Earthing explained: what it does, how it is installed and how it is measured

Short answer: Earthing connects the metal bodies of appliances and the earth pin of sockets to the ground through a low-resistance path. In a fault, the voltage on the casing flows away through this path, the RCD or breaker disconnects the circuit and nobody gets a shock. Earthing is provided by a foundation earth, copper-clad rods or tape in the ground, and carried to the consumer unit and sockets by the green-yellow protective conductor. Its resistance is measured with an earth tester, and in RCD-protected homes it must satisfy RA × IΔn ≤ 50 V; in practice the lowest achievable value is targeted. Local rules apply.

What does earthing do?

• Safety: if the insulation of a metal appliance (washing machine, oven, EV charger) fails, the casing becomes live. Without earthing, the person touching it becomes the current's path.
• Protective devices: RCDs and breakers trip quickly only when the fault current has a path to earth.
• Electronics and lightning: surge protectors and lightning protection only work with good earthing.

Earthing systems: TT and TN

• TT: the building has its own earth electrode and the supply neutral is earthed separately. Fault current returns through the soil; protection relies on the RCD. Common in detached houses and rural areas.
• TN-C-S / TN-S: the protective conductor is connected to the supplier's earthed neutral. Fault currents are high and breakers can trip quickly. Common in cities and apartment buildings, usually with a foundation earth as well.

The system is determined by the distribution company's connection and the installation design.

How is earthing installed?

• Foundation earth: galvanised or copper tape cast into the foundations of new buildings; the best long-term solution.
• Rod electrodes: copper-clad steel rods driven 1.5–3 m deep; if resistance is too high, several rods are connected, spaced at least one rod length apart.
• Tape electrodes: copper or galvanised tape laid horizontally 50–80 cm deep.
• Connection: a green-yellow conductor of suitable size from the electrode to the main earthing terminal; joints are protected from corrosion and left accessible for inspection.
• Equipotential bonding: water and gas pipes and metal bathroom parts are bonded to the earthing terminal.

Soil type (rocky, sandy, damp) changes the resistance greatly; dry and rocky ground needs more electrodes.

What earth resistance is acceptable?

• In an RCD-protected TT system: RA × IΔn ≤ 50 V. With a 30 mA RCD this theoretically allows a very high value (1,666 Ω), but because resistance varies with moisture and season, much lower values are targeted in practice.
• Lightning protection systems usually require 10 Ω or less.
• The exact limit depends on the installation type and design.

How is earthing measured?

With an earth resistance tester using two auxiliary stakes (three-pole method) or a clamp-type tester. Plug-in socket testers only show whether earth is connected, not its resistance. Workplaces typically have earthing tested periodically (often yearly) by qualified persons, with a report.

Earthing and EV chargers

The charger body and the car's chassis are connected to the protective conductor. Many chargers will not start charging if earthing is missing or poor. Earthing is measured before installation and improved if needed; the charging circuit is also protected by a type B RCD or 6 mA DC leakage detection.

Earthing and testing must be done by a qualified electrician. In Bursa, ENGN provides earthing, electrical installation and EV charger installation.

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