Determined under standard conditions, the "interrupt rating" specifies the maximum amount of current a protective device can cut off safely ... i.e. without harm to personnel or resulting damage to equipment, the premises or the device itself. For example, a circuit breaker that trips "safely" successfully interrupts the fault, can be reset and will function properly afterward. To safely stop the fault current calculated for our chiller-motor scenario, the interrupt rating of the circuit breaker or fuses selected must be at least 31,400 amps.

Before leaving this topic, let's dispel a common misconception: "An overcurrent protection device with a comparatively high interrupt rating limits current to other components." Not so -- not unless it's also a true current-limiting device. Even though the device successfully breaks the circuit, all components in the circuit will be exposed to the full magnitude of fault current (as well as the severe thermal and magnetic stresses that accompany it) for the time it takes the device to respond.

Continue on to Withstand Rating, n.