What is a PEN fault, and what does an O-PEN device do?

Glossary · EV charging

How a PEN fault works

A protective earthed neutral (PEN) conductor is a single conductor that has the combined function of the neutral and the protective earth in a TN-C-S earthing arrangement. The Institution of Engineering and Technology (IET) says it is normally, but not exclusively, used with a PME supply service earthing system.^{[1]The Institution of Engineering and Technology, Broken PEN (Wiring Matters, March 2021)} In that arrangement the supplier’s cable carries neutral and earth together, and the earthing of your installation relies on it. See earthing systems for the types.

An open PEN fault, also called a broken PEN, happens when that combined conductor goes open circuit somewhere in the supplier’s network.^{[2]The Institution of Engineering and Technology, IET 01 FAQ's} The IET says the most common causes are damage to cabling from heavy equipment or machinery, and corrosion linked to environmental conditions.^{[2]The Institution of Engineering and Technology, IET 01 FAQ's}

Why it is dangerous

When the PEN conductor breaks, the IET says diverted neutral currents and dangerous touch voltages can appear on metalwork connected to the installation’s main earthing terminal.^{[1]The Institution of Engineering and Technology, Broken PEN (Wiring Matters, March 2021)} The risk of shock is higher outdoors, where a person is likely to be in contact with the ground, and the article names outside water taps and Class I equipment as examples. It also says fire can be a risk, because the diverted current can heat metal water and gas pipes.^{[1]The Institution of Engineering and Technology, Broken PEN (Wiring Matters, March 2021)}

The IET describes the EV case this way. During an open PEN fault, a dangerous potential can develop between metalwork connected to the protective conductor, here the metallic body of the car, and true earth.^{[2]The Institution of Engineering and Technology, IET 01 FAQ's} Someone standing on the ground and touching the car could be at risk. That is why chargepoints get extra protection.

What an O-PEN device does

An O-PEN device is the protection that an EV chargepoint uses to cope with an open PEN fault. The IET’s FAQ explains that BS 7671 requires protection against electric shock by a device that electrically disconnects the vehicle from the live conductors of the supply and from protective earth within 5 seconds, if the voltage at the charging point between line and neutral is greater than 253 V or less than 207 V.^{[2]The Institution of Engineering and Technology, IET 01 FAQ's} In plain terms, the charger watches the supply voltage and shuts the vehicle connection off if it goes outside that window.

The IET says different manufacturers use different methods to meet the requirement, and it is not always clear to the installer which method a given charger uses or whether it suits a particular location.^{[2]The Institution of Engineering and Technology, IET 01 FAQ's} Choosing and fitting the right charger is a job for a registered installer. Details are on our EV charger installation page.

What to do if you suspect one

The fault is in the distributor’s network. The IET FAQ says that where an open PEN fault is detected, the DNO must be contacted immediately so that the fault is reported, assessed and addressed.^{[2]The Institution of Engineering and Technology, IET 01 FAQ's} The Wiring Matters article tells people working on an installation to report a suspected open-circuit PEN conductor by telephoning 105, which routes to the local DNO’s emergency number.^{[1]The Institution of Engineering and Technology, Broken PEN (Wiring Matters, March 2021)} The IET says further research is needed on how often these faults occur.^{[2]The Institution of Engineering and Technology, IET 01 FAQ's}

Example

Tomasz has a charger fitted at his home in Kirkstall. His installer explains that the unit includes open PEN protection and checks how it responds to the supply voltage. If the supply voltage ever moves outside the limits, the charger will disconnect the car. His installer also tells him that the unit protects the vehicle connection and nothing else, so a suspected supply fault still goes to the network operator.

What people get wrong

The common mistake is to think an O-PEN device removes the PEN fault risk. The IET says a charger specified to its standard provides methods of protection in an open PEN fault, but the risk of a dangerous potential between any earthed metalwork and true earth continues to exist.

The second mistake is to think the risk is limited to EV chargers. The IET says it is not, and that any metalwork connected to the protective conductor may present a shock risk.^{[2]The Institution of Engineering and Technology, IET 01 FAQ's}

The bottom line

A PEN fault is a break in the supplier’s combined neutral and earth conductor. It can put dangerous voltage on earthed metalwork and divert current through pipework. An O-PEN device in an EV charger detects it by monitoring supply voltage and disconnects the vehicle, but it does not make other metalwork safe. If a fault is suspected, the network operator must be called without delay.