What must the interrupting rating of a circuit breaker at nominal circuit voltage be equal to?

The interrupting rating of a circuit breaker at nominal circuit voltage must be equal to the available fault current. This rating indicates the maximum amount of fault current that the circuit breaker can safely interrupt without being damaged or failing to operate effectively.

When a fault condition, such as a short circuit, occurs, the fault current can exceed normal operating levels dramatically. The circuit breaker needs to be capable of interrupting this high level of current to protect the electrical system and prevent potential hazards like fires or equipment damage. Ensuring that the interrupting rating matches or exceeds the available fault current is vital for the safety and reliability of the electrical system. This requirement reflects the importance of selecting circuit breakers that are appropriately rated for the specific electrical environment they are installed in.

Other options relate to current under normal operating conditions or average usage scenarios, but they do not account for the potential extreme conditions presented by fault currents, which is why they do not fulfill the requirement for interrupting capacity.