Overvoltage and Undervoltage Monitoring Relays

What is an overvoltage and undervoltage monitoring relay? It is a measuring relay that continuously compares a single- or three-phase supply voltage against one or two adjustable thresholds — typically 80-90% of nominal for undervoltage and 105-120% for overvoltage — and switches an SPDT or DPDT output contact when the voltage leaves that window, per IEC 60255 and IEC 60947-5-1. Miss the drop and a contactor coil chatters, a VFD trips on a DC-bus undervoltage fault, or a motor draws excess current trying to hold torque at low volts; miss the rise and insulation ages faster, lamps fail early, and electronic loads see stress they were not rated for. This article covers how the relay measures and compares the voltage, how threshold, hysteresis and trip delay interact, single-phase versus three-phase versions, output wiring and reset logic, typical applications, and how to pick the right unit for a panel.

How the Relay Detects Over- and Undervoltage

The relay taps the monitored line through an internal voltage divider or a small isolation transformer, rectifies or samples the waveform, and compares the resulting DC level to a reference set by the front-panel dial or DIP switches. Basic units average the rectified sine wave. Higher-end models in our monitoring and control relays range, such as the Schneider RM35 series and ABB CM-UFD/CM-UFS, sample true RMS instead — that matters when the supply carries harmonics from VFDs or switch-mode loads, because an averaging relay reads a distorted waveform incorrectly and can trip, or fail to trip, at the wrong voltage.

Some three-phase versions derive their own operating power from the monitored lines and need no separate auxiliary supply; the Schneider RM17TE and ABB CM-MPS families work this way. Single-phase voltage relays such as the RM17UB usually take a separate control-voltage input, which matters when you are laying out the panel's power distribution.

Threshold, Hysteresis and Trip Delay Settings

Three settings define how the relay behaves at the edge of its window. The threshold is the voltage at which the fault condition starts — for undervoltage protection this typically sits at 80-90% of nominal, for overvoltage 105-120%. Hysteresis is the gap between the trip point and the reset point, and it exists because a supply hovering exactly at the threshold would otherwise make the output relay chatter on and off dozens of times a minute.

For overvoltage the sign flips: V_reset = V_trip x (1 - h). Trip delay is separate from hysteresis — it is a timer, not a voltage band. A 0.1-1 s delay rides through motor starting sag or a brief utility transient without tripping; a longer delay, up to tens of seconds on some ABB CM units, tolerates a generator's voltage recovery after a load step. Set the delay too short and the relay nuisance-trips on every motor start nearby. Set it too long and it stops protecting against the fault it was bought for.

What we see in the field: installers often leave the factory-default hysteresis, usually 2-3%, on a supply that sags 5-8% during motor starts elsewhere in the building, then blame the relay for nuisance tripping. The fix is a wider hysteresis and a short delay, not a looser threshold.

Single-Phase vs Three-Phase Voltage Monitoring

Single-phase relays, such as the Schneider RM17UB or ABB CM-ESS voltage variants, monitor one line-to-line or line-to-neutral voltage against over and under thresholds. They fit control-transformer secondaries, single-phase pump circuits, and lighting feeders.

Three-phase relays add window monitoring on all three lines simultaneously plus, on most models, phase sequence and phase loss detection in the same housing — see our three-phase voltage monitoring relay article and the dedicated phase sequence and phase failure relay guide for that side of the function. A three-phase undervoltage relay on a motor branch catches a single failed phase before it becomes a single-phasing event that burns a winding — a different failure mode from the level and hysteresis settings covered above, but the same measuring principle.

Wiring and Output Logic

The output is a change-over contact, not a load-carrying switch — it drives a contactor coil, a PLC digital input, or an alarm horn, never the motor or the panel main circuit directly. Two logic choices matter at commissioning: normally-energized (fail-safe) versus normally-de-energized, and automatic versus manual (latched) reset.

Fail-safe wiring means the relay's output coil is energized in the healthy state and drops out on fault or on loss of its own supply, so a dead relay reads the same as a fault, which is what you want on a critical circuit. Manual reset forces an operator to acknowledge the fault at the panel before restart, useful where an unattended automatic restart could be dangerous. Auto-reset suits nuisance-prone supplies where downtime matters more than an operator walking to the panel every time voltage dips for two seconds.

Where Overvoltage and Undervoltage Protection Is Used

Motor branch circuits use undervoltage protection to stop a motor drawing excess current at reduced torque before the thermal overload relay has to intervene — see the thermal overload relay engineering guide for how the two protections divide the work. The voltage relay catches the supply-side fault; the thermal relay catches the resulting overcurrent if it slips through. VFD input circuits use undervoltage monitoring ahead of the drive to avoid a DC-bus undervoltage fault mid-run. Generator and standby-power circuits use overvoltage protection to catch a runaway AVR before it damages connected loads. Sensitive electronics and lighting circuits use narrow-window monitoring, where both a sag and a surge matter equally.

Selecting the Right Relay for the Application

Start with the number of phases and whether the relay needs its own auxiliary supply — a self-powered three-phase unit like the RM17TE simplifies panel wiring when there is no spare control transformer. Confirm the threshold range covers your actual supply tolerance, not just nominal voltage; a rural feeder that regularly sags 12% needs a wider adjustable range than a stiff urban supply. Check whether true-RMS measurement matters — any circuit downstream of a VFD, UPS, or large switch-mode power supply carries harmonics that an averaging relay misreads. Match the output contact rating and count to what you are actually switching: one contactor coil needs one change-over contact, a coil plus a PLC input plus an alarm needs two.

Our guide to selecting a phase and voltage monitoring relay walks through this in more depth, and the companion how to set a voltage monitoring relay article covers the commissioning steps once the unit is chosen. For the broader function set — level, temperature, and current monitoring alongside voltage — see the monitoring relay engineering guide.

This depends on how noisy the supply actually is: a panel fed from a dedicated transformer with light loading rarely needs true-RMS sensing, but a panel sharing a feeder with several VFDs almost always does.

Conclusion

Overvoltage and undervoltage monitoring relays do one job well: watch a threshold, apply hysteresis so the output does not chatter, and hold off tripping long enough to ride through a transient but not so long that the fault does damage first. Getting the threshold, hysteresis, and delay right for the actual site supply, not just the nameplate voltage, is what separates a relay that protects equipment from one that gets bypassed after the third nuisance trip. Pair it with the right output logic — fail-safe, auto or manual reset — for the duty, and it becomes one of the cheapest insurance policies in the panel.