What must an OCPD's interrupting rating be relative to the equipment it protects?

• A. It must be equal to the service disconnect rating.
• B. It must be chosen to be able to interrupt the available fault current at the point of installation.
• C. It must be smaller than the fault current.
• D. It must be rated for DC only.

Answer: (B)

The main idea is that an overcurrent protective device must be able to safely interrupt the maximum fault current that could occur at the location it protects. The available fault current depends on the source and the wiring up to that point. If the OCPD’s interrupting rating is not high enough to handle that fault current, the device could fail during a short circuit, creating a safety hazard. So the device is chosen to be capable of interrupting the fault current at the installation point, ensuring the protective device can trip and clear the fault without damage.

The other considerations aren’t the primary requirement here: the protective device doesn’t have to match the service disconnect rating, and it should not be smaller than the potential fault current. Also, many devices are rated for both AC and DC or for the relevant system types; being DC-only is not the determining factor—the key is having an interrupting rating adequate for the expected fault current.