What is SOTF protection? Operation and Purpose to use

SOTF protection stands for Switch On To Fault. If any fault is persisting in the transmission line during energization of Circuit Breaker or fault happens within 20 ms from the CB closing, the SOTF protection will activate and it will trip the Circuit Breaker immediately to isolate the fault.

Why SOTF Protection?

We know that Distance Protection is responsible to isolate the fault of the transmission line. Under Zone-1 protection, the CB will trip immediately in the fault condition and it will protect up to 80% of the total length of the line. Under Zone-2 protection, the CB will trip after 300 to 500 ms in the fault condition and it will protect up to 120% of the total length of the line.

If any persisting fault detects beyond 80% of the line length, the fault will detect under Zone-2 and it will operate after a delay of  300 to 500 ms. Which is not healthy for the connecting equipment and system stability. In this situation, the SOTF protection will cover whole the line and during the persisting fault, it will immediately isolate the line.

Further, after maintenance work of transmission line, it also found many cases, the line charged from the one end whereas the line earth switch of the line not opened at the other end due to human error. In this case, also SOTF function will operate and trip the CB immediately.

sotf protection
sotf protection


SOTF will operate by detecting the fault current and line voltage. In the persisting fault of the transmission line, the line CVT voltage will not increase after closing the CB. In that case, the SOTF function will activate as per the value of the setting of Voltage and current in the relay.

Further, the CB closing command is also used to activate the SOTF protection in the relay. After closing of CB, if any fault is detected within 500 ms, the CB will immediately trip under SOTF protection. After 500 ms, the SOTF function will automatically deactivate.

SOTF Enable logic

Under the following 02 conditions, SOTF can enable the relay.

If any manual or remote CB close command is issued from the control panel, SOTF protection is enabled for 500 ms and after that, it will reset automatically.

When all the 3 poles’ CB open status is detected by the relay for longer than SOTF delay (generally delay is 110 ms), under this condition, after closing of all CB poles, SOFT is enabled in the relay for 500 ms and then resets.

ANSI code

ANSI code of SOTF is 50.

SOTF & TOR protection

TOR stands for Trip on Reclose Protection. TOR is provided in a distance relay for immediate clearance of any line after auto reclosure of the circuit breaker.

SOTF activates after manual closing of the CB and TOR (Trip of Reclose) activates after auto recloses of the CB. During auto reclose, if any fault persists in Zone-2, in that situation, the fault will isolate after the zone-2 time delay in distance protection. However, if the TOR function is enabled in the relay, the fault will isolate immediately.


ANSI code of SOTF protection is 50.

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