SLH Series Phase Loss, Under Voltage and Phase Sequence Monitor Relay

The SLH Series Phase Loss, Under Voltage and Phase Sequence Monitor/Relay from the ATC Diversified Electronics division of Marsh Bellofram Corporation is designed to protect equipment against PHASE LOSS (single phasing), UNDER VOLTAGE (brown outs), and PHASE REVERSAL (improper sequence).  The SLH is unique in that it has a field-adjustable hysteresis.

When correct phase sequence and line voltage are present, the internal relay of the SLH will energize (PICK UP). When there is a phase loss, under voltage or phase reversal condition, the internal relay will de-energize (DROP-OUT). When conditions return to normal, the SLH will automatically reset.

The voltage setting is adjusted to the desired pick-up point indicated by the dial setting. Then, the hysteresis adjustment is set to the desired percentage to achieve the preferred drop-out point. When models up to 300 VAC are set at 0%, they will pick-up and drop-out at the same point, when set at 10%, the drop-out will be an average of the phase-to-phase voltages 10% below the dial setting pick-up point. Models over 300 VAC are adjustable from 0% to 15%.

The SLH is available in the standard voltage ranges (see table below). It has an LED indicator that glows when all conditions are normal. The SLH Series is UL Listed under UL File Number E55826.



  • Models available up to 480 VAC
  • Protects 3-Phase Equipment Against:
    • Phase Loss
    • Under Voltage
    • Phase Reversal
  • Lock Shaft Adjustment for:
    • Voltage Pick Up
    • Drop Out Hysteresis
  • Delta or Wye Systems