The relay type NA21 is typically used in HV, MV and LV radial networks as feeder or power transformer protection.
In solidly grounded systems the residual overcurrent protection can be used on feeders of any lenght, while in ungrounded or Petersen coil and/or resistance grounded systems, the residual overcurrent protection can be used on feeders of small lenght in order to avoid unwanted trippings due to the capacitive current contribution of the feeder on external ground fault.
Beside to the phase and residual overcurrent protection, the following protective functions are provided:

  • thermal image protection of lines and power transformers

  • undercurrent protection for monitoring of CB opening

  • negative sequence protection against asymmetrical short circuits and unbalanced loads

  • I2/I1 protection against phase interruption under low-load condition

  • automatic reclosing

NA21 - Pro-n ()


Key:   standard standard optional optional alternative alternative


  • Feeder
  • Transformer

Protection functions

  • Pt100 thermometric probes (26)
  • Phase undercurrent (37)
  • Negative sequence overcurrent (46)
  • Thermal image (49)
  • Phase overcurrent (50/51)
  • Residual overcurrent (50N/51N)
  • Automatic reclosing (79)
  • High impedance restricted earth fault (87NHIZ)
  • Breaker failure (BF)
  • Phase interruption under low-load condition (I2/I1)

Monitoring and control functions

  • CB diagnostic
  • Cold Load Pickup (CLP)
  • CTs monitoring (74CT)
  • Digital Fault recording (DFR)
  • Fault and event logging (SER & SFR)
  • Logic selectivity
  • Multiple setting profiles
  • Programmable Logic Controller
  • Second harmonic restraint
  • Self-test
  • Trip circuit supervision (74TCS)


  • 2 binary inputs + 6 output relays + 8 LEDs
  • 5 binary inputs + 6 output relays + 8 LEDs
  • Auxiliary power supply 110-230 Vac/dc
  • Auxiliary power supply 24-48 Vac/dc
  • Flush, projecting, projecting, rack, with separate MMI
  • Network module with Ethernet FX port (optical fiber)
  • Network module with Ethernet TX port (copper RJ45) + RS485 port
  • Nominal current 1/5 A selectable by DIP-switch
  • RS485 port (unavailable with Ethernet FX port)
  • Standard CT amperometric inputs 1/5 A


  • IEC 61850 Protocol
  • Modbus RTU RS232 protocol
  • Modbus RTU RS485
  • Modbus RTU RS485 / IEC 870-5-103
  • Modbus RTU RS485 / IEC 870-5-103 / DNP3 protocol
  • Modbus TCP/IP protocol (network board must be provided)