A substation earthing system provides a controlled fault-current path, bonds exposed conductive parts and limits dangerous potential differences. The switchgear earth bar is only one component of the installation grid.

Why this topic matters

Low measured continuity between panels does not prove that touch and step voltages are safe during an earth fault. Grid geometry, soil resistivity, fault-current split, clearing time, surface layer and transferred potentials determine human exposure.

Scope and engineering boundary

IEC 61936-1 provides installation-level earthing requirements within its scope. Projects may also specify IEEE 80 or national methods. The chosen calculation method, physiological limits and local rules must be recorded explicitly.

Core engineering principles

Earth fault current does not all enter the grid

Cable screens, neutrals, overhead earth wires, remote grids and metallic services split the return current. The grid-current input must be derived from the network and bonding arrangement.

Grid potential rise creates local and transferred hazards

A sound equipotential area can still export hazardous voltage through a cable screen, pipe, fence or communication circuit to a remote location.

Touch and step exposure are geometry dependent

Mesh size, conductor depth, rods, surface layer and equipment bonding shape the potential gradient. One earth-resistance number cannot describe all exposure.

Clearing time is part of safety

Permissible exposure and thermal duty depend on protection clearing time, including backup operation. Settings and earthing design therefore need coordination.

Application workflow

  1. Step 1: Measure or justify soil resistivity and define seasonal assumptions.
  2. Step 2: Calculate maximum earth-fault current, grid-current split and clearing time.
  3. Step 3: Design grid conductors, mesh, rods and bonds for thermal and mechanical duty.
  4. Step 4: Evaluate grid potential rise, touch, step and transferred potentials.
  5. Step 5: Coordinate cable screens, fences, structures, neutrals and remote services.
  6. Step 6: Verify installation continuity, selected measurements and as-built configuration before energization.

Practical engineering example

A cable screen bonded at both substations can carry fault current and transfer a portion of the local grid potential rise to the remote site. The screen rating, bonding arrangement and remote touch potential must be assessed together.

Common mistakes

  • Equating low earth resistance with safe touch voltage.
  • Ignoring transferred potential through cable screens.
  • Using main-protection time while backup clearing is slower.
  • Designing the grid before final cable and structure bonding is known.

Design and review checklist

  • Soil model and seasonal conditions are documented.
  • Grid current and fault duration are justified.
  • Touch, step and transferred potentials are evaluated.
  • All structures and cable screens are coordinated.
  • As-built verification is included in commissioning.

Standards basis and official sources

  • IEC 61936-1:2021 — Official IEC installation rules including earthing-system considerations for power installations above 1 kV AC.
  • IEC 60909-0:2026 — Current IEC framework for fault-current calculation feeding earthing-duty studies.

Engineering note: Confirm the contracted edition, amendments, corrigenda, national adoption, project specification and manufacturer instructions before applying a requirement. This article explains engineering use and does not reproduce or replace the standard.

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