About the Breaker Duty Check
Circuit breaker duty check verifies that the interrupting capacity (breaking current in kA) and making capacity (closing current in kA peak) of installed or specified circuit breakers exceed the available short-circuit level at the busbar — a mandatory equipment rating check per IEC 62271.
Formula
I_break_required = I_sc_sym × (dc_component_factor)
- I_sc_sym
- Symmetrical short-circuit current at the busbar (kA rms)
- dc_component_factor
- Per IEC 60909: typically 1.0 for breakers ≥ 2 cycles from source; higher for close-to-generator faults
- I_break_required
- Required breaking capacity (kA rms symmetrical)
I_make_required = 2.5 × I_sc_sym
- 2.5
- Peak factor per IEC 62271 for 50 Hz (approximates √2 × 1.8 asymmetry)
- I_make_required
- Required making capacity (kA peak)
How to calculate it
- 1Obtain the short-circuit current from the fault level study at each busbar.
- 2Check the breaker nameplate: rated short-circuit breaking current (kA rms) must exceed I_sc_sym.
- 3Check rated short-circuit making current (kA peak) must exceed 2.5 × I_sc_sym.
- 4Check the rated short-time withstand current (kA for 1 or 3 s) — the breaker must carry the fault current for the breaker-fail protection operating time without damage.
- 5If the breaker duty is exceeded, upgrade to a higher-rated breaker or add current-limiting reactors upstream.
Worked example
Available fault level at a 33 kV bus = 12.5 kA (symmetrical). Required breaking capacity = 12.5 kA; making capacity = 31.25 kA peak. An existing 12.5 kA, 31.5 kA peak VCB (vacuum circuit breaker) just passes the check. If the DISCOM upgrades the grid infeed and fault level rises to 14 kA, the breaker must be replaced with a 16 kA rated unit.
What you can calculate
- Duty check
- IEC 62271-100
- Asymmetry
- DC offset
Standards & references
Frequently asked questions
What is the difference between a breaker's breaking capacity and its making capacity?
Breaking capacity (kA rms symmetrical) is the fault current the breaker can safely interrupt when opening — it is limited by the breaker's arc-extinguishing capability. Making capacity (kA peak) is the fault current the breaker can close onto — it must withstand the asymmetric peak current that occurs in the first half-cycle when a breaker closes onto a bolted fault. Making capacity is always specified as a peak value and is typically 2.5 × the breaking capacity (rms) for 50 Hz systems.
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