Earthing Diagram: TN-S, TN-C-S, and TT System Types with Electrode Configurations

Earthing Diagram: TN-S, TN-C-S, and TT System Types with Electrode Configurations — interactive diagram. Open it in the editor to customise components and wiring.

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An earthing diagram shows how a building's electrical installation connects to earth — the electrode, conductor paths, and the separation (or combination) of neutral and protective earth in TN-S, TN-C-S, and TT earthing systems. Getting this right prevents shock and satisfies the inspector.

Earthing — the connection of exposed conductive parts of an electrical installation to the general mass of earth — is the primary safeguard against electric shock from insulation failure. When a live conductor contacts an exposed metal part (a fault), a low-impedance earth fault current path causes the protective device (fuse or circuit breaker) to operate quickly, disconnecting the supply before a person contacting that metal part can receive a dangerous shock current.

The IEC 60364 standard defines three main earthing system types, identified by a two-letter (or three-letter) code:

TN-S (Terra-Neutral Separated): The supply neutral and the protective earth (PE) conductor are separate throughout the entire system, from the transformer star point to every outlet. The PE conductor carries no normal load current. This is the cleanest earthing arrangement for sensitive electronic equipment because the PE is always at true earth potential and carries no neutral current noise.

TN-C-S (Terra-Neutral Combined then Separated): The neutral and PE are combined in a single PEN conductor from the transformer to the installation's point of supply (the incoming service head or main distribution board). Inside the installation, they separate at the main earthing terminal (MET) into a separate PE and a separate neutral. This is also called PME (Protective Multiple Earthing) in the UK. The risk in TN-C-S is that an open PEN conductor on the supply side can raise exposed metalwork in the installation to near-line voltage, which is why external metalwork connected to the PME terminal must meet specific requirements.

TT (Terra-Terra): The supply source has its own earth but provides no earth connection to the installation. The installation's exposed metalwork is earthed via its own independent electrode — a ground rod, plate, or ring — at the installation itself. This system is common in rural areas where an earth electrode on the distribution network is not available. TT systems typically require an RCD (residual current device) as the primary fault protection because the earth fault loop impedance through a soil electrode can be high enough that overcurrent devices alone may not disconnect fast enough.

Earth electrodes take several forms: the driven ground rod (most common, typically copper-bonded steel, driven to a depth of 1.2 m or more), the buried plate electrode (larger area, used where soil resistivity is high), the foundation electrode (reinforcing steel in concrete, where permitted), and the ring electrode (a conductor buried around the perimeter of the building). Electrode resistance to earth must be measured with a purpose-built earth electrode tester — not a standard multimeter — and must meet the value required by the applicable standard (IEC 60364-5-54, BS 7671, or equivalent).

The main earthing terminal (MET) is the hub of the earthing system — all protective earth conductors, bonding conductors, and the earth electrode connection terminate here. It must be accessible, labelled, and must include a link that can be disconnected for electrode resistance testing.

How to wire earthing diagram

Identify the earthing system type of the supply Contact the supply utility or inspect the incoming supply to determine whether the installation is served by a TN-S, TN-C-S (PME), or TT network. This determines whether you need an installation earth electrode, what electrode type is appropriate, and whether you need to rely on RCD protection for earth fault disconnection.
Design the earth electrode (TT systems or supplemental electrode) Select the electrode type based on the site's soil resistivity measurement. A single driven ground rod of 1.2–2.4 m depth suits most soil types. High-resistivity soils (rock, dry sand) may require multiple rods, a ring electrode, or a plate electrode. Measure soil resistivity before specifying the electrode configuration.
Install the earth electrode Drive the electrode to the required depth, keeping it clear of building foundations, buried services, and any area subject to excavation. In TT systems, the electrode must be installed in soil that remains moist — resistivity increases significantly in dry conditions. Connect the earth electrode conductor (EEC) from the electrode to the main earthing terminal using a conductor protected against corrosion and mechanical damage.
Install the main earthing terminal (MET) Mount the MET in an accessible location, typically adjacent to the main distribution board. Install a removable link between the MET and the earth electrode conductor so the electrode can be disconnected for testing. Label the MET clearly: 'Safety Electrical Connection — Do Not Remove.' All protective earth conductors and bonding conductors terminate at the MET.
Install main protective bonding conductors Connect bonding conductors from the MET to all incoming metallic services: water pipes, gas pipes, oil supply pipes, structural metalwork, and any other extraneous conductive parts entering the building. The bonding conductor size is specified in IEC 60364-5-54 or BS 7671 based on the earthing conductor cross-section. Bonding connections must be made before the pipe enters the building, on the consumer's side of any meter or insulating section.
Run protective earth conductors to distribution boards and circuits Run the main protective earth conductor (PE) from the MET to each distribution board, and from each board to every circuit's earth terminal. PE conductors must be green-yellow in IEC countries (green in some other jurisdictions). They must not be switched, and they must not be interrupted by any device other than a test link at the MET.
Measure and verify the earthing system Measure the earth electrode resistance using an earth electrode tester (fall-of-potential method or clamp-on method). Measure the earth fault loop impedance at the origin and at each circuit using a loop impedance tester. Verify all values comply with the applicable standard. For TT systems, verify the RCD disconnects in the required time at the measured loop impedance.

