Earthing Systems Explained — TN-S, TN-C-S, TT for London Homes

TN-S has a dedicated earth conductor from the substation transformer to your property — separate from the neutral. Sometimes called 'cable sheath earth' because in older London installs it's the lead sheath of the supply cable.

Most pre-1965 inner London supplies are TN-S. The earth comes back as a metallic conductor right from the substation.

Pros: stable earth potential. RCDs operate cleanly. EV chargers integrate without earth fault loop issues.

Cons: rare in new builds. Some old TN-S installations have corroded sheath connections — bad earth continuity reading on EICR test.

TN-C-S, also called PME (Protective Multiple Earthing), combines neutral and earth into a single conductor (CNE) from the substation. At the property, this splits into separate neutral and earth at the cut-out.

Most UK domestic supplies post-1980 are TN-C-S. The DNO's cost-effective standard.

Pros: cheap to install, low earth impedance, very common.

Cons: in rare circumstances (broken neutral upstream), the PME earth can rise to dangerous potential. This is why EV chargers on PME supplies need either a separate TT earth electrode or DC fault detection — Reg 722.411.4.1 in BS 7671.

TT has no earth from the DNO. You drive an earth rod into the ground next to the property, and that's your earth. Common in rural areas and in some London outer-borough supplies where the DNO won't provide an earth.

Pros: independent of DNO. Avoids PME upstream-fault risks. Always compatible with EV chargers without modification.

Cons: high earth impedance (typical 30-150 ohms vs 0.35 for TN-C-S). Means RCDs are mandatory on every circuit, not just specials.

Earth rod degrades over time. EICR engineer should re-test the earth electrode resistance every cycle — typical rod has a 10-15 year reliable life.

Look at the supply cut-out and meter cabinet. TN-S has a single brown insulated tail (live), single blue (neutral), separate green/yellow (earth) connected to the cable sheath.

TN-C-S has live, neutral, plus an earth wire that runs back to the cut-out and joins the neutral terminal on the DNO side.

TT has live, neutral, plus an earth wire that disappears into the wall and connects to an outdoor earth rod (often in a chamber by the front door).

On the EICR cover sheet, the 'system type' field shows TN-S, TN-C-S or TT. If you have an EICR, check the cover.

TN-S — install standard EV charger, no extras needed.

TN-C-S (PME) — install with TT earth electrode OR a Type B RCD with DC fault detection. Cost: £150-250 extra for the electrode and rod test.

TT — install standard, plus verify earth electrode resistance under load. May need a second rod for low-impedance EV charging compliance.

Reg 722 in BS 7671:2018 + A4:2026 is now explicit on this. Any EV install from October 2026 that doesn't address the PME risk is non-compliant.

C1 — exposed earth conductor making no physical contact, or earth electrode broken on a TT system. Danger present.

C2 — earth fault loop impedance over the BS 7671 maximum for the circuit's protective device. Potentially dangerous.

C3 — TN-S with corroded sheath connection that still tests within limits. Recommend re-establishing a clean PME earth.

FI — earthing system unclear from inspection, requires DNO confirmation. Common in old converted London Victorians where supply has been altered.