Using Voltage Monitor Relays as Under or Overvoltage Relays or Both

Monday: August 8, 2016 : Voltage Monitor Relays


How do you use the Voltage Monitor Relay as an overvoltage relay? Undervoltage relay? Or Both?


Voltage Monitor Relays can be used as either overvoltage or undervoltage relays, depending on the output contact used:

Overvoltage Relay

Provides protection to equipment that cannot handle excess voltages.  Uses a normally closed contact (N.C.).  As long as the monitored voltage remains below the maximum voltage the equipment can withstand (Pick-Up Setting), the relay remains de-energized and the N.C. contact remains closed, keeping the load energized.  If the operating voltage increases beyond the maximum rating of the equipment, the relay energizes and the N.C. contact opens, turning off the load.  When the voltage falls below the Drop-Out Setting (hysteresis), the relay de-energizes and the N.C. contact re-closes, turning on the load.

Undervoltage Relay

Provides protection to equipment that is required to operate above a certain minimum voltage.  Uses a normally open contact (N.O.).  As long as the monitored voltage is above the minimum value required (Pick-Up Setting), the relay will energize and the N.O. contact closes, turning on the load.  If the voltage drops below the Drop-out Setting (the minimum voltage required minus the hysteresis), the relay will de-energize and the N.O. contact will re-open, turning off the load.

Over & Under Voltage Relay (Voltage Band)

Voltage Band Relays provide protection to equipment that is required to operate within an upper and lower voltage limit. As long as the operating voltage remains within an OVER & UNDER voltage range, the internal relay stays energized. If the operating voltage falls outside this range, the relay will drop-out.

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