EV charger PEN fault detection

An open PEN fault is a break or high-resistance failure in the combined protective-earth and neutral conductor used on the network side of a PME supply. The same arrangement is described as TN-C-S at the consumer’s installation.

If the PEN fails upstream, the installation earth and everything connected to it can rise to a dangerous voltage above true Earth. An EV being charged outdoors is a particular risk because a person can touch the PME-connected vehicle while standing on the ground.

Why the normal protections are not enough

Main protective bonding helps keep conductive parts within the building at a similar potential. It does not hold them at true-Earth potential during an open PEN fault.

A conventional RCD has another job. It compares current in the live conductors and trips when enough is missing from the expected return path. During some open PEN faults, load current can remain balanced from the RCD’s point of view while the protective system has moved to a dangerous voltage. RCD protection and open-PEN protection are therefore separate requirements.

Network multiple earthing reduces the likelihood and consequences of a PEN failure but cannot make it impossible. The charging installation has to deal with the residual risk where Section 722 of BS 7671 applies.

Start with the actual parking position

The assessment is not limited to the box on the wall. It should include every position in which the vehicle could be connected, the length of the charging lead and nearby conductive parts.

The risk assessment uses:

• the earthing arrangement confirmed for the supply: PME, TT or TN-S
• whether the connected vehicle can be outdoors or in contact with true Earth
• possible simultaneous contact between the vehicle and PME-connected metalwork, or metalwork on another earthing system
• neighbouring chargepoints, street furniture, gates, pipes and outbuildings within the usable area
• the property phase arrangement and the protective device’s suitability for it

A charger mounted inside a garage is not automatically outside the risk if its lead reaches a vehicle on the drive. Conversely, a rod beside a charger does not prove that the circuit is TT.

The broad protective approaches

BS 7671 permits defined ways of controlling the risk. The designer should select the measure from the edition applicable to the work rather than copy an old list of regulation indents.

The approaches broadly include:

Use an earthing arrangement that does not rely on the PME terminal. A properly designed TT charging installation can do this, but it must remain separated from PME-connected metalwork above and below ground. A shared pipe, cable armour or data screen can defeat the separation.

Use an electrode as part of a calculated protective measure. An electrode intended to limit touch voltage may need a resistance that is difficult to achieve and maintain. An electrode used only as a measurement reference has a different duty. Neither is specified by choosing a familiar “earth rod target”.

Use an open-PEN detection device. An OPDD monitors defined electrical conditions and isolates the vehicle when its fault criteria are met. It can be a separate device or part of the chargepoint. IET 01:2024 gives manufacturers and designers a common specification for these devices.

Use another permitted measure giving the required safety. Any alternative has to be designed, justified and documented. “Equivalent” is an engineering claim, not a marketing phrase.

OPDD protection boundaries

“Built-in PEN protection” on a product page is not enough information. The installer should obtain the declaration and instructions that identify:

• the standard and category to which the device is declared
• whether it suits the supply phases and network arrangement
• what it measures and whether a reference electrode is required
• which conductors and connections it disconnects
• its operating, reset and fail-safe behaviour
• any permitted circuit voltage drop and supply-voltage conditions
• whether it protects one charging point or a defined group
• the tests required at commissioning and during maintenance

Voltage measured at a charger includes the drop along its final circuit. The IET has highlighted that a long or heavily loaded cable can therefore affect a voltage-based OPDD. Cable design and device selection must be coordinated so normal network variation and circuit voltage drop do not cause avoidable trips or move the device outside its declared conditions.

Three-phase systems need particular care because a neutral displacement can affect phases differently. A device designed for one single-phase chargepoint should not be assumed to protect a group of chargers or a three-phase installation.

The earth-rod misconception

An earth electrode can be a valid part of an EV design, but “rod fitted” does not define:

• whether it is the circuit’s protective earth, a supplementary electrode or a sensing reference
• whether the charger remains connected to PME
• the resistance or measurement performance required for its actual duty
• separation from PME-connected buried metalwork
• the completed test method and result

The ground also contains services. Any proposed electrode requires a planned and safe ground-work method, not just a convenient patch of soil near the wall.

Commissioning and handover

The electrical certificate and charger handover should identify the supply earthing arrangement and the exact open-PEN measure used. Where an OPDD is fitted, keep its declaration of conformity, wiring details, settings and prescribed functional-test result. Where an electrode is involved, record its purpose, location, test method and result.

The installer should also check the complete connected-vehicle area for simultaneous contact with metalwork on another earthing system. This matters at shared parking, flats and cross-pavement installations, where a vehicle may be closer to street furniture or a neighbour’s charging vehicle than the wallbox suggests.

Do not test an OPDD by creating a hazardous supply condition. Use the manufacturer-approved method and suitable test equipment.

If the charger reports a PEN fault

The indication and reset behaviour vary by product. Follow the charger’s instructions and do not bypass the protection to restore charging.

If there are abnormal voltages, unusually bright or dim lights, tingling from metalwork or another sign of a network safety problem, keep clear and call 105. It connects callers to the local electricity network operator. If there is an immediate danger to life or fire, call 999 as well and say that electricity is involved.

Related entries

• PME earthing systems
• TT earthing arrangements
• Earth electrode and rod installation
• RCD protection for EV chargers
• EV chargepoint commissioning
• IET EV charging Code of Practice