Voltage Monitoring Relay: Under/Over Voltage and Phase Protection
Indian industrial sites often deal with unstable supply: voltage dips when large loads start, high voltage at night, unbalanced phases on rural feeders, and phases lost to blown fuses. A voltage monitoring relay watches the supply and disconnects equipment, or blocks it from starting, when the supply goes out of limits.
What a voltage monitoring relay checks
- Under-voltage: low voltage makes motors draw more current and overheat, and contactors chatter.
- Over-voltage: stresses insulation, electronics, transformers and lighting.
- Phase loss: one phase missing on the incoming supply.
- Phase unbalance: unequal phase voltages cause much larger current unbalance and heating in motors.
- Phase sequence: wrong rotation after supply or cable changes.
Most relays allow adjustable thresholds and a time delay so that short dips do not cause trips.
Where to use one
| Location | Purpose |
|---|---|
| Panel or MCC incoming | Stop all motors starting on a bad supply; trip on sustained low or high voltage |
| Generator / mains changeover | Confirm the supply is healthy before transfer |
| Pump houses on rural feeders | Protect against low voltage and unbalance common on long lines |
| Sensitive equipment (UPS, drives, HVAC) | Disconnect before over-voltage causes damage |
| Data centres and utilities | Supply quality monitoring and alarms |
Voltage monitoring vs motor protection relay
The two complement each other:
- A voltage monitoring relay looks at the supply, usually at the incoming. It can prevent starting on a bad supply, but it cannot see what happens in each motor feeder.
- A motor protection relay such as the EOCR 3DE / 3EZ looks at the current in each motor. It catches overload, phase loss anywhere in the feeder, current unbalance, locked rotor, dry run and earth fault.
When voltage is unbalanced, the current unbalance in a motor is several times larger, so a current-based relay with unbalance protection directly protects the motor from the effect of a poor supply. Read more in phase unbalance protection and phase failure relay vs single phasing preventer.
Setting tips
- Set the under-voltage threshold above the level at which your largest motor would stall or contactors drop out.
- Set the over-voltage threshold below the rating of your most sensitive equipment.
- Use a time delay of a few seconds to ride through motor starting dips.
- Decide whether reset is automatic (unattended pumps) or manual (where restart needs a check).
- Wire an alarm contact to the SCADA or PLC so supply problems are logged.
Need help with supply protection?
Concord can advise on supply monitoring and motor protection together, so that the panel incoming and every motor feeder are covered. Share your supply conditions and motor list with our team.
Frequently Asked Questions
What does a voltage monitoring relay do?
It monitors supply voltage and trips or blocks equipment on under-voltage, over-voltage, phase loss, unbalance or wrong phase sequence.
Do I need a voltage monitoring relay if I have a motor protection relay?
They do different jobs. A voltage relay at the incoming prevents starting on a bad supply; a motor protection relay protects each motor based on its actual current. Many panels use both.
Why is phase unbalance harmful to motors?
A small voltage unbalance causes a much larger current unbalance, which overheats the windings. Current-based unbalance protection, as on the EOCR 3DE, trips before damage occurs.
Talk to Concord
Concord Automation & Controls has supplied motor protection relays and industrial relays to Indian industry since 1996 and is the exclusive ComatReleco (Switzerland) distributor for India, Bangladesh and Mauritius. Share your motor, pump or panel details and we will recommend the right model: call or WhatsApp +91 96670 40511, email contact@concordrelay.com, or use our contact page.