9. SHORTCIRCUIT CURRENT CALCULATIONS（9。）.pdf

9. SHORT-CIRCUIT CURRENT CALCULATIONS 9.1 Purpose the MCCB, generally, the short-circuit current is cal- Japanese and international standards require, in sum- culated from the impedance on the supply side of the mary, that an overcurrent protector must be capable breaker. of interrupting the short-circuit current that may flow Fig. 9.1 represents a summary of Japanese standards. at the location of the protector. Thus it is necessary to establish practical methods for calculating short-cir- 9.3 Impedances and Equivalent Circuits of cuit currents for various circuit configurations in low- Circuit Components voltage systems. In computing low-voltage short-circuit current, all im- pedances from the generator (motor) to the short-cir- 9.2 Definitions cuit point must be included; also, the current contrib- 1. % Impedance uted by the motor operating as a load. The method is The voltage drop resulting from the reference current, outlined below. as a percentage of the reference voltage (used for 9.3.1 Impedances short-circuit current calculations by the % impedance 1. Power Supply Impedance (Z ) L method). The impedance from the power supply to the trans- voltage drop at capacity load former-primary terminals can be calculated from the % impedance =