Earthing Arrangements Explained (TN-S, TN-C-S, TT)

The earthing arrangement describes how an installation is connected to earth. It is fundamental to protection against electric shock, because it determines how quickly the supply will disconnect if a fault occurs.

The main systems

• TN-S — the supply provides a separate earth conductor all the way back to the source. A traditional arrangement, often with a cable sheath earth.
• TN-C-S (PME) — Protective Multiple Earthing: the earth is derived from the supply neutral at the origin. Common in modern UK supplies, but with specific requirements (and limitations for some uses, such as certain EV charging installations).
• TT — no earth is provided by the supplier; the installation uses its own earth electrode. Common in rural areas, and it relies heavily on RCD protection for safety.

Why it matters on an EICR

The inspector confirms the earthing arrangement is correctly identified, adequate and safe, that the earthing conductor is properly sized, and that disconnection times can be met. Problems here often attract a C2, because they matter under fault conditions.

Related considerations

Earthing works hand in hand with protective bonding and RCD protection. It is also central to EV charging earthing considerations, where open-PEN faults on PME supplies need specific mitigation.