# What Is a PEN Fault and Why Does It Matter for EV Charging? By EV Charge Cables ยท Published 18 October 2026 **The short version** A PEN fault is a failure in the combined protective earth and neutral conductor on a PME supply, which can make a charging car dangerously live. - Most UK homes use a PME (TN-C-S) supply, which carries a PEN fault risk if the shared conductor breaks. - UK wiring regulations (BS 7671) require a PEN fault protection device on any wallbox connected to a PME supply. - A qualified, OZEV-registered installer will assess your supply type and fit the correct protection automatically. A PEN fault is a break or failure in the combined protective earth and neutral conductor in an older type of domestic electrical supply, and it matters for EV charging because a wallbox drawing high current for an extended period can expose any underlying weaknesses in the supply in a way that an ordinary household load would not. If the PEN conductor fails while a charger is running, dangerous voltages can appear on parts of the car and charger that should be at safe potential. This is a technical topic that your installer will address, but understanding it helps you ask the right questions. For cable guidance once your installation is complete, start with [our cable finder](https://www.evchargecable.co.uk/). | Supply type | PEN fault risk | Required protection | | --- | --- | --- | | TN-C-S (PME), most common in UK | Present if PEN conductor fails | PEN fault protection device (PFD) required by BS 7671 | | TN-S (separate earth and neutral) | Lower risk; separate earth to substation | Standard RCD/RCBO protection sufficient | | TT (local earth rod) | No PEN conductor, so no PEN fault risk | RCD required; often found in rural properties | ### What is PME and how does it relate to PEN faults? Most UK homes are connected to the grid via a Protective Multiple Earthing (PME) supply, also known as a TN-C-S system. In this arrangement, the neutral and the protective earth share a single conductor back to the distribution network. That combined conductor is called the PEN (Protective Earth and Neutral). Under normal conditions this is perfectly safe and is used in millions of homes without issue. The concern arises if the PEN conductor breaks somewhere between your home and the substation. In that situation, the earth in your home can rise to a dangerous voltage relative to true earth, which poses a shock risk to anyone touching the car or charger. ### Why does EV charging make this more significant? The risk from a PEN fault has always existed in PME supplies, but a typical home draws a modest and variable load. An EV wallbox draws a large, sustained current for many hours, which places a more consistent stress on the supply and increases the consequences if a fault develops during charging. A person touching the car bodywork, which would be energised in a PEN fault scenario, could receive a serious or fatal electric shock. The 18th edition of the IET Wiring Regulations (BS 7671) requires that EV charging equipment on a PME supply must include a PEN fault protection device to detect this condition and disconnect the charger before a dangerous situation develops. ### What does a PEN fault protection device actually do? A PEN fault protection device (sometimes abbreviated as PFD) continuously monitors the voltage between neutral and earth. If the neutral rises relative to earth by more than a safe threshold, which is what happens during a PEN fault, the device disconnects the charger. This prevents the car from becoming live while someone might be approaching or touching it. Some wallboxes have this protection built in as a single integrated unit. In other installations it is a separate device fitted by the electrician. Your OZEV-registered installer is required to assess the supply type at your property and fit appropriate protection before connecting any wallbox. ### How do I know what type of supply my home has? You do not need to determine this yourself. A competent electrician or OZEV-registered wallbox installer will carry out a supply assessment as part of the installation survey. They will check the earthing arrangement and confirm whether PME is in use. If you are curious, the type of supply is often indicated on your electricity meter or consumer unit documentation, or your installer can tell you after the survey. Properties in rural areas, including many farms and older houses, are sometimes supplied via a TT arrangement with a local earth rod, which does not carry a PEN fault risk in the same way. ### Are there situations where a wallbox cannot be installed safely on a PME supply? In practice, the vast majority of PME-supplied homes can have a wallbox installed safely with appropriate PEN fault protection. A small number of older properties or properties with particularly aged supply infrastructure may require additional work by the network operator before installation. Your installer's survey will flag any issues. The regulations are designed to be practical: the goal is to make EV charging safe, not to prevent it. If you are getting a wallbox installed, choosing an OZEV-registered installer is important precisely because they understand these requirements and will not cut corners on protection. ### Frequently asked questions #### Do I need to do anything myself to protect against PEN faults? No. This is handled by your installer as part of a compliant wallbox installation. A reputable, OZEV-registered installer will assess your supply, confirm whether PME is in use, and fit appropriate PEN fault protection as required by the wiring regulations. You do not need to source or specify the protection device yourself. #### Can I use a 3-pin plug charger on a PME supply without worrying about PEN faults? A 3-pin granny charger is typically used at lower power and for shorter periods than a wallbox, but the same underlying supply issue applies. For occasional use it is unlikely to be a significant concern in practice, but for regular or overnight charging a properly installed wallbox with appropriate protection is the right long-term solution. #### Is a TT supply safer for EV charging? A TT supply uses a local earth rod rather than the shared PEN conductor, so the specific PEN fault risk does not apply. However, TT supplies have their own characteristics that affect charger installation, including the need for a suitable RCD. Your installer will know how to work with whichever supply type you have. #### What happens if an installer skips PEN fault protection? An installation without required PEN fault protection would not comply with BS 7671 and could fail an electrical inspection. More importantly, it leaves a genuine safety gap if the supply ever develops a PEN fault during charging. Always use a qualified, OZEV-registered installer who will certify the work is compliant. #### Does the car itself provide any protection against PEN faults? The car's own systems are not designed to detect or protect against a supply-side PEN fault. Protection must be in the charging equipment and supply infrastructure. This is why the wiring regulations require it to be addressed at the installation stage rather than relying on the vehicle. #### Does this affect public charging as well as home charging? Public charging installations are subject to their own compliance requirements and are installed and maintained by professional operators. The PEN fault concern is primarily relevant to domestic installations where the supply arrives from the local network in a PME arrangement. If you charge primarily at home, the relevant protection should be in place as part of a compliant wallbox installation. PEN fault protection is not a detail you need to manage yourself: it is your installer's responsibility, and a good OZEV-registered installer will handle it as a matter of course. What you can do is choose your installer carefully, ask them to confirm the supply type at your property, and keep a copy of the installation certificate. 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