Short-Circuit Duty: ANSI vs IEC

One fault bus, two methods. ANSI/IEEE C37 gives a symmetrical current adjusted for X/R against the breaker's test circuit; IEC 60909 gives Ik″, ip, Ib and Ith with the voltage factor c. Each is compared with the ratings of its own standard family.

Screening

System

Fault bus voltage (V, line-to-line)FrequencyUtility sourceSource short-circuit (MVA)Source X/R

Transformer

IncludeRating (kVA)Impedance (%Z)X/R

Cable to the fault bus

Include

Motor contribution at the bus

IncludeRunning motors (kVA)Xd'' (% on motor kVA)Motor X/RANSI first-cycle multiplier

Breaker — ANSI / IEEE C37 ratings

Breaker typeInterrupting rating (kA sym)Test X/R

Breaker — IEC ratings

Breaking Icu / Isc (kA)Making Icm (kA peak)Short-time Icw (kA)Icw duration (s)Minimum break time tmin (s)Fault duration Tk (s)LV voltage tolerance

Breaker test X/R and ANSI motor multipliers are typical values prefilled for convenience — confirm them against the standard edition you apply.

Result

Describe the path to the fault bus and the breaker ratings; both the ANSI and the IEC results are shown.

RokBench is a screening tool intended to assist qualified engineers. It does not constitute engineering services, does not produce sealed or certified deliverables, and does not replace applicable codes, standards, manufacturer documentation, or the judgment of a licensed professional responsible for the design. Verify all results independently before use. Report a problem with this result.