Control Tab

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 in 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.

I_DC = Ö2 I_{AC 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 I_DC / S I_{AC 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.

Reference:
Parise G., A new approach to calculate the decaying AC contributions to short-circuit: the ‘characteristic’ currents method; IEEE Transactions on Industry Applications, Vol. 31, No. 1, January/February 1995.

Sets the lower limit for flagging breaker violations in SmartDuty™. If the threshold is set to -10 percent, SmartDuty™ will flag all equipment which has short circuit duties within 10 percent of their maximum rating (greater than 90% of their rating).

Note: The ability to automatically check equipment duties during analysis is only available if you have purchased the SmartDuty™ option to EasyPower.

For example, a GE AM-13.8-500 air blast breaker has a momentary and interrupting duty rating of 19.56/37 kA at 13.8 kV. SmartDuty™ provides a warning flag if the current exceeds 17.6/33.3 kA in either the momentary or interrupting rating. If the current exceeds 19.56/37 kA, a violation is flagged.

Note: You must click Equipment Duty from the Short Circuit focus for this field to have any effect on the one-line result output. 
For this field to have any effect for text result output, you must select the Equipment Duty check box in the Text Output tab of the Short Circuit Options dialog box.