IEC 60364-1 classification system

IEC 60364-1 classifies low-voltage earthing systems using two letters. The first letter describes how the power system (supply transformer) is related to earth. The second letter describes how the exposed conductive parts of the installation are connected to earth. A third letter (C, S or C-S) appears in TN systems to describe how the protective and neutral functions are combined or separated.

TN-C: combined PEN conductor

In a TN-C system the protective earth (PE) and neutral (N) functions are combined into a single conductor called PEN (protective earth neutral) from the supply transformer all the way to the final consumer. There is no separate PE wire — the PEN conductor carries both return current and provides the fault current path.

Supply transformer → PEN conductor (combined) → Consumer
  PEN = grey or black conductor, marked PEN at each end
  No separate PE conductor in cable runs

Minimum cross-section:
  Copper PEN: 10mm² minimum (IEC 60364-5-54)
  Aluminium PEN: 16mm² minimum

Applications:
  Fixed industrial installations only
  Overhead distribution lines (older rural networks)
  PROHIBITED for socket outlet circuits < 10mm² copper
  PROHIBITED in new EU residential installations

TN-S: separate PE and N throughout

TN-S is the modern European standard for all new residential and commercial installations. The protective earth (PE) and neutral (N) conductors are separate from the supply transformer star point all the way to every final circuit. The PE conductor carries no load current under normal conditions — only fault current during a line-to-earth fault.

Line conductors (L1, L2, L3): brown, black, grey
Neutral conductor (N):         blue
Protective earth (PE):         green/yellow striped

PE and N are separate from transformer star point.
PE is bonded to:
  - Transformer case and enclosure
  - Main earth bar (MEB) at main panel
  - All equipment exposed conductive parts

Under TN-S, fault current returns via the low-impedance
PE conductor — MCB or RCD clears the fault quickly.

TN-C-S: combined supply, split at main panel

TN-C-S is the most common configuration for UK and northern European residential supply. The distribution network operator (DNO) delivers a TN-C supply — a single PEN conductor in the street — which is split into separate PE and N conductors at the main panel (consumer unit). The building installation is therefore TN-S internally.

Split point = Main Equipotential Bonding (MEB) bar in main panel

At the split point:
  PEN from DNO → MEB bar
  PE → from MEB bar to all circuits (green/yellow)
  N  → from MEB bar to all neutral conductors (blue)
  Earth electrode → bonded to MEB bar

PE and N MUST NOT be reconnected anywhere downstream
of the split point. Reconnecting downstream creates
a TN-C segment, which is prohibited in new installations.

Protective Multiple Earthing (PME):
  UK-specific TN-C-S variant where the PEN is earthed
  at multiple points along the distribution network.
  PME prohibits additional earth electrodes in some
  outdoor locations without DNO approval.

TT: local earth electrode required

In a TT system the power supply transformer star point is directly earthed (first T), but the installation exposed conductive parts are connected to a completely independent local earth electrode (second T). There is no metallic low-impedance return path from the installation back to the supply transformer. Fault current must flow through both the local earth electrode and the transformer earth electrode, through the soil between them.

IEC 60364-4-41 requirement for TT:
  RA × Id ≤ 50V

Where:
  RA = resistance of installation earth electrode (Ω)
  Id = RCD operating current (A)
  50V = maximum permitted touch voltage

Example with 30mA RCD and 200Ω electrode:
  200Ω × 0.030A = 6V  ✓  (well below 50V limit)

Example with 100mA RCD and 200Ω electrode:
  200Ω × 0.100A = 20V  ✓  (acceptable)

Example with 300mA RCD and 500Ω electrode:
  500Ω × 0.300A = 150V  ✗  (exceeds 50V — earth
  electrode resistance must be reduced or 30mA RCD used)

IT: isolated power system

In an IT system the power system is either completely isolated from earth or connected only through a high impedance (typically greater than 50kΩ). Exposed conductive parts are earthed locally. The critical property: during the first line-to-earth fault, no dangerous fault current flows because there is no complete circuit back to the supply. Normal operation continues. This makes IT the system of choice wherever continuity of supply during a first fault is critical.

Earth electrode types and resistance

The earth electrode establishes the connection between the installation earth and the general mass of the earth. Electrode type, material and installation method determine the achieved earth resistance, which must meet system-specific requirements.

Earth fault loop impedance

Earth fault loop impedance (Zs) is the total impedance of the path that fault current takes during a line-to-earth fault: from the source, along the line conductor to the fault, back along the protective conductor to the source. IEC 60364-4-41 requires that Zs is low enough for the overcurrent protective device to disconnect within the required time (0.4 seconds for final circuits up to 32A in TN systems).

Maximum Zs for MCB disconnection in 0.4 seconds (TN system):

  Zs = U0 / Ia

Where:
  U0 = nominal line-to-earth voltage (230V in Europe)
  Ia = current causing disconnection in 0.4s

MCB Type B, 16A:  Ia = 5 × 16A = 80A   → Zs ≤ 230/80   = 2.88Ω
MCB Type B, 32A:  Ia = 5 × 32A = 160A  → Zs ≤ 230/160  = 1.44Ω
MCB Type C, 16A:  Ia = 10 × 16A = 160A → Zs ≤ 230/160  = 1.44Ω
MCB Type C, 32A:  Ia = 10 × 32A = 320A → Zs ≤ 230/320  = 0.72Ω
MCB Type D, 32A:  Ia = 20 × 32A = 640A → Zs ≤ 230/640  = 0.36Ω

Note: Type C and D MCBs require lower Zs (better earth paths)
For motor circuits: verify with manufacturer's trip curve data

Measure Zs with a calibrated loop impedance tester on site.
Record measured value; compare with maximum permitted value.
Account for temperature correction (conductors warm in use).

Earthing system comparison summary