What is phase sensitivity?

Line monitoring relays in applications are sensitive to phase loss, unbalance and wrong sequence in the systems. Phase loss can occur due to breaking of cable, blowing of a single fuse, open connection, etc. and requires immediate corrective action as phase loss in a single phase can lead to increase in current in two other phases in the three phase line. The increase in current in the other two phases can reach about 200% the initial current value leading to an immediate burnout. Also, an unbalance in the three phases can result in unequal single phase loads and open delta connections to radically increase the current in the motor windings in the system. For this reason, monitoring relays are usually set to a certain voltage range to detect any deviation in phase. Also, changes in phase sequence or the wrong phase sequence can result in the monitoring relays to changeover to rest position. In addition to these three areas of sensitivity for line monitoring relays, they are also sensitive to over-voltage, under-voltage, over-current, under-current and faults in neutral conductors.