Working Principle of Undervoltage (27) and Overvoltage (59) Relays


Power system Under-voltages may occur due to system faults, an increase in system loading, loss of an incoming transformer etc.

Where a transformer is supplying 3-phase motors a significant voltage drop e.g. to below 80% may cause the motors to stall. If it is not tripped by protective relay causes they will draw higher currents at reduced voltage levels. This leads to excessive heating of the coils and insulation will get damaged.


Power system Over-voltages may occur due to sudden loss of load, a tap changer run-away condition occurs in the high voltage direction, Generator AVR equipment malfunctions or Reactive compensation control malfunctions.

System over-voltages can damage component insulation.

Under Voltage Relay

U/V O/V Relay

Working of Undervoltage Relay:

It 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.

Working of Overvoltage Relay:

It 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.

Example settings of Under/Over Voltage Relay:

Pick-up: Adjustable from 85-115% of nominal voltage

Drop-out: Adjustable from 80-95% of pick-up setting

A time delay is required to ensure voltage fluctuations due to remote system faults do not result in an unnecessary disconnection of loads.

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