SLU-100 Series Universal Phase Monitoring Relay

The SLU-100 series universal monitor relay is designed to provide cost-effective protection against premature equipment failure or downtime caused by voltage faults or untimely outages on three-phase systems (Delta and Wye). They are designed to support those systems typically found in motor control centers or other critical motor and pump monitoring applications.

Designed for the convenience of electricians, maintenance managers and engineers, the SLU-100 series monitors up to 480 VAC, with easy adjustment capabilities on a single relay for voltage, imbalance percentage and fault delay time and restart delay requirements. These capabilities allow for high-reliability protection against unbalanced voltages or single phasing, regardless of any regenerative voltages which may be present in the system.

Each Marsh Bellofram SLU-100 series unit is equipped with an LED fault indicator for easy visual troubleshooting, and is offered with a voltage range of 550V AC or 700V AC. Units are available with either universal DIN-rail (ASD model) or 8-pin plug-in connection and with choice of automatic or manual reset. For requirements in Wye system, connections to neutral are not required. In addition, an adjustable manual or rapid cycling feature prevents unwanted motor cycling due to load induced fault conditions.

Notes: Both Delta and Wye systems may be monitored. In Wye systems, connections to neutral are not required.

Not recommended for generator or variable frequency drive applications. Call technical support for assistance.



  • Monitors up to 480 VAC
  • DIN-rail mounted or 8-pin plug-in versions
  • Automatic or manual reset
  • LED fault
  • Adjustments for:
    • Unbalance percentage
    • Fault delay time
    • Restart delay


  • Critical motor and pump monitoring
  • Water and wastewater plants
  • Sewage plants
  • Booster stations
  • Grinders
  • Irrigation equipment
  • Petrochemical
  • Machine Tool
  • Conveyors
  • HVAC/R
  • Elevators/escalators
  • Utilities
  • Processing equipment
  • Agricultural machinery and equipment
  • Renewable energy – wind power turbine monitoring