Grounding Types Explained: TN-C, TN-S, TN-C-S, TT, IT

What is an earthing system

The earthing arrangement determines how the source neutral and exposed conductive parts are connected to earth. Safety, protection device operation, and EMC all depend on this choice.

IEC 60364 defines 5 types. In India — IS 3043.

Letter meanings

First letter — source neutral to earth:

• T — earthed (solidly grounded neutral)
• I — isolated

Second letter — exposed parts to earth:

• class="ml-4 mb-1 text-text-secondary">T — independently earthed
• N — connected to source neutral

For TN only:

• C — Combined PEN conductor
• S — Separate N and PE

TN-C (old system)

Common PEN conductor. Cheaper but dangerous — PEN break puts phase voltage on exposed metal. Banned in new buildings.

TN-S (modern standard)

5-wire system (L1, L2, L3, N, PE). Maximum safety. Standard in Europe, recommended for hospitals, data centers.

TN-C-S (compromise)

TN-C before building entry, TN-S after PEN split at main panel. Split PEN into PE and N at the incoming device. Requires re-earthing of PE (≤10 Ω for 400/230 V). PEN minimum 10 mm² copper.

Used in 90% of Russian residential buildings. Common in India for multistory apartments.

TT (for private houses)

Independent earth electrode for exposed parts. Mandatory RCD on incomer (IΔn ≤ 300 mA). Used when utility earth is unreliable.

IT (hospitals, mines)

Isolated neutral or high-impedance earthed. First fault — alarm only, no trip. Used in operating rooms, ICUs — where power continuity is critical.

Selection guide