Technical guide — for electricians
Diverted neutral faults
On a TN-C-S (PME) supply, a high-impedance or broken neutral upstream of the intake doesn't just dim the lights — it pushes return current down your bonding, lifts earthed metalwork above true earth and, at its worst, puts line voltage on every tap, radiator and appliance chassis in the building. This page covers what causes it, what it looks like on site, the tests that confirm it, and the safe isolation rules that change when you suspect it.
The mechanism
Why neutral current ends up in the bonding
In a PME supply, neutral and earth are one conductor — the PEN — from the substation star point to the intake, and the installation's main earthing terminal is deliberately bonded to it. The PEN is a current-carrying conductor for every amp the property uses. So when a joint in that path corrodes, loosens or breaks, the load current doesn't stop: it finds another way back to the substation. The next best path is the one the installation itself provides — the main earthing terminal, the main protective bonding to gas and water, and the general mass of earth.
Everything downstream still "works". Loads run, RCDs stay healthy, and the standard certificate figures look normal. That is what makes the fault dangerous: the evidence is a warm clamp, a few volts to true earth and a persistent current on a conductor that should carry none.
Examples
Four ways it shows up on real jobs
Corroded or loose neutral joint at the service head
The most common source. The DNO's neutral terminates in the cut-out or service head, often decades old, and aluminium-to-copper or corroded joints slowly increase in resistance. Every amp the house draws now has to fight through that joint — and part of the return current finds an easier route through the installation's bonding.
Tell-tale: Household lights that dim when a large appliance starts, warm bonding clamps, occasional tingles from taps.
Shared services between neighbouring properties
Two houses fed from a common service, or flats whose metallic water and gas services are interconnected below ground, provide a parallel path between separate installations. When one property's neutral impedance rises, its return current flows down the other property's bonding — so one neighbour's fault becomes everyone's problem.
Tell-tale: Dimming and flicker in both properties, bonding current that changes when the neighbour switches loads on or off.
PME exported to an outbuilding or EV charge point
Where PME is exported to a detached building through steel wire armour, or to an outdoor EV charge point, a broken PEN upstream leaves the whole remote earthing arrangement — including the charge point body — depending on that single combined conductor. This is exactly why BS 7671 Section 722 restricts PME for outdoor charge points.
Tell-tale: Charge point or outbuilding metalwork at a measurable voltage to true earth; open-PEN detection device shutting the charger down repeatedly.
Metal services as the return path
An interconnected metallic water main, gas service pipe or structural steelwork across a street can quietly carry several amps of diverted neutral current continuously, under normal load, with nothing visibly wrong.
Tell-tale: Discoloration or heat at bonding clamps, corroded pipe fittings, currents measured on bonding where there should be none.
Dangers
Why this fault is treated as urgent
Fire at bonding clamps and connections
Bonding conductors were sized for fault current, not for carrying load current indefinitely. Continuous diverted current heats clamps on gas and water pipes, dries out and degrades the joint, and has caused fires where a clamp on a plastic gas service riser or an old water pipe worked loose.
Energised metalwork
Every volt dropped on the faulty neutral raises the installation's earthed metalwork above true earth. Taps, radiators, appliance chassis and shower parts sit at a touch voltage against the ground or a damp floor — the classic unexplained tingle.
Broken PEN — full line voltage on earthed parts
If the neutral fully breaks, the only return path is the bonding and the mass of earth, and the installation earth can rise toward line potential. Anyone touching earthed metal while standing on real ground is across 230 V. This is the failure mode Section 722's open-PEN measures for EV charge points exist to defeat.
Equipment damage on three-phase systems
On a three-phase supply with a failing neutral, unbalanced loads no longer share a solid star point. Voltages between lines and neutral can swing severely — some circuits see well above nominal while others brown out — killing motors, electronics and heating elements.
Danger hidden inside an otherwise tidy installation
The consumer's wiring can test perfectly. R1+R2, Zs and insulation resistance all look fine while the hazard sits upstream in the DNO's network. An electrician who tests only inside the installation will certify a dangerous supply as healthy.
Not the same as a borrowed neutral
A borrowed (shared) neutral is a wiring error inside the installation — one circuit's neutral tied to another circuit's — which makes safe isolation dangerous because isolating one circuit leaves a neutral live. A diverted neutral is a supply-side fault on the DNO's network. Both are found with neutral-to-earth testing and both demand care, but the borrowed neutral is fixed by the electrician rewiring; the diverted neutral is fixed by the DNO. NICEIC's guidance on shared neutrals covers the first case; this page covers the second.
Testing
How an electrician confirms it
Clamp the main protective bonding conductors
With loads running, clamp-meter each main bonding conductor. Any persistent current on a bonding conductor is diverted neutral current (or another parallel path) — bonding should carry nothing in normal operation. Then switch off at the main switch and re-clamp: current still flowing with the installation switched off confirms the path is live from outside the installation.
