If the source has inductance, the fault current has to start at zero because you can't change current instantaneously in an inductor. If the fault voltage is > 0, however, at the instant on the sine wave when the fault starts, the current starts out with a high rate of change (V = L?di/dt) and it overshoots the normal peak, then eventually settles down to the steady state (symmetric) value. The current averages more than zero until it settles down and can be represented by a sine wave plus a decaying dc component. It is not symmetrical about the zero axis (the positive part is different from the negative part), so is called asymmetrical fault current.