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What Is a PME Fault Detection Device - And How Does It Work?
Posted by: Michael Hickey -

PME (Protective Multiple Earthing) is one of the most common earthing systems used in the UK. It’s reliable in most cases, but it comes with a big risk: if the combined neutral and earth (PEN) conductor fails, exposed metal parts can suddenly become live. That includes EV chargers, car bodywork, and outdoor equipment.

A PME Fault Detection Device solves this by monitoring the supply and disconnecting power the moment a PEN fault is detected. For example, without PEN protection, a broken neutral on a PME supply can leave a car body sitting at full mains voltage. That means the chassis you’re about to plug into could be live at 230V — a direct shock risk to the user. PME detection devices cut the supply before that can ever happen.

Here’s what you need to know about how these devices work, why they’re needed, and which options are available.

Looking for compliant, ready-built boards? Explore our full range of Consumer Units - including EV-ready, PME-ready and surge-protected options.

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Why Do We Need PME Fault Detection?

When the PEN conductor fails, dangerous voltages can appear on accessible metalwork. In the context of EV charging, that could mean a live voltage on the chassis of the vehicle itself.

Amendment 2 of the 18th Edition wiring regs requires EV chargers connected to PME networks to have protection against open PEN faults. BS 7671:2018+A2:2022 (Reg. 722.411.4.1) specifically prohibits connecting outdoor EV charge points directly to PME earthing unless open PEN protection is provided. That’s why PME Fault Detection Devices are now a requirement, not an optional extra.

What Is a PME Fault Detection Device?

A PME Fault Detection Device constantly monitors the supply voltage on a TN-C-S (PME) system. If it detects an imbalance or an abnormal voltage that indicates the PEN has been lost, it automatically disconnects the supply.

  • In simple terms: it stops dangerous touch voltages from ever reaching the vehicle or charger.

Most units combine the detection system with other protection devices like RCBOs, SPDs, and contactors. That way, you get overcurrent, earth leakage, surge, and PEN protection all in one enclosure.

Is PME the Same as PEN?

Not exactly.

  • PEN conductor = the physical wire that carries both Protective Earth (PE) and Neutral (N).
  • PME system = the wider UK earthing arrangement (TN-C-S) where the DNO distributes that PEN conductor and provides multiple earthing points along the network.

PEN faults are the hazard. PME fault detection devices are the protection.

How Does a PME Fault Detection Device Work?

  • Monitors the voltage between live, neutral, and earth.
  • Detects abnormal conditions (like loss of neutral in a PEN).
  • Trips a built-in contactor or RCBO to cut power to the charger or board.
  • Some models self-reset when the supply stabilises, while others need manual reset.

This action prevents exposed conductive parts (like the car body or charger casing) from sitting at dangerous voltages.

Why Is PME Fault Protection Critical for EV Charging?

EV chargers are almost always installed outdoors and connected to PME supplies. Without PEN protection, the risk is that a fault in the supply could energise the car itself.

  • Regs demand it: Amendment 2 requires PEN fault protection for PME-fed chargers.
  • Earth rods aren’t always practical: While driving a rod is an option, it’s not reliable in all soil types or locations.
  • Installers save time: PME fault devices are faster and cleaner to fit than installing an earth electrode.
  • Customers stay safe: It avoids hazardous conditions and failed inspections.

What Are the Problems with PME Earthing?

According to UK DNO data, around 400 PEN failures are reported every year. That’s not a huge number compared to the millions of connections — but every failure carries serious safety consequences if no detection is in place.

  • Single point of failure: If the PEN breaks, the whole installation is compromised.
  • Voltage rise: Live potential can appear on accessible parts of the system.
  • Outdoor equipment risk: EVs, heat pumps, and metal-cased equipment outdoors are especially vulnerable.

FAQs on PME Fault Detection

What does a PEN fault device do?

It disconnects the power supply if the PEN conductor fails, preventing dangerous voltages from reaching exposed parts.

Are PEN and PME the same?

No. PEN is the conductor. PME is the earthing system that uses it.

Do I still need an earth rod?

Not if a compliant PME fault detection device is fitted. That’s one of the main reasons installers use them.

What are the four types of earthing systems?

TN-S, TN-C, TN-C-S (PME), and TT.

Is PME protection built into some chargers?

A few premium chargers include integrated PEN fault detection, but most do not. Always check the manufacturer’s specification. Even as more chargers come with integrated PEN protection, standalone PME devices remain the safer choice in most installs. They give you visibility, reset options, and upgrade flexibility that built-in solutions can’t match — and they cover you if the charger is ever swapped out.

Recommended PME Fault Detection Units

The simplest way to stay compliant is to use a pre-assembled PME Fault Detection Unit. These come pre-wired with RCBO, SPD, and contactor modules, so they’re fast to fit and ready to go.

Popular options include:

Wrap-up

PME fault detection devices are now essential for EV charging on PME supplies. They monitor the PEN conductor, disconnect when a fault is detected, and prevent dangerous voltages from reaching the charger or vehicle.

For installers, they’re the fastest way to stay compliant with the 18th Edition while avoiding the hassle of earth rods.

👉 Browse our full range of PME Fault Detection Units — all stocked in the UK and available for next-day delivery.