First-cycle duty of fused low-voltage power circuit breaker = MF × first-cycle symmetrical RMS current

MF = 1.0 is applicable whenever the first-cycle short circuit X/R ratio at the fault point (i.e., source side of breaker) is 4.9 or less. If the first-cycle short circuit X/R ratio is greater than 4.9, the following formula from IEEE Std C37.13-2008 can be used to find the MF.

The 3-phase rated short circuit current in RMS kA must exceed the first-cycle duty in asymmetrical RMS kA.

Note: On 3-phase systems where the voltage across a single pole under a fault condition may exceed 58% of the rated maximum voltage (e.g., corner-grounded delta under single line to ground fault), the single-pole interrupting capability shall be 87% of the 3-phase rated short circuit current.