The short circuit is essentially an abnormal condition within a power system in which a heavy amount of current flows through the circuit.
A short circuit usually occurs as a result of a fault in a power system. The fault may be a conductor breaking and falling to the ground, or two or more electrical conductors coming in contact with each other.
Such faults result in the formation of a low resistance path for the current. This is a short circuit condition.
That is why short circuit calculations are mandatory as well as recommended practice of electrical preventive maintenance according to NFPA. (NFPA 70B chapter 9)