#### 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

## Molded- and Insulated-Case Circuit Breakers

First-cycle duty of molded- or insulated-case breaker = MF × first-cycle symmetrical RMS current

**Interrupting ratings 10,000A symmetrical or less:**

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 less than 1.7. If the first-cycle short circuit X/R ratio is greater than 1.7, the following formula can be used to find the MF.

**Interrupting Ratings between 10,001A and 20,000A symmetrical:**

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 less than 3.2. If the first-cycle short circuit X/R ratio is greater than 3.2, the following formula can be used to find the MF.

**Interrupting ratings exceeding 20,000A symmetrical:**

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 less than 4.9. If the first-cycle short circuit X/R ratio is greater than 4.9, the following formula 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.