How to Set a Voltage Monitoring Relay: Thresholds and Delay

How do you set a voltage monitoring relay? You configure three interdependent values on the dial or DIP switches: the trip threshold (the overvoltage or undervoltage limit, in volts or percent of nominal), the hysteresis (the reset band around that threshold, typically a few percent, per IEC 60255 general measuring-relay practice), and the trip delay (the on-fault time delay, commonly 0.1-10 s). Get any one of the three wrong and the relay either chatters on and off near the threshold or lets a real undervoltage ride through long enough to stall a motor. This guide covers reading the protected load's actual voltage tolerance before touching a dial, setting over- and undervoltage thresholds correctly, sizing hysteresis against how noisy the supply actually is, choosing a trip delay that filters transients without masking faults, and the extra asymmetry and phase-loss settings that come with three-phase units.

Start With the Load, Not the Relay

The threshold isn't a number you pick off a chart. It comes from the equipment the relay protects. A motor nameplate typically states a voltage tolerance of ±10% of rated voltage per IEC 60034-1; a VFD input stage may tolerate less on the low side before it trips on undervoltage internally anyway. If the relay's threshold is looser than the equipment it guards, the equipment trips first and the monitoring relay never earns its keep. If the threshold is tighter than necessary, the installation nuisance-trips on ordinary grid sag that the load would have ridden through fine.

Write the target window down before opening the relay's cover: nominal voltage, acceptable +/- band, and whether over, under, or both matter for that circuit. A submersible pump feeder usually cares most about undervoltage and phase loss; a sensitive control panel supply may care about both directions equally.

Setting the Overvoltage and Undervoltage Thresholds

Most Schneider Zelio Control RM17/RM35 and ABB CM-series voltage monitors set the threshold as a percentage of nominal supply voltage (commonly adjustable from roughly 70% to 120% of Un, depending on the model and function), read via a labeled dial, DIP-switch bank, or a digital display on higher-end units. Set overvoltage and undervoltage independently if the relay supports window mode; some single-function units only monitor one direction, so confirm the model's function before wiring, not after.

Set the threshold to the equipment's actual limit, not a round number that happens to be convenient. 207 V line-neutral protects a 230 V ±10% load correctly; 200 V is not the same setting and leaves the load exposed longer before the relay reacts.

Hysteresis: Why the Reset Point Isn't the Trip Point

A relay that resets at the exact same voltage it trips at will chatter — energize, drop just below threshold, trip, recover just above, re-energize, repeat — every time the supply sits near the setpoint. Hysteresis moves the reset point away from the trip point so the output stays in one state until the supply has clearly recovered (or clearly failed), not just crossed a hairline.

For an overvoltage function the same logic runs the other way: V_reset = V_trip × (1 + h), so the relay waits for the voltage to drop back below trip level by the hysteresis margin before re-energizing. Narrow hysteresis gives tighter protection but tolerates less supply noise before it starts chattering; wide hysteresis rides through a noisier supply but lets the voltage drift further from nominal before the relay resets.

What we see in the field: installations on a stiff, clean utility feed can run hysteresis near the low end of the adjustable range without issue. Sites downstream of welders, VFDs, or large motor starts need it opened up, or the output relay clatters every time a neighboring load starts.

Trip Delay: Filtering Transients Without Hiding Real Faults

The trip delay is a timer that has to elapse continuously in the fault condition before the output changes state. Its job is to let the relay ignore a voltage dip caused by a large motor starting nearby, a brief utility switching event, or an inrush transient — none of which represent a sustained supply problem worth stopping the line for.

Set it too short and every DOL motor start on the same feeder trips the monitoring relay. Set it too long and a genuine sustained undervoltage — a blown fuse on one phase, a failing contactor, a loose terminal heating up and dropping voltage — runs unprotected for that whole window. A pump motor single-phasing for 8 seconds at a 5-second delay setting is 8 seconds closer to a burned winding than it needs to be.

A practical starting point: set the delay just longer than the worst-case starting transient of the largest motor sharing that supply, then verify with an actual start under load rather than trusting the datasheet number alone. This depends heavily on what else is on the same feeder — a soft starter produces a very different voltage dip profile than an across-the-line start, so the same delay setting that works on one panel can be wrong on the next.

Asymmetry and Phase Loss on Three-Phase Units

Three-phase voltage monitors such as the Schneider RM17TE or an ABB CM-MPS add asymmetry detection on top of over/undervoltage: they compare the three phase voltages to each other, not just to a fixed threshold, and trip when the deviation between phases exceeds the set percentage.

Typical asymmetry thresholds sit in a low single-digit-to-teens percentage range depending on the model and application; motors are more sensitive to unbalance than resistive loads, since a few percent voltage unbalance produces a much larger current unbalance in the windings. Set asymmetry tighter for motor-heavy panels than for general lighting or control supplies.

Phase loss and phase sequence are usually built into the same three-phase unit and don't need separate threshold tuning — they're detection functions, not adjustable setpoints. Confirm the relay is wired to see all three phases correctly during commissioning, not after a rotation fault burns a pump. Related setup detail: see our guide on phase sequence and phase failure relays for what these functions actually detect versus what voltage monitoring covers.

Output Logic and Reset Mode

Before commissioning, confirm two settings that aren't thresholds at all but change what the relay does on a fault: normally-energized (fail-safe — the output de-energizes on fault or on loss of auxiliary supply) versus normally-de-energized, and automatic versus manual (latched) reset.

Fail-safe wiring means a tripped relay, or one that has simply lost its own power, drops the downstream contactor — this is the safer default for most protection duties. Manual reset forces someone to acknowledge the fault at the panel before the circuit re-energizes, which is worth specifying on unattended equipment where an automatic restart after an undervoltage event could restart a machine into an unsafe state.

Commissioning: Verify the Settings, Don't Just Trust Them

A voltage monitoring relay set on the bench with a variac and never checked on the actual supply is a guess, not a commissioned setting. Where practical, verify the threshold with a controlled voltage source, confirm the delay timing with a stopwatch against the actual value on the label, and watch the output contact on a start of the largest motor on that feeder to confirm hysteresis and delay together don't nuisance-trip under real conditions.

Document the final settings on the panel drawing or a label inside the enclosure. Relays get swapped during maintenance, and the replacement unit needs the same threshold, hysteresis, and delay values — not the factory defaults — or the protection quietly changes on the next service visit.

For the broader selection decision — which function and range to specify before you're at the setting stage — see how to select a phase and voltage monitoring relay. For the underlying mechanics of how these devices sense and switch, see how a monitoring relay works, and for the broader three-phase function set see our guide to three-phase voltage monitoring. Stock for both Schneider Zelio Control and ABB CM-series monitoring and control relays is available for panels that also carry motor protection circuit breakers and contactors on the same feeder. See the full monitoring relay engineering guide for how threshold, hysteresis, and delay fit into relay selection generally.

Conclusion

Setting a voltage monitoring relay is three decisions, not one: where the threshold sits relative to the load's actual tolerance, how much hysteresis margin the supply's noise level requires, and how long a delay filters out normal transients without hiding a real fault. Get the threshold from the equipment nameplate, size hysteresis and delay from the supply's actual behavior rather than the factory default, and verify all three together during commissioning with the largest motor on the feeder running. The setting, once verified, belongs on a label inside the panel — not just in the technician's memory.