Systems & Electronics

UAR4L – Digital under and overvoltage and frequency multifunction protection relay

The protection relay UAR4L belongs to SIGMA-L digital protection line and it performs overvoltage and undervoltage protection, over-frequency and underfrequency protection, residual overvoltage protection. The protection relay implements the functions indicated in the following table; their availability depends on how the relay is inserted into the system.


Overvoltage protection 59

Undervoltage protection


Positive sequence undervoltage protection


Negative sequence overvoltage protection


Residual overvoltage protection (Vo)

59N (59Vo)

Stator earth fault 95% (residual voltage)


Overfrequency protection


Underfrequency protection


Voltage balance


All the functions of the relay are fully programmable by front panel keyboard or through a RS485 serial interface using a personal computer with set-up program; furthermore the relay can be programmed through a remote control and monitoring system.

Thresholds – UAR4L manages the following thresholds:

  • 2 two-phases/three-phases overvoltage thresholds
  • 2 two-phases/three-phases undervoltage thresholds
  • 3 pairs of 2 unipolar overvoltage thresholds
  • 3 pairs of 2 unipolar undervoltage thresholds
  • 2 positive sequence undervoltage thresholds
  • 2 negative sequence overvoltage thresholds
  • 2 residual overvoltage thresholds
  • 2 over-frequency thresholds
  • 2 underfrequency thresholds
  • remote trip

The first thresholds of under and over voltage protections can be programmed either definite time or time dependent; other thresholds are definite time only.

Each definite time threshold delay can be combined with an additional delay controlled by the digital inputs.

Trip condition is shown and recorded with LEDs and with a message on the display.