Short-circuit studies convert the network’s source, transformer, cable, motor and grounding data into currents that equipment must safely make, carry and interrupt. For MV switchgear, the calculation result must be mapped to several different ratings rather than compared with one kA value.

Why this topic matters

A switchboard can have adequate symmetrical breaking current yet be inadequate for peak making current, short-time duration or the earthing circuit. Incorrect assumptions about transformer impedance, utility contribution, motor infeed or operating topology can also hide the worst case.

Scope and engineering boundary

IEC 60909-0:2026 provides a calculation method for short-circuit currents in three-phase AC systems within its scope. Equipment selection then uses the relevant IEC 62271 ratings and switching-device evidence.

Core engineering principles

Different current quantities serve different decisions

Initial symmetrical current supports breaking and withstand checks; peak current governs electrodynamic stress and making duty; thermal equivalent and duration influence conductors and earthing circuits.

Topology and operating scenario matter

Bus couplers, parallel transformers, embedded generation and motor contribution can change maximum fault level. Minimum fault current can govern protection sensitivity and clearing time.

X/R and DC offset affect peak and interruption

The network impedance angle influences asymmetry and peak current. It also affects CT transient performance and breaker duty around the interruption interval.

Rating selection includes duration

A 25 kA 1 s assembly is not automatically equivalent to 25 kA 3 s. Protection clearing time, backup operation and mechanical withstand must be coordinated with the declared duration.

Application workflow

  1. Step 1: Build a validated network model with utility, transformers, cables, generators, motors and grounding impedances.
  2. Step 2: Define credible maximum and minimum operating configurations.
  3. Step 3: Calculate three-phase and relevant unbalanced faults at each switchboard location.
  4. Step 4: Extract initial symmetrical, peak, breaking and thermal-equivalent duties with assumptions recorded.
  5. Step 5: Compare results with breaker making/breaking, assembly short-time/peak and earthing-circuit ratings.
  6. Step 6: Recheck protection clearing time and future expansion margin before freezing the specification.

Practical engineering example

Two 20 MVA transformers operating in parallel can produce a much higher bus fault level than either transformer alone. If the bus coupler is normally closed, the study must use that topology or define an interlock and operating rule that prevents the parallel condition.

Common mistakes

  • Copying the transformer terminal fault current to every downstream bus.
  • Checking only symmetrical rms current.
  • Ignoring parallel-source and motor contribution.
  • Using a 1 s rating where backup protection can clear later.

Design and review checklist

  • Network model and source data are approved.
  • Maximum and minimum scenarios are evaluated.
  • Peak and short-time duration are checked.
  • Earthing circuit and cable screen duties are included.
  • Protection clearing time is coordinated with ratings.

Standards basis and official sources

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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