An open PEN fault isn’t a typical short circuit. There’s no bang, no obvious overcurrent, and no guarantee a breaker will trip immediately. But the risk can be serious - especially on PME (TN-C-S) supplies feeding outdoor or EV equipment.
This guide explains what actually happens during a PEN failure, step by step, and why standard protective devices don’t always deal with it on their own.
Quick Answer: When a PEN conductor fails, the installation can lose its stable earth reference. Neutral voltages may shift, exposed metalwork can rise in potential, and dangerous touch voltages may appear - without a traditional fault current large enough to trip an MCB or fuse. That’s why PEN/PME fault detection devices are used on certain installs.
👉 Compare PME Fault Detection Units (IP65, SPD & 3-Phase Options)
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Normal Operation on a PME (TN-C-S) Supply
Under normal conditions, the PEN conductor provides both neutral return and earth reference up to the service position. Inside the installation, neutral and earth are separated, and bonding keeps exposed metalwork at the same potential.
Everything sits at a stable reference. Voltages are balanced. Protective devices operate as designed.
Step 1: The PEN Conductor Becomes High Resistance or Open Circuit
The failure usually occurs upstream - within the DNO network or service cable. The combined protective earth + neutral path becomes compromised.
At this point, the installation hasn’t “faulted” in the usual sense. Loads are still connected. Current may still flow - but the reference point has changed.
Step 2: Neutral Voltage Shift & Earth Potential Rise
With the PEN compromised, the neutral reference floats. On single-phase supplies this can cause voltage instability. On three-phase systems, imbalance can become significant.
Crucially, because the installation earth is tied to that same reference, exposed conductive parts can rise in voltage relative to true earth.
- EV charger casings
- Metal enclosures
- Bonded pipework
- Vehicle bodywork
This is where the touch voltage risk appears.
Step 3: Why MCBs, Fuses & RCDs May Not Clear It
Overcurrent devices operate when current exceeds rating. RCDs operate on residual imbalance.
An open PEN event does not automatically produce a large overcurrent or residual fault. The installation may continue operating in an unsafe condition.
Key point: Open PEN risk is a supply reference problem - not necessarily a short-circuit or earth leakage event.
Why This Matters Most on EV & Outdoor Installs
Indoor bonding reduces exposure to true earth potential differences. Outdoor installs are different.
When a person stands on the ground and touches a vehicle or charger connected to a PME earth, any voltage rise can become hazardous.
How PEN / PME Fault Detection Devices Respond
PEN fault detection units monitor voltage relationships and supply conditions. When they detect parameters consistent with an open PEN scenario, they disconnect the protected circuit.
- Supply voltage monitoring
- Neutral shift detection
- Automatic isolation of load
In practice, this means the EV charger or downstream equipment is isolated before a prolonged touch voltage condition can exist.
Installer Tip: Using a dedicated PME fault detection board keeps the compliance layer, isolation, and overcurrent protection contained in one enclosure.
PEN Detection Units Designed for This Scenario
If you’re protecting against neutral shift, voltage imbalance and open PEN conditions, use a dedicated PME fault detection unit matched to the supply:
Single-Phase (Domestic EV Installs):
👉 IP65 PME Consumer Unit – 40A Type A RCBO + SPD
Three-Phase (Commercial / Multi-Charger):
👉 3-Phase PME Fault Detection Unit – 40RSP
👉 3-Phase PME Fault Detection Unit – 63A with SPD
Each unit monitors supply reference conditions and disconnects all live conductors if unsafe parameters are detected.
FAQs
Quick, straight answers to the questions that usually come up on site or during design stage. These cover common misunderstandings around open PEN faults, voltage shift, and how protection devices behave.
Does an open PEN always cause overvoltage?
Not always extreme overvoltage, but voltage instability and reference shift are common - particularly under load imbalance.
Is this mainly a three-phase issue?
Three-phase imbalance can make effects more dramatic, but single-phase PME supplies can still present dangerous touch voltage risks.
Can you test for open PEN with a basic tester?
No. Detecting supply-side PEN integrity requires proper test procedures and equipment. Standard voltage pens are not suitable.
Final Word
An open PEN event doesn’t look like a typical electrical fault. The danger comes from earth reference instability, not just current flow.
- PEN failure can raise exposed metalwork potential.
- Breakers may not clear it automatically.
- EV and outdoor installs increase touch voltage exposure.
👉 Need compliant protection? Compare PME Fault Detection Units for single-phase and three-phase installations.