Short-circuit calculating procedure for low-voltage A-C systems

FAULT currents in electrical systems depend upon the impedance of the circuit for their intensity. Stresses, both thermal and mechanical, occur due to such currents and at various time intervals following initiation of the fault. Such stresses are of primary interest in system and equipment design. The possible amount of short-circuit current in any system equals the quotient of the voltage divided by the impedance. The latter term usually includes certain elements which formerly were indeterminate but now can be estimated with reasonable accuracy. Short-circuit currents measured in present-day (600 volts and less) types of equipment were found to check closely with currents calculated from the impedance of the circuit and the applied voltage.¹ It is, therefore, advisable to discard “rule-of-thumb” or “discounting” procedures for predetermining fault currents. In the discussion which follows, a method is given whereby the symmetrical current may be calculated from the E/Z relation; the amount of d-c component is shown to decrease from a theoretical maximum at zero time at various rates according to the relation of the circuit reactance to its resistance; a simplified and convenient step-by-step procedure is given for the determination of balanced three-phase short-circuit currents using the foregoing relationships; the time is shown at which currents that determine the interrupting and short-time duties of circuit interrupting devices are identified in order that the corresponding value of reactance and d-c component can be used for calculation of the imposed current.