Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the available fault current ___.

Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the available fault current ___.

• A. at the output terminals of the power source
• B. both a and b
• C. at the line terminals of the equipment
• D. neither a nor b

Answer: (C)

The main idea is that equipment that interrupts fault current must be able to safely break the exact amount of current that could flow through it at its point of operation. That means the interrupting rating must be at least equal to the available fault current on the line side of the equipment, at the nominal circuit voltage. This is why the line terminals are the reference: they’re where the device actually experiences and interrupts the fault current. The output terminals of the power source aren’t the point the device is protecting, and sizing follows the current available at the line side, not upstream or elsewhere.

The main idea is that equipment that interrupts fault current must be able to safely break the exact amount of current that could flow through it at its point of operation. That means the interrupting rating must be at least equal to the available fault current on the line side of the equipment, at the nominal circuit voltage. This is why the line terminals are the reference: they’re where the device actually experiences and interrupts the fault current. The output terminals of the power source aren’t the point the device is protecting, and sizing follows the current available at the line side, not upstream or elsewhere.