Feeder protection

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 length, while in ungrounded or Petersen coil and/or resistance grounded systems, the residual overcurrent protection can be used on feeders of small length 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 unbalance loads
  • I2/I1 protection against phase interruption under low-load condition
  • Automatic reclosing.


Standard Protective & control elements
  • Phase overcurrent side H and side L
  • Maximum Residual Overcurrent
  • Automatic Recloser
  • Breaker Failure
  • CT Supervision
  • Trip Circuit Supervision
  • Under Current
  • Negative sequence overcurrent
  • Thermal Image Transformer
SW Pack optional configuration
  • Thermal with Pt100 Probes


  • One USB Type B local port for communication with ThyVisor setup software