PME earthing systems

How PME and TN-C-S earthing work, what an open PEN fault changes, and where additional design measures are needed.

Protective multiple earthing, or PME, is the network operator’s application of a TN-C-S earthing arrangement. On the network side, one PEN conductor performs the combined protective-earth and neutral functions. At the supply position, those functions are separated into neutral and earth for the customer’s installation.

The combined function is not continued through the home’s wiring. The Electricity Safety, Quality and Continuity Regulations prohibit consumers from using a PEN conductor in their installation.

What “multiple” means

The network neutral is connected to Earth at the supply transformer and at additional points on the low-voltage network. Those multiple connections help control the PEN conductor’s voltage to Earth and reduce the consequences of faults.

The network operator designs and maintains that system under the statutory regulations and ENA Engineering Recommendation G12. The consumer is responsible for the installation after the supply boundary, including suitable earthing conductors, protective bonding and any additional measures required by the equipment being installed.

An earth terminal at the service head should not be altered, and a missing or damaged terminal should not be improvised. The DNO must confirm or provide the network earthing facility.

The open-PEN risk

If the PEN conductor becomes open circuit upstream of a property, it loses its intended neutral and protective path. Load current can then seek a return route through bonded water or gas pipes, structural metal, neighbouring installations and the ground.

This can cause:

dangerous voltage between PME-connected metalwork and true Earth
diverted current in bonding conductors and shared metallic services
overheating and fire risk at poor or undersized connections
abnormal line-to-neutral voltages, particularly on an unbalanced three-phase network
damage or unexpected behaviour in connected equipment

The risk is especially important outdoors. A person touching a PME-connected car, tap, heat-pump casing or other exposed metalwork while standing on the ground can bridge the voltage difference.

A conventional RCD is not, by itself, an open-PEN protective device. It may see outgoing and returning load current as balanced even while the installation earth has risen above true Earth. Main protective bonding also has a different job: it reduces voltage differences between bonded parts inside the equipotential zone, but does not hold them at true-Earth potential.

Warning signs and what to do

Possible signs of a neutral or PEN problem include lights becoming unusually bright or dim, erratic voltages, equipment behaving abnormally, or a shock or tingling sensation from taps or other metalwork. These signs can have other causes, but they need urgent attention.

If metalwork may be live, do not keep touching it or try to prove the fault with a DIY tester. Keep other people away and call 105, which routes the call to the local electricity network operator. If there is fire, serious injury or an immediate danger to life, call 999 as well and say that electricity is involved.

An electrician who finds diverted neutral current should not simply disconnect the bonding path. Current can remain present even when the customer’s main switch is off, and removing the path may make accessible metalwork more dangerous. The network operator needs to investigate the supply.

EV chargepoints

An EV being charged outdoors combines PME-connected metalwork with a person standing on true Earth, so BS 7671 contains specific measures for that risk. The designer must select a permitted protective arrangement under the edition of Section 722 applicable to the work and coordinate it with the charger and its instructions.

Modern chargepoints often include, or are paired with, open-PEN detection that disconnects the live conductors and protective connection when the required fault conditions are detected. IET 01:2024 gives a specification for open-PEN detection devices. A product claim is not enough on its own: the installer still has to verify the device’s declared conformity, installation conditions and complete circuit design.

An earth electrode can be part of some permitted arrangements, but a casual rod is not a universal answer. An electrode used to limit touch voltage may need a resistance that is difficult to achieve, while one used as a measurement reference has a different duty. Nearby PME-connected buried metalwork can also raise the local ground potential and undermine a supposedly separate TT area.

The dedicated charger article covers the protective options and RCD requirements in more detail.

Outbuildings and outdoor equipment

PME can be exported to an ordinary detached building where the complete design allows it, but the decision cannot be made from the cable type alone. The installer should assess:

extraneous-conductive-parts and required main protective bonding
whether cable armour or another protective conductor can safely perform every assigned duty
people touching connected metalwork while in contact with the ground
shared pipes, structural metal and data links between buildings
any special-location requirements
whether a properly separated TT arrangement would reduce or create risk

There is no sound blanket rule that every shed should be TT, or that PME should always be exported. Both arrangements can become unsafe when bonding paths and simultaneous contact are missed.

Outdoor heat pumps and other fixed equipment also need an individual assessment. Being outside is not, by itself, a ban on PME. The equipment construction, location, circuit protection and accessible conductive parts determine the risk.

Solar and battery systems

A normal grid-connected inverter uses the installation earthing arrangement while it is connected to the network. It must disconnect or cease energising the grid when the supply fails in accordance with its connection protection.

A battery that supplies backup circuits has an additional problem: island mode cannot depend on the grid’s neutral-to-earth reference remaining available through disconnected live conductors. The changeover, neutral-to-earth switching, consumer electrode and protective devices must form a valid arrangement in both connected and island modes.

That does not mean every PME home with a battery becomes TT. A designed island may use a local TN-S arrangement. The decisive questions are which conductors are switched, where the source reference is created and whether faults disconnect safely in each mode.

Confirming the supply type

Cable appearance is not reliable proof of TN-S. Network repairs and conversions can produce mixed cable construction. IET guidance says a public-network supply should be presumed TN-C-S unless the DNO confirms TN-S in writing.

For a new charger, battery or other design sensitive to open-PEN risk, record the DNO’s confirmation rather than relying on a visual guess at the service head.

Bonding and inspection

PME main protective bonding is sized to account for possible diverted neutral current, not only ordinary circuit fault current. An electrician should confirm the bonding arrangement before adding new work and use appropriate precautions before disconnecting any conductor.

Useful handover records include:

the DNO-confirmed earthing arrangement
main earthing and bonding conductor details
the open-PEN measure used for relevant equipment
any electrode purpose and test result
island-mode switching and test results where backup is provided
the applicable BS 7671 edition and product instructions

These records prevent a later installer from undoing a deliberate protective arrangement or mistaking a sensing electrode for a TT earth.