Earthing System Types TN-S, TN-C-S, TT, IT — IEC 60364 Guide

The earthing system is the single most fundamental safety decision in any electrical installation. It determines how fault currents return to the source, how quickly protective devices operate, and whether exposed metalwork becomes dangerously live during a fault. IEC 60364-1, Clause 312 defines the internationally recognised classification system for earthing arrangements, using a letter code that describes the relationship between the power supply, the installation earth, and the protective conductors.

Every country that bases its electrical standards on IEC 60364 uses this classification system, including BS 7671 (UK), AS/NZS 3000 (Australia/New Zealand), and the national standards of most European, Asian, and African countries. Understanding the earthing system type is essential for:

• Selecting the correct protective device strategy (overcurrent vs RCD)
• Sizing earth conductors and earth electrodes
• Calculating earth fault loop impedance
• Determining touch voltage exposure during faults
• Designing equipotential bonding systems

This guide explains the letter code, describes each earthing system type with fault current path diagrams, and provides application guidance for selecting the appropriate system for different installation types.

The IEC earthing system classification uses a two- or three-letter code. Each letter has a specific meaning defined in IEC 60364-1, Clause 312.2:

Earthing system code structure:
  [Supply earth] - [Installation earth] - [Conductor arrangement]

  T-N-S   = Supply earthed, installation via supply earth, separate PE and N
  T-N-C-S = Supply earthed, installation via supply earth, combined then separate
  T-N-C   = Supply earthed, installation via supply earth, combined throughout
  T-T     = Supply earthed, installation via independent earth
  I-T     = Supply isolated, installation via independent earth

In a TN-S system, the supply source (transformer neutral) is connected to earth, and the protective earth (PE) conductor is separate from the neutral (N) conductor throughout the entire installation, from the transformer to the final circuit.

Characteristics of TN-S:

• Fault current path: Earth fault current flows through the PE conductor back to the transformer neutral point. The path has low impedance because it uses metallic conductors throughout — no soil resistance in the fault loop.
• Fault current magnitude: High (typically thousands of amperes), enabling fast operation of overcurrent protective devices.
• Touch voltage during fault: Moderate — limited by the impedance of the PE conductor. Equipotential bonding reduces touch voltage further.
• EMC performance: Excellent — the separate PE and N conductors minimise circulating neutral currents in the earth system, reducing electromagnetic interference.

TN-S fault loop impedance:
  Z_s = Z_source + Z_line + Z_PE

Where:
  Z_source = transformer impedance (typically 0.01–0.05 ohm)
  Z_line   = phase conductor impedance (ohm/m × length)
  Z_PE     = protective earth conductor impedance (ohm/m × length)

Example (32 A circuit, 30 m, 4 mm² Cu):
  Z_source = 0.03 ohm
  Z_line   = 0.30 ohm (4 mm² × 30 m)
  Z_PE     = 0.30 ohm (4 mm² × 30 m)
  Z_s      = 0.63 ohm

  Fault current = 230 / 0.63 = 365 A
  (Sufficient to trip a 32 A Type B MCB instantaneously)

Where TN-S is used: New-build commercial and industrial installations, data centres (for EMC reasons), and any installation where clean earthing is important. TN-S is considered the safest and most desirable earthing arrangement because the PE and N are never combined, eliminating the risk of neutral current flowing in the earth system.

TN-S requires a dedicated earth conductor from the supply transformer to the installation — typically the cable armouring or a separate conductor in the supply cable. This makes TN-S more expensive than TN-C-S for distribution networks, which is why many utilities supply TN-C-S instead.

In a TN-C-S system (also known as Protective Multiple Earthing or PME in the UK and Australia), the neutral and earth functions are combined in a single PEN conductor in the supply network, then separated into distinct N and PE conductors at the installation’s origin (typically the main switchboard).

This is the most common earthing system in the UK (provided by Distribution Network Operators under the Electricity Safety, Quality and Continuity Regulations 2002) and in Australia (where it is called MEN — Multiple Earthed Neutral, per AS/NZS 3000, Clause 5.6).

Characteristics of TN-C-S:

• Fault current path: Same as TN-S within the installation (PE conductor back to the main earthing terminal). The PEN conductor carries both neutral load current and fault return current in the supply network.
• Fault current magnitude: High — similar to TN-S, with low-impedance metallic fault loop.
• Shock risk if PEN conductor breaks: This is the critical weakness. If the PEN conductor becomes open-circuit (e.g., broken neutral in the supply cable), the neutral voltage rises and all exposed metalwork connected to the PME earth may become live at a dangerous voltage. The touch voltage can reach up to full phase voltage (230 V) under worst-case conditions.
• Equipotential bonding: Essential. IEC 60364-4-41, Clause 411.4 and BS 7671, Regulation 411.3.1.2 require main equipotential bonding of all extraneous conductive parts (water, gas, structural steel) to mitigate the broken-PEN risk.

