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 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.