Voltage Monitoring Relays
250A Series
3-Phase Voltage/Phase Monitor
Description
The 250A series is a three-phase, auto-ranging, dual-range voltage monitor that protects 190–480 V ac, 50*/60 Hz motors
regardless of their size. This monitor offers protection from low- and high-voltage, voltage unbalance, single-phase, and
phase reversal. The 250A provides a user-selectable nominal voltage setpoint and will automatically select between the
200 V and 400 V range. This monitor also features adjustable or manual restart delay.
This unique microcontroller-based voltage and phase-sensing device constantly monitors the three-phase voltages to
detect harmful power line conditions. When a harmful condition is detected, the output relay is deactivated after a
specified trip delay. The output relay reactivates after power line conditions return to acceptable levels. The 250A monitor
includes advanced, single LED diagnostics. Five different light patterns distinguish between faults and normal conditions.
LED indications include normal operation, power-up restart delay, reverse-phase trip, unbalance/single-phase trip, and high/
low voltage trip.
*Note: 50 Hz will increase all delay timers by 20%.
Features & Benefits
FEATURES
BENEFITS
Constant monitoring of single-phase, low-voltage, voltage unbalance,
phase reversal, harmful power line conditions
Proprietary microcontroller-based circuitry
Automatically senses system voltage between 190–480 V ac.
Saves setup time
Auto-sensing wide voltage range
Advanced LED diagnostics
Adjustable trip delay
Quick visual indicator for cause of trip
Prevents nuisance tripping due to rapidly fluctuating power
line conditions
DPDT relay output
Manual reset
Allows for versatility to meet wide application needs
Allows for inspection of equipment before system is re-energized
Applications
• Fan motors
• Air conditioners
• Compressors
• Heat, well, and sump pumps
• Small conveyer motors
Littelfuse.com/250A
©2023 Littelfuse, Inc.
Specifications are subject to change without notice.
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Revised: 07/14/2023