The VMD258 is an undervoltage and overvoltage relay for monitoring three-phase AC systems (window discriminator function). It features pure analog technology without a microprocessor, making it particularly suitable for critical applications with high safety and reliability requirements, such as power plants and nuclear facilities.
The device is self-powered directly from the monitored system, requiring no separate auxiliary supply. The power supply for the electronics, the relays and the connection of an external energy storage device are isolated from the system by double insulation. Special input transformers effectively attenuate interference from the monitored system. No neutral connection is required, making the device suitable for all 3AC systems.
This model features screw‑type terminals and is identified by order number B93010067.
Technical Specifications
· Rated system voltage (Uₙ): 3AC 690 V, 45…66 Hz
· Operating range: 0.5…1.5 × Uₙ
· Setting range: Undervoltage 0.7…0.95 × Uₙ; Overvoltage 1.05…1.3 × Uₙ
· Response delay: 0…5 s (adjustable)
· Response time: Approx. 100 ms (reset time after voltage returns to the set range)
· Alarm outputs: Two independent changeover contacts each for undervoltage and overvoltage
· Operating mode: Undervoltage relay N/C operation; overvoltage relay N/O operation
· Mounting: 35 mm DIN rail
· Protection class: IP30 (internal components) / IP20 (terminals)
· Operating temperature: –20 °C … +70 °C
· Weight: Approx. 825 g
Key Features
· Undervoltage and overvoltage monitoring: Continuously monitors three-phase AC system voltage; triggers alarm when voltage exceeds preset window values
· Pure analog technology: No microprocessor design, eliminating software-related failures and providing higher reliability in critical safety applications
· Self-powered design: No external auxiliary supply required; powered directly from the monitored system
· Independent alarm outputs: Separate alarm relays for undervoltage and overvoltage, allowing easy fault type identification
· Continuously adjustable parameters: Response values and response delay can be adjusted continuously to suit different applications
· Optional external energy storage: ES258 optional, provides at least 5 seconds bridging time during power failure
Typical Applications
· Voltage protection in critical infrastructure such as power plants and nuclear facilities
· Power supply monitoring for machinery and electrical installations
· Monitoring of standby and emergency power supply systems
· Power supply voltage monitoring for portable loads
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