PME restrictions: Because of the broken-PEN risk, some locations prohibit or restrict PME earthing:

In Australia, the MEN system is universal for utility-supplied installations. The PEN conductor is earthed at multiple points along the distribution network (hence “Multiple Earthed Neutral”), and the MEN bond at the main switchboard connects the neutral bar to the earth bar. This bond is mandatory per AS/NZS 3000, Clause 5.6.2.2.

In a TT system, the supply source is earthed at the transformer, but the installation has its own independent earth electrode with no metallic connection to the supply earth. The earth fault current must flow through the soil between the installation’s earth electrode and the supply transformer’s earth electrode.

Characteristics of TT:

• Fault current path: Earth fault current flows through the PE conductor to the installation’s earth electrode, through the soil to the supply transformer’s earth electrode, and back to the transformer neutral. The soil resistance is the dominant impedance in the loop.
• Fault current magnitude: Low — typically tens of amperes, not hundreds or thousands. Soil resistance (R_A) is typically 10–200 Ω, compared to fractions of an ohm for metallic conductors.
• Protective device: Overcurrent devices (MCBs, fuses) cannot reliably clear earth faults in TT systems because the fault current is too low. RCD (residual current device) protection is mandatory per IEC 60364-4-41, Clause 411.5.

TT system earth fault loop:
  Z_s = Z_source + Z_line + R_A + R_B

Where:
  R_A = resistance of installation earth electrode (ohm)
  R_B = resistance of supply earth electrode (ohm)

Example:
  Z_source = 0.03 ohm
  Z_line   = 0.30 ohm
  R_A      = 50 ohm (driven rod in clay soil)
  R_B      = 5 ohm (transformer earth)
  Z_s      = 55.3 ohm

  Fault current = 230 / 55.3 = 4.2 A
  (Far too low for MCB trip — RCD required)

RCD requirement (IEC 60364-4-41, Clause 411.5.3):
  R_A × I_delta_n ≤ 50 V

  For 30 mA RCD: R_A ≤ 50 / 0.030 = 1,667 ohm
  For 300 mA RCD: R_A ≤ 50 / 0.300 = 167 ohm

Where TT is used: TT systems are common in:

• Rural France and other European countries where the distribution network does not provide a PME earth
• Locations where PME is prohibited (caravan sites, construction sites, marinas in the UK)
• Developing countries where the utility infrastructure does not guarantee a reliable earth connection
• Standalone installations fed from private generators where the supply earth must be independently established

The advantage of TT is simplicity and independence from the supply network’s earth integrity. The disadvantage is the dependence on RCDs for all earth fault protection and the need to install and maintain earth electrodes with adequate resistance.

In an IT system, the supply source is either completely isolated from earth or connected through a high impedance (typically 1,000–2,000 Ω). The installation’s exposed metalwork is earthed via independent earth electrodes. This is the only earthing system where a single earth fault does not create a dangerous condition.

Characteristics of IT:

• First fault: When a single phase develops an earth fault, the fault current is extremely small (limited by the insulation impedance of the other two phases to earth, or by the deliberately inserted high impedance). The first fault current is typically a few milliamperes to a few amperes — not enough to operate any protective device and not enough to create a dangerous touch voltage.
• Second fault: If a second earth fault develops on a different phase before the first fault is repaired, a phase-to-phase fault exists through the earth system, and a large fault current flows. This second fault must be cleared by overcurrent protective devices.
• Insulation monitoring: Because the first fault does not trip anything, an insulation monitoring device (IMD) is mandatory per IEC 60364-4-41, Clause 411.6. The IMD continuously monitors the insulation resistance between the live conductors and earth. When the first fault occurs, the IMD raises an alarm — but does not disconnect supply. This allows maintenance staff to locate and repair the fault without interrupting power.

IT system first-fault current (simplified):
  I_fault = V_phase / Z_insulation

  For a 400 V three-phase system:
  V_phase = 230 V
  Z_insulation ~ 100 kΩ (healthy phases to earth)
  I_fault = 230 / 100,000 = 2.3 mA

  Touch voltage = I_fault × R_A
  If R_A = 50 ohm:
  V_touch = 0.0023 × 50 = 0.115 V (negligible)

Where IT is used:

IT systems are more complex and expensive to design, install, and maintain than TN or TT systems. They require insulation monitoring devices, trained maintenance staff to respond to first-fault alarms, and careful design to ensure second-fault protection. For these reasons, IT is only used where the cost of a power interruption exceeds the cost of the additional infrastructure.

The following table summarises the key characteristics and selection criteria for each earthing system type:

Selection rule of thumb:

• For most commercial and industrial installations where the utility provides a reliable earth: TN-C-S (or TN-S if available)
• For installations where PME is prohibited or no utility earth is available: TT
• For critical installations where power continuity during the first fault is essential: IT
• For new installations where EMC is a priority (data centres, sensitive electronics): TN-S

The earthing system type must be determined at the very beginning of the design process, as it affects virtually every other design decision: protective device selection, conductor sizing, bonding requirements, and the overall protection philosophy.