PME earthing and how it normally operates
Protective Multiple Earthing (PME) is the standard earthing arrangement used by UK distribution network operators for the vast majority of low-voltage supplies. In a PME system, the protective earth (PE) and neutral (N) conductors are combined into a single PEN conductor at the distribution level, with the neutral bonded to earth at multiple points along the network. At each consumer's premises, the incoming PEN conductor provides both the return current path and the protective earth terminal, to which all exposed metalwork is bonded.
Under normal operating conditions, the PEN conductor remains at close to earth potential throughout. The exposed metalwork of all connected equipment — and the earthing terminal itself — sits at or near zero volts relative to true earth. Touch voltages between exposed metalwork and ground are negligibly small, and the arrangement is considered safe for conventional indoor and workplace electrical installations.
The open PEN fault and its consequences
The critical failure mode for PME earthing is the open PEN fault — a condition in which the PEN conductor develops a break or high-impedance connection upstream of the consumer's installation. This can result from corrosion of an aged cable joint, mechanical damage to the supply cable, or a fault within the distribution network itself. The probability of any individual PEN conductor failing in this way at any given moment is low. But the consequences are severe, particularly in the context of outdoor EV charging.
When the PEN conductor opens, the supply neutral is lost. Current returning through the connected loads can no longer flow back to the source via the network neutral — instead, it can only return via the earth path. The voltage that appears on the combined neutral-earth terminal at the consumer's installation, and hence on all PME-bonded metalwork, is determined by the impedance of the connected loads between line and the now-floating neutral. In practical terms, this voltage can approach mains voltage: potentially 115 V to 230 V above true earth potential.
Why on-street and residential EV charging is specifically at risk
For most indoor electrical installations, an open PEN fault creates an unpleasant condition — equipment may malfunction, and any person touching exposed metalwork will receive a shock — but the person is likely to be standing on an insulated floor (carpet, tiles, a wooden floor) that provides some measure of protection against touch voltage, and the exposed metalwork they can reach is typically part of fixed equipment inside a building.
On-street and residential EV charging changes this picture significantly. A person connecting a charging cable to a vehicle parked on a public street, or to a charger mounted on the external wall of a house, is standing on a conductive surface — pavement, tarmac, or wet ground — that is at true earth potential. They are simultaneously able to touch the vehicle or charger body, which under a PME open PEN fault would be at elevated voltage. This is the simultaneous contact hazard: the person becomes the path between the fault voltage and true earth.
Mitigation strategies
The three principal mitigation strategies are TT earthing, open PEN fault detection devices, and physical segregation.
TT earthing involves providing a local earth electrode for the EV charging circuit, disconnected from the PME earth terminal. If the PEN conductor opens, the TT-earthed equipment remains at true earth potential — the fault voltage does not appear on it. A TT installation requires an effective earth electrode (confirmed by measurement), appropriate RCD protection to provide fault clearance (since overcurrent protection will not operate under the high fault-loop impedance of a TT circuit), and care to ensure that no metallic path exists between the TT earth and any PME-earthed metalwork.
Open PEN fault detection devices (commonly marketed under the O-PEN or similar designation) monitor the voltage between the neutral-earth terminal and a true earth reference. When the neutral voltage rises above a safe threshold — indicating a PEN fault condition — the device disconnects the supply before the voltage on the bonded metalwork can reach a dangerous level. These devices allow EV charging equipment to be earthed to the PME terminal while providing automatic disconnection if the PME earthing fails.
Physical segregation — barriers, enclosures, or installation geometry that prevents a person from simultaneously touching the charging equipment and a true-earth surface — is generally difficult to achieve reliably in an outdoor setting, and is not accepted as a primary mitigation strategy in most current guidance.
The appropriate strategy for any specific installation depends on the earthing arrangement available, the physical configuration of the site, and the requirements of the relevant network operator. It should be determined at the design stage, before the installation is committed to a particular earthing arrangement.