Ground Rules by John Cadick: Molded Case Circuit Breaker Ratings
We often make assumptions. We assume that the police radar won't be where we are speeding. We assume that the IRS won't notice that "interesting" little deduction that uncle Harry told us to take. We assume that we understand the ratings of the various pieces of equipment that we apply in our power systems.
The interrupting rating of a protective device is something that we assume we understand. We know that the NEC, among other standards, requires that interrupting devices must be capable of opening the short circuit current that they may be called upon to clear. For years, many of us assumed that the rating that is marked on the nameplate of the device is, in fact, the amount of current that it can interrupt. However, special care must be exercised in evaluating the interrupting ratings of molded case circuit breakers (MCCB).
Interrupting ratings of molded case circuit breakers are evaluated using a circuit similar to that shown in Figure 11. A power source calibrated to precise voltage, short circuit capacity, and power factor is connected to the breaker under test via a maximum length of four (4) feet of rated cable. The actual short circuit is connected to the breaker via a ten (10) inch length of rated cable.
If the circuit breaker successfully interrupts the short circuit (subject to the conditions imposed by the test parameters), the breaker is then listed with an interrupting rating equal to the source short-circuit capability. In Figure 1, for example, the breaker would be rated at 14,000 Amperes.
Obviously the breaker is actually tested at a current lower than 14,000 Amperes. In fact, the smaller the breaker frame size, the greater will be the discrepancy between the rating and the actual tested value. For example, a breaker, which uses AWG #12 wire, may be tested at a value of only 85% of the rated current.
This means that a circuit breaker applied within its published rating may, in fact, be over-dutied. This topic is covered in greater detail in the following two IEEE papers:
- Short Circuit Ratings Labels and Fault Withstandability of Molded Case and Insulated Case Circuit Breakers and Combination Motor Starters by Arthur J. Smith III.
- Short-Circuit Ratings and Application Guidelines for Molded-Case Circuit Breakers by William M. Hall and George D. Gregory
1This drawing is shown for illustration only is not an attempt to precisely re-create actual test conditions. For specific procedures refer to UL test standard 489.
A registered professional engineer, John Cadick has specialized for three decades in electrical engineering, training, and management. In 1986 he created Cadick Professional Services (forerunner to the present-day Cadick Corporation), a consulting firm in Garland, Texas. His firm specializes in electrical engineering and training, working extensively in the areas of power system design and engineering studies, condition based maintenance programs, and electrical safety. Prior to the creation of Cadick Corporation, John held a number of technical and managerial positions with electric utilities, electrical testing firms, and consulting firms. Mr. Cadick is a widely published author of numerous articles and technical papers. He is the author of the Electrical Safety Handbook as well as Cables and Wiring. His expertise in electrical engineering as well as electrical maintenance and testing coupled with his extensive experience in the electrical power industry makes Mr. Cadick a highly respected and sought after consultant in the industry.