ABB REL 356

ABB REL 356 Current Differential Protection
4-13
Measuring Elements and Operational Logic
Timer OST3 starts timing after the inner blinder operates. If the timer has timed out, then tripping will be allowed
immediately (with a 20 msec time delay) once the outer blinder resets and OST is set to trip on the way out, WAYO.
Otherwise, OST has to time out first.
The OSOT timer is an out of step override timer that bypasses OSB after it has timed out and lets the relay trip.
Loss of Potential (LOPB)
Loss of potential logic is used to supervise the distance measurements in the distance backup system. When this
condition exists, all impedance measuring units well have their output blocked.
The [V0 x notI0] logic will detect one or two blown fuses, but will fail to detect the unlikely simultaneous failure of all three
phase fuses. The operating threshold are 3V0 > 7 V and 3I0 < 0.5 A.
Loss of Current Monitoring
The loss of current logic will indicate loss of one or two current inputs.