Switch on to fault protection (SOTF)
Switch on to fault protection (SOTF) is used to provide high-speed tripping when energizing a power line under fault condition. Exactly, switching on the circuit breaker without fault clearance (fault exist in the line). Switch on to fault protection trips the circuit breaker immediately without time delay.
Switch on to fault protection Working Principle:
Consider If you are switching on the circuit breaker under fault condition, the line Voltage does not rise to its rated and the line current goes very high due to the fault. Many Line protections include this SOTF protection as a built in logic. The closing of CB enables the logic for a set time say 200ms.If the Voltage does not rise to 50-60 % of the rated voltage in one cycle & if the current is more than the set current it is sensed as a SOTF and a three phase tripping impulse is given to the tripping circuit.
SOTF Protection will be activated in the event of Line faults still exist in the transmission line and accidental line energization with maintenance earthing clamps left in position.