The interrupting rating is primarily determined by which factor in a system?

• A. Fault levels in the system
• B. Ambient temperature
• C. Conduit fill
• D. Voltage rise under no-load conditions

Answer: (A)

The interrupting rating is defined by the maximum fault current the protective device can safely interrupt. In a system, this rating is driven by the available fault current (the fault levels) at the device’s terminals. You must choose a device whose interrupting rating is at least equal to the highest prospective short-circuit current that could occur, otherwise a fault could cause equipment damage or failure to interrupt.

Ambient temperature, conduit fill, and voltage rise under no-load affect other aspects of performance (like heat/ampacity or voltage behavior) but do not determine the device’s fundamental ability to interrupt a fault current.