The name of short-circuit current ik is the effective value of steady-state short-circuit current. Short-circuit impulse current appears in the first time period after short-circuit fault, and short-circuit current is the sum of steady-state component and transient component. When they are added together, there is a maximum value, which is called short-circuit impulse current. In high voltage system, ish=2.55Ik, Ik is the calculated short circuit current.