The total short circuit current is the summation of two components - the symmetrical RMS current , and the DC component. The DC component is a function of the stored energy within the system at the initiation of the short circuit. It decays to zero after a few cycles due to I2
R losses in the system, at which point the short circuit current is symmetrical about the zero axis. The RMS value of the symmetrical component may be determined using Ohm`s Law. To determine the asymmetrical component, it is necessary to know the X/R ratio of the system. To obtain the X/R ratio, the total resistance and total reactance of the circuit to the point of fault must be determined. Maximum thermal and mechanical stress on the equipment occurs during these first few cycles. It is important to concentrate on what happens during the first half cycle after the initiation of the fault.
A simple explanation of the origin of the transient component is that in power systems the inductive component of the impedance is high. The current in such a circuit is at zero value when the voltage is at peak, and for a fault at this instant no direct current (dc) component is required to satisfy the physical law that the current in an inductive circuit cannot change suddenly. When the fault occurs at another time, there has to be a transient current whose initial value is equal and opposite to the instantaneous value of the ac short-circuit current. This transient current can be called a dc component and it decays at an exponential rate.