Specifications

Earthing system types (IEC 60364)	TN-S, TN-C, TN-C-S, TT, IT
Protective earth conductor colour (IEC)	Green-yellow striped
Minimum earth electrode conductor (IEC 60364-5-54, buried, mechanically protected)	16 mm² copper
Minimum earth electrode conductor (bare, buried, not mechanically protected)	25 mm² copper or 50 mm² steel
Typical driven earth rod length	1.2–2.4 m
RCD trip threshold (general circuits)	30 mA
Applicable standard (international)	IEC 60364-5-54

Safety warnings

All earthing and bonding work must be designed, installed, tested, and certified by a licensed electrician to IEC 60364-5-54, BS 7671, NEC/NFPA 70, AS/NZS 3000, or the applicable local standard. Incorrect earthing does not protect against electric shock — it may give a false sense of security.
Never remove the removable link at the MET while the installation is energised. With the link removed, exposed metalwork in the installation loses its earth connection and may rise to a dangerous potential if an insulation fault exists.
In TN-C-S (PME) systems, the PEN conductor failure on the supply side can raise the building's earthed metalwork to near-line voltage. Do not connect the installation MET to external metalwork (water tanks, above-ground external structures) unless specifically assessed and approved under the applicable standard.
Earth electrode resistance must be measured — it cannot be assumed. A rod driven into dry, rocky, or sandy ground may have a resistance orders of magnitude higher than the same rod in moist clay. High electrode resistance in a TT system can prevent RCDs from operating and leave the installation unprotected.

Tools needed

Earth electrode resistance tester (fall-of-potential method or clamp-on type)
Earth fault loop impedance tester
RCD tester (for TT systems)
Insulation resistance tester (megohmmeter)
Torque driver or wrench (for MET connections)
Cable crimping tool (for large-section conductor lugs)
Approved voltage indicator (for verifying dead before working)

Common mistakes

Installing a single short earth rod (under 1 m) in dry or rocky soil and assuming it provides adequate earth resistance — soil resistivity must be measured and the electrode design validated against the required resistance value.
Connecting bonding conductors to the supply side of a water meter or gas meter rather than the consumer side — bonding must be on the installation side of any insulating section to be effective.
Interrupting a protective earth conductor with a switch, fuse, or any other device except a test link at the MET — the PE must be a continuous conductor that cannot be opened under any operating condition.
Confusing main protective bonding with supplementary bonding — main bonding connects extraneous conductive parts to the MET; supplementary bonding is local bonding between simultaneously accessible parts within a specific zone (such as a bathroom). Both are required by IEC 60364 and BS 7671.
Using a standard multimeter to measure earth electrode resistance — a multimeter cannot perform the fall-of-potential or stakeless test needed to measure resistance to remote earth. Only a dedicated earth tester gives a valid reading.

Troubleshooting

RCD trips immediately on power-up in a TT system: Cause: Earth fault current is flowing — insulation failure on a connected appliance or in the installation wiring, or the RCD is oversensitive to leakage current from a large number of connected appliances Fix: Disconnect all loads and sub-circuits. If the RCD still trips with nothing connected, the fault is in the wiring — test insulation resistance between each live conductor and earth on the installation. If the RCD resets with all loads disconnected, reconnect circuits one by one to identify which load carries the fault.
Earth electrode resistance measurement is very high (above several hundred ohms): Cause: Soil resistivity is high (dry, sandy, or rocky soil), electrode is too short, or poor contact at the electrode top connection Fix: Drive additional electrode rods in parallel (electrode resistance reduces in parallel) or use a plate electrode with greater contact area. Water the soil around the electrode in dry conditions to measure the resistivity at expected worst-case values. Use electrode enhancement compounds in persistent high-resistivity conditions (consult a specialist).
Shock felt from metal pipework despite bonding being installed: Cause: Bonding conductor is broken or has high resistance, or the bonding connection is made on the supply side of an insulating section rather than the consumer side Fix: Test continuity of the bonding conductor from the MET to the pipe connection point. Verify the bonding clamp is making firm contact with the bare pipe — not on a painted or coated section. Relocate the bonding connection to the correct side of any meter or insulating union.

Frequently asked questions

What is the difference between TN-S and TN-C-S earthing systems?

In TN-S, the neutral (N) and protective earth (PE) are separate conductors throughout the entire system from supply source to final circuit. In TN-C-S (PME), they are combined as a PEN conductor from the transformer to the installation's main distribution board, then separated internally. TN-S is preferred for sensitive electronics; TN-C-S is common in UK and many European urban networks.

What is a TT earthing system and when is it used?

A TT system has an independently earthed supply neutral and a separate independent earth electrode at the installation. It is used where no earth connection is available from the supply network — common in rural areas, older installations, and many countries outside Europe. TT systems require RCD protection as the primary fault protection because the earth fault loop impedance through soil can be too high for overcurrent devices alone.

What types of earth electrode are used in an earthing diagram?

The main electrode types are: the driven ground rod (copper-bonded or solid copper, typically 1.2 m to 2.4 m long), the buried plate electrode (used in high-resistivity soil), the ring electrode (buried conductor around the building perimeter), the tape electrode (horizontal buried strip), and the foundation electrode (reinforcing steel in concrete footings, where permitted by the standard).

What is the main earthing terminal (MET) and why does it need a removable link?

The main earthing terminal (MET) is the common connection point for all protective earth conductors, equipotential bonding conductors, and the earth electrode conductor. A removable link in the connection between the MET and the earth electrode allows the electrode to be isolated for resistance-to-earth testing without disconnecting the entire earthing system from the installation. The MET must be accessible and clearly labelled.

Does every electrical installation need its own earth electrode?

Not always. In TN-S and TN-C-S systems, the earth path is provided by the supply utility's earthed neutral. An installation served by a TN network does not require its own electrode as the primary earth — though an electrode may still be installed for additional protection or where required by the local utility. In a TT system, an installation electrode is mandatory.