SLD Series Phase and Under-Voltage Monitor

The SLD Series Phase and Under-Voltage Monitor from the ATC Diversified Electronics division of Marsh Bellofram Corporation is designed to protect 3-phase equipment against Phase Unbalance, Phase Loss, Under Voltage and Phase Reversal conditions. With normal operating voltage present on all three phases in the proper phase sequence, the internal relay will energize (PICK-UP). When an incorrect phase sequence or phase loss occurs or the three-phase line voltages fall outside the preset unbalance or under voltage settings, the internal relay will de-energize (DROP-OUT). When all conditions return to normal, the relay will reset. The Adjustable Release Delay is provided to ignore momentary voltage fluctuations that cause nuisance tripping. Both Delta and Wye systems may be monitored. In Wye Systems, connections to neutral are not required.

NOTE: When a phase is lost while the motor is running, a condition known as regeneration occurs where a voltage is induced into the open phase nearly equal in magnitude to the normal phase-to-phase voltage. The SLD series is designed to detect this condition when properly adjusted.



  • Models available up to 480 VAC
  • Protects 3-Phase Equipment Against:
    • Phase Loss
    • Under Voltage
    • Phase Unbalance
    • Phase Reversal
  • Adjustable:
    • Phase Unbalanced Percent
    • Under Voltage Drop Out
    • Release Time
  • Lock Shaft, or Screwdriver Adjustment
  • Automatic Reset
  • Several Enclosure Styles
  • Delta or Wye Systems