Power system circuit breakers

To explain the difference between typical and series amp-interrupting capacity (AIC) ratings, let’s review the typical method.

By Steven L. Pay, PE, LEED AP, FEA Consulting Engineers, Henderson, Nev. October 1, 2008

View the full story , including all images and figures, in our monthly digital edition . To explain the difference between typical and series amp-interrupting capacity (AIC) ratings, let’s review the typical method. Fault current calculations are needed in an electrical system to ensure that the system can safely handle and protect the specified equipment during a fault. The interrupting capacity of a device is the “the maximum short-circuit current that a protective device can safely clear.” The electrical power system design should provide safe operation under all conditions and allow non-effected buses of the system to continue operation under an isolated fault at a given bus.

Asymmetrical fault currents typically are composed of two components, symmetrical (steady-state) and asymmetrical (transient and subtransient currents). Steady-state current is the current supplied in the system during normal operation. Subtransient current is the current supplied in the system from the time the fault occurs to two cycles after the fault occurs. Transient current is the current supplied in the system from two cycles after the fault occurs until the steady-state value is reached.

In general, there are two types of power distribution systems, single-phase and three-phase. We’ll discuss three-phase systems in this article. In the three-phase power distribution system, there are two major ways to calculate fault current: the base, or ohmic, method and the per-unit method.

Ohmic calculation

The ohmic calculation method typically is used for simple calculations. This method is from “Engineering Dependable Protection for an Electrical Distribution System — Part I A Simple Approach to Short Circuit Calculations” from Bussmann Mfg. Initially, the utility reactance (the utility resistance is neglected) and the utility transformer resistance and reactance must be determined. This is accomplished using the following equations:

The next step is to gather the impedances of the large switches, circuit breakers, and transformers in the system. The manufacturer provides the circuit breaker and transformer values. Basic fault current calculations typically do not consider the minor effects of the impedance of circuit breakers and switches. The feeder impedances are given in tables for the type of conductors and raceway installation.

The calculation method next requires that we determine the sum of the resistance and reactance values to the point of the fault. At that point, the total impedance per phase can be determined to the point of the fault by:

With that established, we must now determine the short circuit symmetrical root mean squared amps at the fault. It is calculated by:

Next, determine the motor load of the system. Motor load contribution usually is approximated by using a percentage of the transformer full load current. In general, 50% motor load is assumed. The asymmetrical motor contribution is calculated by:

We also have to calculate the symmetrical motor contribution of the system. The asymmetry factor depends on the motor design of the system; the factor typically is approximated as 1.25. Therefore, the symmetrical motor contribution is determined by:

The total symmetrical short circuit is calculated by:

The asymmetrical factor is then determined by tables based upon the X/R ratio calculated below. This enables calculation of the asymmetrical short circuit current, using the following equation:

Per-unit calculations

Since we’ve discussed the base method in detail, we won’t discuss the per-unit method in depth. The per-unit method is used for complex calculations and also is from the Bussmann Mfg. book. The same assumptions are made in the per-unit method as in the base method, and both methods have similar calculation steps. The per-unit method equations are:

Series method of AIC ratings

A series AIC rating is defined by Cutler-Hammer as “A short-circuit interrupting rating assigned to a combination of two or more over-current protection devices. The short-circuit rating of the upstream device (main) must be at least equal to the available system fault current. The high interrupting capacity rating of the downstream device can be less, providing factory tests have been conducted to verify the combination rating.”

Typically, the fault current is calculated at every point within the system. The main switchboard will have an AIC rating calculated by the methods shown above. Then, all of the devices at the main switchboard will have an AIC rating that is the same based on the interrupting capacity determined in the calculations. For example, if the fault current calculation was 72,300 amps at the main switchboard, the next standard AIC rating would be selected. A 100,000 AIC rated switchboard usually is specified. All of the circuit breakers at the main switchboard, including the main and the distribution circuit breakers, have the same AIC rating of 100,000.

In the series rating method, the main breaker would have an AIC rating of 100,000, but the distribution breakers on the main switchboard can have a rating less than 100,000, based on factory tests of the combination of the breakers. The combination of the two breakers is rated at 100,000.

Cutler-Hammer defines three types of systems: selectively coordinated system, fully rated system, and series-connected system.

A selectively coordinated system is a system in which “all breakers are fully rated, and upstream breakers must have adequate short-time withstand ratings and short time delay adjusting capabilities.” This type of system has the highest reliability, requires calculations for the entire system, and is the most expensive of the three noted by Cutler-Hammer. In addition, fuses are sometimes required to obtain selective coordination.

A fully rated system is a system in which “all the over-current protective devices have a short circuit interrupting rating at least equal to the available short circuit current.” This type of system is less reliable than a selectively coordinated system because multiple breakers may open during a fault and it requires calculations for the entire system. A fully rated system is less expensive than a selectively coordinated system.

In a series-connected system, “low-level arcing faults are still cleared by the downstream breaker alone.” However, under high fault conditions, both the upstream and downstream breakers would open, eliminating service to the affected portion of the system. This type of system operates similarly to the fully rated system where multiple breakers may open to clear the fault; however, it does not require calculations for the entire system because tables are used to determine the required combination. This system is the least expensive of the three types. If a series-connected system is used, it also must comply with National Electric Code 240.86 requirements, specifically marking, motor contribution, and engineering supervision in existing installations.

Testing requirements

The tests required by UL for a single breaker are:

Calibration of the test breaker

“O” test in which the fault is initiated with the breaker in the closed position

“CO” test in which the same breaker is closed on the fault

Calibration test to ensure the breaker is still functional

Dielectric test.

The testing required for a series rating is the same testing as noted above for the main and branch circuit breakers individually—the interrupting ability and the intermediate ability tests. Depending on the results of these tests, additional testing may be required. In addition, UL may require follow-up testing.

Author Information

Pay is principal and electrical department head of FEA Consulting Engineers. He is an expert in low- and medium-voltage distribution system design.


National Electric Code 110.9 requires overcurrent protective devices to have an interrupting rating sufficient for the nominal circuit voltage and the available short-circuit current at the line terminals of the device. The 2005 NEC 240.86 added another type of series-rated system. However, confusion and controversy have surrounded the codes that involve interrupting ratings as related to the types of systems and how to protect buildings, workers, and equipment according to the types of systems selected. To understand the concepts behind fully rated, series-rated, and selectively coordinated systems, a refresher on calculating fault current for each type of system describes the physics behind the codes.