Purpose of power system protection. Criteria of detecting faults: overcurrent, differential current, difference of phase angles, over and under voltages, power direction, symmetrical component of current and voltages, impedance, frequency and temperature. Instrument transformers: CT and PT. Electromechanical, electronic and digital relays: basic modules, overcurrent, differential, distance and directional. Trip circuits. Unit protection schemes: generator, transformer, motor, bus bar, transmission and distribution lines. Miniature circuits breakers and fuses. Circuit breakers: principle of arc extinction, selection criteria and ratings of circuit breakers, types, oil, SF6 and vacuum.