What does 'interrupting rating' refer to and how is it used?

• A. The voltage a device can withstand before insulation breaks
• B. The temperature rating of a protective device
• C. The physical size of a protective device
• D. The maximum fault current a protective device can interrupt safely; used to select devices appropriate for system fault currents

Answer: (D)

Interrupting rating is the maximum fault current a protective device can interrupt safely. This rating tells you how much current the device can break open without sustaining damage, warping or welding contacts, or creating unsafe conditions during a fault. It’s the key factor when selecting protective devices because you need a device whose interrupting capability meets or exceeds the worst-case fault current that could occur at the point of installation. In practice, you determine the available fault current for the system and choose a breaker or fuse whose interrupting rating is at least that large (at the operating voltage). The other options describe unrelated properties—voltage withstand (dielectric rating), temperature rating, and physical size—which do not describe the device’s ability to interrupt fault currents.