection device fails or is bypassed, the protection is lost
• Some devices have false-trip issues in certain supply conditions; discuss with the DNO if nuisance tripping is experienced
• Not all chargepoints are compatible with all PEN detection devices; check compatibility

Which Solution to Use?

Use TT earthing when:

• A suitable location for an earth electrode exists (soil, not solid concrete throughout; distance from other metalwork)
• The EV chargepoint is in a garage, driveway, or external location where an earth rod can be installed
• The installer is confident in earth rod installation and testing
• This is the simplest and most reliable solution

Use PEN fault detection when:

• An earth rod is not feasible (basement flat, solid concrete throughout, very high ground resistance)
• The chargepoint is in an apartment building where installing an individual earth electrode is impractical
• The chargepoint manufacturer has an integral PEN detection solution
• The DNO has confirmed it is acceptable

Both solutions require:

• An Electrical Installation Certificate (EIC)
• Testing of the earth fault path (earth loop impedance, RCD operation)
• Documentation of the earthing system type

Testing and Documentation

After installing the TT earth electrode:

1. Earth rod resistance test — using a dedicated earth resistance tester (stakeable instrument or 2-terminal method per BS 7671); confirm resistance is adequate for the RCD protection chosen
2. Earth fault loop impedance — measure Ze at the chargepoint terminals; confirm RCD will operate
3. RCD test — test the RCD (30mA, trip within 200ms); confirm operation
4. Record on EIC — document earthing system type, solution chosen, test results

Frequently Asked Questions

Is all UK domestic electricity supply on PME?

No, but the vast majority is. TN-S (separate neutral and earth throughout) is found on some older properties, particularly in London and other areas where the underground distribution network has remained unchanged for decades. TT earthing is found in rural areas and some other situations where the DNO has not provided a PME supply. Always check before assuming.

Can I just use a Type B RCD instead of TT earthing for PME?

No. A Type B RCD addresses the issue of DC fault current components from EV chargepoints, but it does NOT address the PME PEN conductor fault risk. A Type B RCD will still have its earth connected to the PME terminal; if the PEN conductor fails, the RCD earth and the EV chassis will still be at dangerous potential. The RCD will not see a fault current in this scenario (because the fault is upstream of the consumer's installation) and will not trip. TT earthing or a PEN fault detection device is required in addition to the Type B RCD.

How deep should the earth rod be?

The minimum recommended length is 1.2m. A standard single-piece copper-bonded steel earth rod is typically 1.2m long. For better performance, a 2.4m rod (or two 1.2m rods connected in series with a rod coupler) achieves lower resistance in most UK soils. Test after installation to confirm resistance is adequate.

What if the earth rod resistance is too high?

If the measured earth rod resistance is too high (typically >200Ω), consider:

• Drive a second earth rod parallel to the first, connected at the top; separation should be minimum 3m (rods too close don't significantly reduce resistance)
• Treat the ground around the rod with bentonite clay or specialist earth enhancement compound (reduces soil resistance)
• Use a longer rod (3.0m)
• If none of these are feasible, consider whether a PEN fault detection device is a better solution

Regulations & Standards

• BS 7671:2018+A2:2022 — Regulation 722.411.4.1: PME protection requirements for EV charging

• IET Guidance Note 7 — EV charging; PME earthing guidance

• BS EN 62305 — Lightning protection; relevant where separate earth electrodes are installed

• ESQCR 2002 — Electricity Safety, Quality and Continuity Regulations; DNO obligations regarding earthing systems

• Building Regulations Part P — electrical installation safety in dwellings

• IET BS 7671:2018+A2 — Chapter 722 — PME protection regulations

• Garo EV Protection Device — approved PEN detection device information

• NICEIC Technical Guidance — EV Charging and PME — practical PME guidance for installers

• bs 7671 ev wiring requirements — full Chapter 722 requirements

• ozev approved installer — OZEV registration requirements

• ev charger installation types — chargepoint hardware selection

• three phase ev supply — three-phase EV charging and earthing considerations