If you install EV charge points, outdoor supplies, or any equipment on a PME (TN-C-S) supply, you’ve probably heard the phrase “open PEN” or “PEN conductor failure”. It gets mentioned in regs guidance and DNO conversations, but it’s not always explained in plain installer terms.
This guide breaks down what a PEN conductor is, why its failure is dangerous, and why PEN/PME fault detection devices exist - especially on EV installs where exporting PME and touch voltage risk become a real issue.
Quick Answer: A PEN conductor is a combined Protective Earth + Neutral conductor used in TN-C-S (PME) systems. If the PEN goes open-circuit, metalwork that should be at earth potential can rise towards line voltage, creating a dangerous touch voltage risk. That’s why PEN/PME fault detection is commonly specified for EV charge points and outdoor equipment on PME supplies.
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What Is a PEN Conductor?
In plain terms: a PEN conductor is a single conductor that acts as both Protective Earth (PE) and Neutral (N).
In a TN-C-S system (commonly called PME in the UK), the supply network uses a combined earth/neutral conductor for part of its route. At the service head / cut-out, that combined conductor is split into separate N and PE for the installation.
Under normal conditions, the arrangement is reliable and widely used - but the key point is this: if the PEN becomes open-circuit upstream, the installation “earth” is no longer sitting safely at true earth potential.
Where PEN Is Used (TN-C-S / PME Supplies)
Most domestic and light commercial sites in the UK are TN-C-S / PME. You’ll typically see this where the DNO provides a PME terminal at the cut-out and bonding is in place to bring exposed conductive parts to the same potential.
The PEN conductor exists on the supply side - in the distribution network and service cable - before it is separated into neutral and earth at the intake position.
- TN-C-S (PME): Combined PEN in the network, separated at the cut-out.
- TN-S: Separate earth provided by the supply (no combined PEN in the same way).
- TT: No earth from supply; installation relies on an earth electrode.
Regulation Reminder: EV installs on PME supplies are a hotspot under BS 7671 because an open PEN can raise touch voltage on accessible metalwork. The accepted methods of protection typically involve PEN fault detection (or other compliant arrangements) where required. Always confirm the current EV charging requirements and manufacturer instructions for the specific installation.
Why an Open PEN Conductor Is Dangerous
The danger is touch voltage. When the PEN is intact, bonding keeps everything referenced to the same potential. When the PEN goes open-circuit upstream, the return path for neutral current is compromised and the installation’s “earth” can float upwards in potential.
That creates a scenario where:
- Exposed metalwork connected to PME earth (charger casing, metal enclosure, bonded pipework) can rise in voltage.
- A person touching that metalwork while also in contact with true earth (standing outdoors, touching a metal fence, damp ground) can be exposed to a dangerous potential difference.
- This can happen without a traditional overcurrent fault that would instantly clear via MCB/fuse.
On paper, it’s sometimes described as “earth rising towards line potential”. On site, the takeaway is simpler: open PEN risk is about accessible metal becoming live-ish in the wrong conditions.
Installer note: This is why “just stick it on the existing PME” can be a problem for outdoor and EV installs. The risk is not a normal earth fault - it’s the supply reference itself disappearing.
What Happens During a PEN Failure (Step-by-Step)
Here’s the practical sequence. Not every open PEN event looks identical, but the pattern is consistent enough to understand why protection is needed.
- PEN goes high impedance or open-circuit somewhere on the DNO network / service cable.
- Neutral reference becomes unstable - voltages can shift, especially under load or with uneven loading.
- Installation earth can rise in potential because it’s tied to the compromised supply reference.
- Accessible metalwork becomes a touch voltage risk relative to true earth.
- Danger exists before a “fault” trips anything - because MCBs/fuses are not designed to detect this condition.
That’s the reason you’ll see dedicated terms like open PEN detection, O-PEN devices, and PME fault detection units.
Why EV Chargers Are a Focus for PEN Protection
EV charge points combine outdoor conditions with accessible metalwork. Even when the unit is plastic, you often have connected metalwork in the environment - vehicle bodywork, metal posts, adjacent metal fences, damp paving, and users who are grounded.
Two common reasons EV installs get special attention:
- Outdoor / external touch risk: People are often standing on the ground while handling the charge connector.
- Exported PME concerns: Many EV setups effectively “export” the PME earth outside the equipotential zone of the building.
