What Happens During a PEN Fault? Step-by-Step Guide

Normal Operation on a PME Supply

In a TN-C-S/PME system, the distributor's PEN conductor carries neutral current and provides the reference from which the customer's protective conductor is derived. Under healthy conditions, bonded metalwork remains close to the expected earth potential.

Step 1: The PEN Conductor Becomes Open or High Resistance

The fault may be a complete break or a deteriorated joint with significant resistance. Because it is on the supply side, the customer's final-circuit protective devices do not necessarily see it as a short circuit or overload.

Step 2: Neutral Voltage Shifts

Connected loads still seek a return path. On a three-phase network, phase imbalance can move the neutral point; on single-phase supplies, return current and parallel paths can also raise the customer's PME earth reference relative to true Earth.

Exposed conductive parts bonded to the PME terminal can rise with that reference. A person standing on the ground and touching a connected vehicle, charger casing or other metalwork may bridge the voltage difference.

Step 3: Why MCBs, Fuses and RCDs May Not Clear It

An MCB or fuse responds mainly to overcurrent. An ordinary RCD responds to imbalance between the live conductors passing through it. An upstream PEN failure may allow current to continue flowing without creating the downstream conditions that either device is designed to detect.

This is why “the breaker will trip” is not a safe assumption. Open-PEN protection looks for a different fault signature.

Why PEN Faults Matter for EV Charging

A vehicle is a large accessible conductive object, often touched outdoors while the user is in contact with true Earth. That combination makes the touch-voltage risk particularly important on PME-fed charging points.

The same discussion should not be extended automatically to every outdoor appliance. Use the requirements for the actual equipment and location.

How a PEN Fault Detection Unit Responds

A suitable unit monitors the parameters declared by its manufacturer. If those parameters cross the operating thresholds for the required duration, the unit commands its contactor or switching device to disconnect the protected circuit.

• It must be suitable for the supply phase arrangement.
• It must disconnect the required live conductors.
• Its load rating and protective devices must be coordinated.
• Its interaction with charger-integrated protection must be checked.

For single-phase applications, consider the IP65 PME Consumer Unit with Type A RCBO and SPD. For multi-charge-point work, see the three-phase PME fault detection unit.

PEN Fault FAQs

Does an open PEN always put exactly 230 V on metalwork?

No. The voltage varies with the network, loads, phase balance, impedance and any parallel paths. It can still be dangerous.

Can an RCD detect a PEN fault?

An ordinary RCD is not a substitute for open-PEN protection and may not see the required residual-current imbalance.

Is a PEN fault only a three-phase problem?

No. Three-phase imbalance is one mechanism, but single-phase PME installations can also present dangerous conditions.

What should an installer do if a supply PEN fault is suspected?

Stop, make the situation safe under the correct procedure and contact the relevant network operator; do not treat it as an ordinary customer-side repair.

Final Word

A PEN failure can move the installation's neutral and PME earth reference without producing a conventional protective-device trip. That is why outdoor EV charging needs a recognised protective arrangement designed specifically for the open-PEN risk.