The ANSI/IEEE calculation method is based on the same per unit quantities as calculated before. However, it differs from the classical method because it makes it possible to study two separate circuits derived from the original one: one resistive only and one reactive only. This will be carried out for both momentary and interrupting network fault impedances.
For each network, Thevenin equivalent resistance and Thevenin equivalent reactance will then be combined in order to obtain the equivalent Thevenin impedance. This is the significant difference between the ANSI/IEEE procedure and the classical calculation method.
As mentioned before, the momentary network fault impedance is based on the subtransient reactances of the rotating machines, which allows for the calculation of the first-cycle peak fault duty. The total fault resistance and reactance values will be calculated separately, following the same formula as the ZFault equation in the classical calculation section, except the Zs must be replaced with the Rs and Xs.
Then they'll be combined as total fault impedance ZFault, which will yield ISC3-phase and IPeak according to the formulas.
The interrupting network fault impedance is based on individual equipment transient reactances. In the previous example, only the reactance of Motor 1 needs to be adjusted. It's acceptable to neglect Motor 2 at medium voltage levels. The resistances of the network, in fact, don't vary with respect to time. ANSI C37.010-1999 identifies the adjustment factor as 1.5.
In this case, the total fault resistance and fault reactance (with adjustments) will be calculated separately as already seen.
ISC3-phase, symmetrical duty is calculated as it was in the classical method. However, it's typically characterized by a smaller magnitude because the Zfault “interrupting” current is larger than the one in the momentary network calculation.
ISC3-phase is essential because a multiplier factor is applied to this quantity for comparison to the breaker interrupting rating.
This multiplier will account for:
- The additive contribution of the DC current component, which might still be “alive” after the time of contact parting.
- The eventual subtractive contribution of the AC current decay, due to the evolution of the reactances toward larger values. This effect is possible when the generation of power is local.
The multipliers, in function of time of contact parting and of the ratio X/R at the point of fault, are described in curves starting from figure A-8, page 60, C37.010-1999 (Figure above).
Once ISC3-phase has been multiplied by this factor (between 1 and 1.25), you have the minimum rating of your equipment. As in the classical method, you can also use Table 1, page 1 in ANSI C37.06-1997 to determine a standard rating.