Though often confused, "interrupt rating" and "withstand rating" are not interchangeable terms. Unlike the interrupt rating, which defines the performance limit of an overcurrent protection device (e.g. circuit breaker or fuse), the "withstand rating" is a performance limit for an enclosure. In other words, it identifies the maximum short-circuit amperage an enclosure can contain without injuring personnel or damaging the premises.

Underwriters Laboratories Inc. (UL) defines the short-circuit test methods and parameters for HVAC equipment. Essentially, the test subjects an enclosure to the recommended current, i.e. 4,000 amps if the unit RLA exceeds 40 amps and 3,500 amps if it's less. If the doors blow open or if it emits flames or sparks, the enclosure fails the test. For those that pass, it's "acceptable"-even probable- that the internal components will be damaged beyond repair. Given the destructiveness and expense of this test, it's not surprising that most manufacturers prefer not to pursue higher-than-normal short-circuit withstand ratings for their equipment unless there's a documented need.

Recall that when a fault occurs, all components in the circuit experience the brunt of the short circuit until it's stopped. Therefore, it's important to assure that all components "at risk" can withstand a fault condition without causing injury or damaging the surroundings. The National Electric Code (NEC) states this requirement in Section 110-10, "Circuit Impedance and Other Characteristics":

The overcurrent protective devices, the total impedance, the component short-circuit withstand ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway.

Commentary in the 1996 National Electrical Code® Handbook further explains Section 110-10:

Overcurrent protective devices (such as fuses and circuit breakers) should be selected to ensure that the short-circuit withstand rating of the system components will not be exceeded should a short circuit or high-level ground fault occur.

System components include wire, bus structures, switching, protection and disconnect devices, distribution equipment, etc., all of which have limited short-circuit ratings and would be damaged or destroyed if these short-circuit ratings are exceeded. Merely providing overcurrent protective devices with sufficient interrupting ratings will not ensure adequate short-circuit protection for the system components. When the available short-circuit current exceeds the withstand rating of an electrical component, the overcurrent protective device must limit the let-through energy to within the rating of that electrical component.

To comply with this section of the NEC, all of the component selections in our chiller-motor scenario must be based on a minimum short-circuit withstand rating of 31,400 amps ... a requirement well above UL's standard ratings.

Continue on to Current Limiting, n.