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.”

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