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Rating of electrical equipment:
Power rating: Power rating of the electrical equipment (say) transformer, circuit breaker depends on the power it is handled during normal operation of the equipment. It will be normal to 35MVA, 70MVA, generation transformer near 500mW generator will be 600MVA.
Short circuit rating: All the equipments are designed to withstand the 3 phase short circuits. So the design of circuit breakers, isolators will be in the range of 20kA, 40kA. It has to withstand the short circuit for 1sec before the relay operates ans isolates the fault.
Importance of impedance during short circuit: During short circuit all the motors (Induction motors, synchronous motors..), generators, capacitors connected in the power system feed the fault (its reason for such large short circuit current ) and impedance such as transformer impedance, static loads connected and cables reduces the short circuit current. So while designing transformer impedance is an important factor to limit the short circuit current. Too much impedance on the other hand results in voltage drop in normal operating condition. So an optimum Z value is chosen for transformer while designing. I hope this helps to understand up to some extent.... Thankyou

Circuit breaker is a device for interrupting circuits under fault or needed conditions. It depends on the momentary and interrupting duties