Testing of Neutral Over-Current Relay (50n / 51N) :

Neutral Over-Current relay is tested with secondary injection set with electronic meter. . Ensure that the CT incoming on the Test Plug MMLB01 is shorted before inserting. . Isolate the Relay Current Measuring Element & Trip /Alarm contacts by inserting the Test Block MMLB01. . Connect correct polarity (rating of DC Voltage through Test Plug MMLB01 to Relay. Ready “LED indicator” Should be illuminated. Press the “Target Reset” button and check that all target LEDs light up. . Ensure Curve selection as required. . Disable the instantaneous element and enable the time delayed element of the Relay. . Set the value and check start value for O/C protection. . Inject single phase current into the current element (terminal # B9 & B10) and slowly increase the current until the Pick-up LED lights. . Connect the Relay time delayed output contacts (terminals # A3 & A6) to both trip the test set, and stop a timer. . Set appropriate time setting. . Inject single phase current at a current level of “n” X rated current ( where “n” being the multiple of current setting of the Relay. . Check operating time for the Relay. . Inject single phase current at different current levels/curve selection & measure the operating times. . For instantaneous element operation, enable the instantaneous element of rhe Relay. . Adjust current setting for instantaneous element and connect instantaneous trip output contact (terminal # A3-A8) to trip the test set. . Time delay element is set to operate in 4s. The current level should be set, and then suddenly applied.