What is in this article?:
- Short Circuit Current Duties of Circuit Breakers and Fuses โ Part 2
- Unfused Low-Voltage Power Circuit Breakers
- Fused Low-Voltage Power Circuit Breakers
- Molded- and Insulated-Case Circuit Breakers
- Low-Voltage Fuses
Understanding the methodologies behind the math
Unfused Low-Voltage Power Circuit Breakers
First-cycle duty of unfused 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 6.6 or less. If the first-cycle short-circuit X/R ratio is greater than 6.6, 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.