During a short circuit, switchgear must withstand intense heating and electrodynamic forces until protection clears the fault. Rated short-time withstand current and rated peak withstand current describe different physical stresses and must be specified together with duration and frequency.

Learning objectives

Differentiate rms short-time current from instantaneous peak current, connect protection clearing time to thermal duty and assess the current path through busbars, joints, supports and earthing conductors.

Core engineering principles

Short-time current represents thermal duty

The rms current and declared duration define the approximate I²t stress imposed on conductors and contacts. A 25 kA rating for one second is not automatically equivalent to 25 kA for three seconds; conversion requires validated thermal assumptions and manufacturer limits.

Peak current represents electrodynamic duty

The first major current peak contains the effect of dc offset and creates the highest mechanical force. Force varies approximately with current squared, so busbar supports, joints, shutters and terminal structures must resist the declared peak without impairing function.

Fault duration must reflect the protection system

The required duration should cover relay operating time, trip-circuit time, breaker interrupting time and credible backup clearing where specified. Faster protection can reduce system stress, but it does not change the declared equipment rating.

Every current path must be considered

The main circuit, earthing switch, earth bar, removable connections and cable screens can experience different portions of fault current. An assembly main-bus rating does not automatically prove the duty of every earthing or neutral path.

Type-test configuration controls applicability

Conductor spacing, supports, joint locations, compartment arrangement and test connections influence mechanical stress. Evidence from a materially different width or busbar geometry should be assessed under the manufacturer’s extension rules.

Engineering application method

  1. Step 1: Obtain maximum initial symmetrical and peak short-circuit currents at the installation point.
  2. Step 2: Define required clearing duration for main and backup protection scenarios.
  3. Step 3: Select assembly short-time and peak withstand ratings equal to or above the calculated duties.
  4. Step 4: Trace fault current through the main and protective circuits, including earthing-switch making duty where applicable.
  5. Step 5: Review test reports and design-extension evidence for the exact busbar and panel configuration.

Practical example

If the calculated fault is 22 kA rms with a 55 kA peak and backup clearing can take 1.5 seconds, an assembly declared 25 kA for 1 second is not adequate merely because 25 exceeds 22. The duration and resulting I²t duty must also be covered by a declared or manufacturer-validated rating.

Common mistakes

  • Omitting the duration when stating short-time current.
  • Assuming the peak factor is always the same.
  • Using transformer terminal fault current without calculating the switchboard location.
  • Ignoring backup protection clearing time.
  • Assuming the earth circuit has the same rating as the main bus.

Design and review checklist

  • Are rms current, peak current and duration all specified?
  • Do calculations include actual source impedance and operating configuration?
  • Is backup clearing duty required by the project?
  • Are earthing and removable-part paths covered?
  • Does evidence match conductor geometry, supports and connections?

Standards basis and official sources

Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.

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