Measure neutral-to-earth voltage at the MET
Between the main earthing terminal and the incoming neutral, a healthy supply with load connected reads a fraction of a volt. Readings of a few volts and above under load point to high neutral impedance upstream — measure with a good load applied, because the symptom disappears with the load off.
MET to true earth, via a temporary electrode
Drive a test spike into real ground away from the building and measure from the MET to it. Zero (or millivolts) is healthy. Several volts, or tens of volts, means the installation earth is riding above true earth — a diverted neutral signature. Compare readings with different loads switched on and off.
Check the bonding clamps for heat
Warm or hot-to-touch clamps, discoloured pipework at the clamp, or melted PVC sleeving at the bonding connection are physical evidence of sustained current. An infrared thermometer or thermal camera makes the finding documentable for the EICR and the DNO.
Separate the suspects: borrowed neutral vs diverted neutral
A borrowed neutral is internal — a final circuit's neutral shared with another circuit — and shows up as neutral-to-earth continuity between ways with everything isolated. A diverted neutral is external — supply-side impedance driving current through bonding. Isolate, test N–E between circuits for the borrowed case; the clamp and spike tests above identify the diverted case. The two can coexist, and each one alone justifies action.
Report it to the DNO — do not attempt a supply-side repair
The service head, cut-out and service cable are DNO equipment. Call 105 (the free national power cut line) or the local DNO and report suspected diverted neutral current or a high-impedance neutral. DNOs attend supply-side neutral faults free of charge. Record your measurements, keep the EICR coding consistent with them, and make the customer's position clear in writing.
Safe isolation
What changes when you suspect a diverted neutral
The standard lock-off sequence still applies — identify, isolate, lock off, prove dead, re-prove, warn — but a suspected diverted neutral adds rules of its own. Work through it with the full safe isolation procedure alongside these points.
Treat the neutral as live — always
On a supply with a suspected diverted neutral, the neutral conductor can sit at a significant voltage to earth, because the MET it is bonded to is itself riding above true earth. Test line–neutral, line–earth and neutral–earth at the point of work, and prove the indicator before and after, exactly as GN3 requires.
Test suspect surfaces with a voltage contact indicator
Touch voltage on taps, radiators, appliance chassis or any suspect surface is checked with a voltage contact indicator — a tester that detects voltage by making physical contact, pen-shaped and often single-pole (e.g. Martindale VT7), proved before and after use as GS38 requires. It looks very similar to a non-contact 'volt stick' pen, but the volt stick senses a field through the insulation: it can miss a voltage held on a damp or high-impedance surface and give false reassurance. Never find out by touching the metal.
Never disconnect the main protective bonding
The bonding is currently the only thing holding the installation's metalwork near earth potential. Open it and the touch voltage on every earthed part jumps immediately — possibly to full line potential. This is not a normal 'isolate and disconnect to test' situation.
Parallel paths change what 'dead' looks like
With bonding and other earthed metalwork providing return paths, proving a conductor dead on a PME supply needs the full L–N, L–E and N–E check. A reading only between line and earth can mislead where current has alternative routes.
Isolate, then decide whether to proceed
Locking off the installation's main switch does not make the supply side safe — the cut-out is live DNO equipment and must not be opened. If the installation itself remains unsafe with the bonding in place (e.g. a full broken PEN), the correct move is to leave the bonding connected, withdraw, keep people away from earthed metalwork and get the DNO out as an emergency.
The paperwork matters as much as the tests
Record the clamp readings, N–E voltages and MET-to-spike voltages, with the loads that were on. On the EICR, a supply-side diverted neutral is normally coded as potential danger (C2) — rising to urgent danger (C1) where metalwork is at a dangerous voltage or clamps are dangerously hot — per Electrical Safety First Best Practice Guide 4. Note that the remedial action sits with the DNO, and give the customer the 105 reference.
Reference documents
- BS 7671:2018+A4:2026 (previous A2:2022+A3:2024 version also current until 15 October 2026) ↗
Section 722 (EV charging, including Regulation 722.411.4.1 on PME/open-PEN), Chapter 54, Table 54.8 and Regulation 544.1.1 (main bonding sizing on PME)
- IET Wiring Matters — BS 7671:2018+A1 ↗
Overview of Regulation 722.411.4.1 and the PME conditions for outdoor EV charge points
- IET 01 — Open PEN detection devices ↗
Standardised behaviour for open PEN conductor detection devices in EV charging equipment
- Electrical Safety First — Best Practice Guide 4 ↗
EICR classification codes for domestic and similar installations (C1/C2/FI)
- NICEIC — Safe isolation and borrowed neutrals ↗
Shared neutrals, Regulation 14 of the Electricity at Work Regulations 1989, and testing for crossed neutrals
- HSE GS38 / HSG85 / HSR25 ↗
Test equipment, safe working practices, and the Electricity at Work Regulations 1989 guidance
- IET Guidance Note 3 ↗
Inspection & Testing — safe isolation and initial verification procedures
Cited by regulation, chapter and edition only — no printed page numbers, as pagination varies between formats of the same publication.