The relay type STR is intended for the complete protection of high power motors, with medium voltage or low voltage supply. Several protection functions, incorporated in a single relay, assure the protection of the motor against all the different fault conditions: overheating caused by mechanical overload or by an abnormal supply, locked rotor, too long starting, interruption of one phase conductor, current unbalance, short circuit between phases, short circuit between turns, earth fault, underload running, too frequent startings.