PLMU Series – Voltage Monitor Series - Littelfuse PLMU Voltage Monitoring Relays Provide Superior Protection

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The PLMU (PLMU11) series is a three-phase voltage monitor that continuously measures the voltage of each of the three phases to provide protection for motors and sensitive loads. Its proprietary microcontroller-based circuitry senses under and overvoltage, voltage unbalance, phase loss, and phase reversal. Protection is provided even when regenerated voltages are present. Universal voltage operation and standard base connection allows the PLMU11 to replace hundreds of competitive part numbers.


Upon application of power, a 0.6 s random start delay begins, and the PLMU11 measures the voltage levels and line frequency and then selects the voltage range. The output relay is energized and the LED glows green when all voltages are acceptable and the phase sequence is correct. LED flashes green during trip delay and glows red when output de-energizes. Undervoltage, overvoltage, and voltage unbalance must be sensed for continuous trip delay before the relay de-energizes. Re-energization is automatic upon fault correction. The output relay will not energize if a fault condition is sensed as three-phase input voltage is applied. The LED alternately flashes red/green when phase reversal is sensed. Line voltage is selected with the knob, setting the over and undervoltage trip points. The voltage range is automatically selected by the microcontroller.

LED Indicator

Steady green = Energized
Steady red = De-energized (tripped on fault)
Flashing green = Trip delay
Alternate flashing red/green = Phase reversal


  • Proprietary microcontroller-based circuitry
  • Quick octal 8-pin mounting
  • LED diagnostics
  • Isolated, 10 A, SPDT output contacts
  • Simple three-wire connection for delta or wye systems – no connection to neutral
  • ASME A17.1 Rule 210.6
  • NEMA MG1 14:30, 14:35
  • IEEE C62.41-1991 Level B

Access specifications, certifications, check availability and order parts below

Catalog #Input
Voltage (V)
Termination TypeMounting MethodDimensionsWeight (lbs/g)Input AC Line Frequency (Hz)Input Phase SequenceInput Tolerance (%)Input TypeInput Voltage Maximum (V)Output Load Rating (VA)Output TypeOvervoltage Reset Voltage (%)Overvoltage Trip Voltage (%)StockSamples
PLMU11200 - 480 VacIsolated SPDTOctal 8-pin plug-inPlug-in socket rated 600V3.03 H x 2.39 W x 1.78 D in. (77 x 60.7 x 45.2 mm)0 Kg50 / 60 ±2; 200 - 240 V ac: 50 / 60; 340 - 420 V ac: 50; 400 - 480 V ac: 60A; B; C±15%3-phase delta or WYE with no connection to neutral55210 A resistive @ 240 Vac; 1/4 hp @ 125 Vac; 1/3 hp @ 250 Vac; 277 Vac maxEnergized when voltages are acceptable-2% of the trip voltage109 - 113% of the adjusted line voltageCheckOrder
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