TCC Options

: Short circuit calculations are based on one of three methods:

• The ANSI Standard method uses a separate R network for the interrupting duty (2-5 cycle) network to determine a conservative Z/R ratio. This ratio is then used as the Thevenin equivalent fault point X/R ratio for determining the appropriate breaker contact parting time multipliers and NACD ratios. Current and voltage calculations are based on a complex (R+jX) network reduction. Both the momentary (1/2 Cycle) and 30 cycle calculations use a complex network reduction for all voltage, current, and X/R ratio calculations.
• The Standard (non-ANSI) calculation uses a complex network reduction for all momentary, interrupting duty, and 30 cycle voltage, current, and X/R ratio calculations.
• The Characteristic Current Method (CCM) calculates the dc component of each branch based on phase angle of the current flowing in it and then sums the dc component is each branch contributing to the fault current. The ratio of the total dc to the total AC is used to determine the equivalent X/R at fault point. Branch current flows having different current phase angles (X/R ratios) will have the current peak at slightly different times before the first-half cycle. To simplify calculations, the dc component is taken at 0.5-cycle for all branches using the expression in equation below.

IDC = Ö2 IAC RMS SYM exp (-p /|X/R|)

After each dc component is determined and totaled, the equivalent X/R ratio is found from the equation below.

Equivalent X/R = -p/ ln (S IDC / S IAC RMS SYM / Ö2)

The CCM method provides a conservative approach to obtain the fault point X/R ratio and appears to do the best overall job without being over-conservative.