Trade shortcut: If you’re installing EV equipment on a PME supply and the install approach involves exporting earth or creates an outdoor touch voltage concern, you’re usually into the territory where PEN fault protection must be considered.
How PEN / PME Fault Detection Devices Reduce the Risk
In one line: these devices monitor supply conditions and disconnect the circuit if a dangerous open PEN condition is detected.
Different manufacturers use different detection logic (voltage monitoring, imbalance detection, reference checks), but the practical outcome is the same: if the supply earth/neutral reference becomes unsafe, the device isolates the load.
That’s why you’ll see PME units built as pre-wired, install-ready boards - they combine:
- PEN/PME fault detection (the protection layer)
- Main switch / isolation
- Overcurrent protection (MCB/RCBO)
- Surge protection (SPD on variants that include it)
For installers, it’s often the simplest way to keep the spec clean: dedicated EV/PME board, clear parts list, and a single enclosure that’s already built around the compliance requirement.
Choosing the Right PME Fault Detection Unit (What to Spec)
Start with the supply type and what else the board needs to do. The main decisions are usually phase, whether you need SPD, and whether load balancing / metering is part of the job.
| Spec Decision | What It Affects | Common Choice |
|---|---|---|
| 1-phase vs 3-phase | Device type + pole arrangement | Domestic EV = 1-phase, Commercial EV = often 3-phase |
| SPD included? | Amendment 2 compliance + equipment protection | Often yes for EV boards |
| MCB vs RCBO | Overcurrent vs combined protection | RCBO where you want earth leakage + overcurrent in one device |
| Load balancing / metering | Commercial / multi-charger management | Load relay + kWh metering for managed installs |
Compare the full range here: PME Fault Detection Units (single-phase & three-phase).
Spec-Compliant PME Fault Detection Units
If you’re specifying PEN protection for an EV or outdoor install on PME, choose a unit matched to the supply and load requirements:
Single-Phase (Domestic EV):
👉 IP65 PME Consumer Unit – 40A Type A RCBO + SPD
👉 IP65 PME Consumer Unit – 40A Type A RCBO (No SPD)
Three-Phase (Commercial / 22kW EV):
👉 3-Phase PME Fault Detection Unit – 63A with SPD
All units are designed for compliant PEN detection on TN-C-S (PME) supplies.
Installer’s Pick: If you want the simplest compliance-friendly setup, choose a dedicated PME fault detection board with SPD included - it reduces parts chasing and keeps the install spec consistent.
View a Single-Phase PME Consumer Unit with Type A RCBO + SPD (IP65)
FAQs
Quick answers for common PME / PEN questions on site.
Is PEN the same as PME?
No. PEN is the combined protective earth + neutral conductor. PME is the earthing arrangement (TN-C-S) where that combined conductor exists in the supply network and is separated at the intake.
What does “open PEN” mean?
It means the PEN conductor has become disconnected or high resistance upstream. That can make the installation earth reference unstable and create a touch voltage risk on accessible metalwork.
Will an MCB or fuse trip in an open PEN fault?
Not reliably. Open PEN issues are not the same as a short-circuit or overcurrent event. That’s why dedicated PEN/PME fault detection is used where required.
Do EV chargers always need PEN fault protection on PME?
Not in every scenario, but EV charging on PME supplies is one of the main applications where PEN protection is commonly required. Always follow BS 7671 requirements and the EVSE manufacturer’s instructions for the install approach.
Do I still need an earth rod if I fit a PME fault detection device?
Often no, depending on the compliant method used. This is covered in full here: Do You Need an Earth Rod with PME Fault Protection?
Is PEN fault protection the same as RCD protection?
No. RCDs/RCBOs monitor residual current to protect against earth leakage faults. PEN/PME fault detection monitors supply conditions linked to an open PEN risk and isolates the circuit when that condition is detected.
Final Word
PEN failure is a supply reference problem, not a standard “fault” your protective devices are guaranteed to clear. That’s why open PEN risk matters most on outdoor and EV installs where users are grounded and exposed metalwork can become a touch voltage hazard.
- PEN = combined protective earth + neutral used on TN-C-S (PME) networks.
- Open PEN can raise the installation earth potential and create dangerous touch voltages.
- PEN/PME fault detection units are designed to monitor and disconnect if the supply reference becomes unsafe.
👉 Ready to spec? Browse PME Fault Detection Units and choose the right single-phase or three-phase option for your install.