Directional overcurrent protection is used for overcurrent or earth-fault protection when selectivity is required, e.g. due to parallel lines meshed networks or networks being supplied from a number of directions.
The RXPDK 21H is used for phase-overcurrent protection. The operating time can be set to be current independent (definite time) or current dependent using the available IEC inverse characteristics. The relay uses memory voltage for polarization when the voltage collapses due to the fault.
Assemblies for earth fault-protection in some solidly earthed networks may use the RXPDK 21H directional relay with independently set time delay. The polarizing voltage for RXPDK 21H must exceed 5 V in order to provide a proper directional sense.
The RXPDK 22H is used for directional earth-fault protection in high-impedance earthed systems. The RXPDK 23H relay is intended for solidly grounded systems where expected polarizing voltages are below 5 V.