Fundamentals
UK earthing systems
The first thing Paul confirms at any survey is the earthing arrangement. Everything downstream — the disconnection times, the RCD strategy, whether an EV charge point can share the property earth — hinges on whether you have TN-S, TN-C-S (PME) or TT.
System TN-S
TN-S — separate earth conductor
- How to identify
- Earth is a separate metallic conductor supplied by the DNO — typically the lead sheath of an older paper-insulated cable or a dedicated earth core.
- Typical Ze
- 0.35 – 0.8 Ω typical
- In practice
- Increasingly rare on new supplies. Where you find it on an old service, check the sheath earth is still continuous — corrosion under the earth clamp is a common finding.
System TN-C-S (PME)
TN-C-S — Protective Multiple Earthing (PME)
- How to identify
- The DNO combines neutral and earth in the incoming cable (a PEN conductor) and provides a single earth terminal at the intake. Standard on the majority of modern UK supplies.
- Typical Ze
- 0.10 – 0.35 Ω typical
- In practice
- Very low earth fault loop impedance is a benefit, but a broken PEN conductor upstream can raise the whole installation earth to a dangerous voltage — this is why PME earthing is not permitted (without specific measures) for EV charge points installed outdoors, or often for outbuildings and caravan pitches. See BS 7671 Section 722 and OZEV requirements.
System PNB
PNB — Protective Neutral Bonding
- How to identify
- A variant of TN-C-S where the neutral and earth are combined in the DNO's supply cable but the neutral-to-earth link is made at the consumer's intake rather than at the transformer or a network point. Usually seen on a single dedicated service from a nearby transformer (e.g. one large property, a farm, or a small commercial unit fed from its own pole-mounted transformer).
- Typical Ze
- 0.10 – 0.35 Ω typical
- In practice
- Electrically behaves like TN-C-S / PME once inside the installation, and the same open-PEN risks apply to outdoor EV, outbuildings and caravan pitches. Paul confirms with the DNO which arrangement is actually in service — the label on the cut-out isn't always right.
System TT
TT — no DNO earth, local earth electrode
- How to identify
- The DNO provides no earth facility. The installation earth is derived from an electrode driven into the ground at the property.
- Typical Ze
- 20 – 200 Ω typical (highly soil-dependent)
- In practice
- Common on rural supplies, overhead services, and by choice for outbuildings/EV where PME cannot be extended. Because Ze is high, disconnection times cannot be met by overcurrent devices — an upfront 100 mA time-delayed RCD is used for the main incomer, with 30 mA RCDs downstream for final circuits.
PME + EV chargers
Under BS 7671 Section 722, an EV charge point installed on a PME supply must either use a separate earth electrode (TT island) or an approved open-PEN detection device. Paul selects the right option after measuring Ze, Ra and PEN integrity on site — never by assumption.
Fault scenario
Diverted neutral currents — the hidden PME hazard
On a TN-C-S (PME) or PNB supply, neutral and earth are combined in the DNO's PEN conductor and only separated at the intake. Because the installation's main earthing terminal is bonded to that PEN, any impedance in the DNO neutral back to the substation causes a share of the return neutral current to divert through the property's earthing and bonding system instead of the intended neutral path.
How it happens
- Corroded, loose or partially fractured DNO neutral joint upstream (service head, cut-out tail, or street joint).
- High neutral impedance forces return current to seek a parallel path.
- That path is your main protective bonding — gas service pipe, water main, structural steel, cable armour, adjacent property's bonding via a shared service.
- Currents of several amps can flow continuously through bonding conductors under normal load.
What you'll see on site
- Warm or hot main bonding clamps on the incoming gas or water pipe.
- Measurable AC voltage (a few volts up to tens of volts) between the MET and true earth on a TT test spike.
- Clamp meter shows persistent current on the main protective bonding conductor with the installation switched off at the main switch.
- Neighbours reporting flickering lights or dimming — a classic sign of a failing shared neutral.
Broken PEN — the dangerous end-state
If the DNO neutral fully breaks (a broken PEN), every earthed metallic part in the property — sink taps, radiators, appliance chassis, an outdoor EV charge point body — can rise to full line voltage relative to true earth. Anyone touching that metal while standing on real ground is across 230 V. This is the specific hazard BS 7671 Section 722 (EV) and Section 705 (agricultural / caravan) mitigate by either creating a TT island for the outdoor equipment or requiring an approved open-PEN detection device.
If diverted neutral current is suspected, do not disconnect the main bonding — that removes the only fault path holding voltages down. Report to the DNO as an emergency (they attend free of charge) and have the supply investigated before further work.
Not the same as your safety earth
Functional earthing — what it is and why it needs its own colour
A protective earth (the green-and-yellow one) exists to keep you alive. Its only job is to hold exposed metalwork at the same potential as true earth and to carry fault current back to the source fast enough for the RCD or MCB to disconnect. If it fails, people get killed. Everything in BS 7671 about CPCs, bonding, disconnection times, R1+R2 and Zs concerns the protective earth.
A functional earth (FE) is completely different in purpose. It is a reference connection that a piece of equipment needs in order to work correctly, not to be safe. Typical examples:
- PV inverters and battery inverters that need a reference for anti-islanding and DC-injection sensing.
- EV charge points with open-PEN detection — the FE provides the reference the controller compares against N.
- Class I ICT equipment (data cabinets, telecoms racks) using filtered/clean earth for EMC and signal integrity.
- SPDs and some RCBO/SPD combinations that use a functional earth path for internal reference.
Losing the FE will typically not electrocute anyone — the equipment simply malfunctions, fails safe, or logs an earth-fault error. But if the FE is confused with the CPC (or if the CPC is confused with an FE and left disconnected because "it's only functional"), the safety earth path is broken and the risk is real.
Colour identification (BS 7671)
- Green-and-yellow — Protective conductor (CPC, main earthing conductor, bonding). Never re-purpose.
- Pink (occasionally cream) — Functional earth conductor. Used to mark FE terminals and sleeving so an engineer can tell it apart from the safety earth at a glance.
Under IEC 60445 the letter designations are PE for protective earth, FE for functional earth and PEF for a conductor doing both jobs. Terminal markings on modern EV chargers, inverters and ICT power supplies use exactly this labelling — check the manual before wiring.