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over current comes from overload but short cicuit comes from short between two or more than two lines in transmision lines or between two windings or three inside the transformer and generator. for protection in both fault it uses overcurent relay and differntial relay in gen. and trans. and less than 30km in the transmision line
usually the method of fault is determining the difference. over-current implies the circuit is still functional but the current has exceeded the safe operating threshold and has entered the "still has margin enough to survive but you should really think about shutting down in a controlled manor" mode.
short circuit is a different type of fault. it is more random in its occurrence (could be line to line, line to ground, 3-phase to ground etc), has virtually zero impedance between energy source and fault location and is considered catastrophic and requires IMMEDIATE attention.