The document discusses relay coordination and grading methods for protective relays in power systems. It describes various coordination techniques including current grading, time grading, and a combination of time and current grading using inverse definite minimum. The selected protection principle affects the operating speed of the protection, which has a significant im-pact on the harm caused by short circuits. The faster the protection operates, the smaller the resulting ha-zards, damage and the thermal stress will be. We shall discuss a few important cases. That is to say, each one must isolate only the faulty section of.
