SLB Series Phase Unbalance Monitor

The SLB Series Phase Unbalance Monitor from the ATC Diversified Electronics division of Marsh Bellofram Corporation is designed to protect three-phase equipment against Phase Unbalance, Phase Loss, and Phase Reversal conditions.

With normal operating voltages in the proper ABC sequence and the 120 VAC control voltage applied, the internal relay will energize (PICK-UP). When any combination of Phase Unbalance exceeding the preset value or Phase Loss or Phase Reversal occurs for longer than the preset drop out time, the output relay will de-energize (DROP-OUT). If the control voltage is removed, the relay will de-energize.

The wide input voltage range permits use on any one of several standard line voltages without recalibrating. The unbalance detection level is adjustable from 2% to 15%.

Both Delta and Wye systems may be monitored. In Wye systems, connections to neutral are not required. The LED indicator glows when conditions are normal.

NOTE: A balanced condition exists and the output relay will energize when there is a complete absence of voltage on all three phases (Terminals 1, 3 and 5) and the control voltage is continuously applied to (Terminals 11 and 12). For this reason, the SLB series is ideally suited for load side monitoring applications.



  • Models available up to 480 VAC
  • Protects 3-Phase Equipment Against:
    • Phase Loss
    • Phase Unbalance
    • Phase Reversal
  • Adjustable Unbalance Percentage and Release Time
  • Delta or Wye Systems
  • Several Enclosure Styles