How to Select a Phase and Voltage Monitoring Relay
How do you select the right phase and voltage monitoring relay? Match the relay's measured function — phase sequence, phase loss, asymmetry, over/undervoltage, or a combination — to what the load actually needs protected, per IEC 60947-5-1 control-circuit ratings. Pick the wrong function and the relay either misses the fault it was bought for or nuisance-trips on conditions the process can tolerate. This guide walks through function selection, auxiliary supply requirements, setting range, output contact logic, and module width, comparing Schneider Zelio Control and ABB CM options at each step.
Start With the Fault, Not the Part Number
Before opening a catalog, write down what actually damages the equipment. A three-phase motor running backwards because of reversed rotation is a phase-sequence problem, not a voltage problem — a plain undervoltage relay will not catch it. A motor that single-phases because one fuse blew needs phase-loss detection. A pump that cavitates on a sagging supply needs undervoltage with a trip delay long enough to ride through a 200 ms sag but short enough to catch a real brownout.
Most panel builders default to a three-phase supply control relay (Schneider RM17TE, ABB CM-MPS) because it bundles phase sequence, phase loss, asymmetry, and over/undervoltage in one 17.5-22.5 mm module. That covers the majority of motor-starter panels in one part. Specify a single-function relay only when the panel already has separate protection for the other faults, or when panel space is tighter than the budget for a combined unit.
Single-Function vs Multifunction: RM17/CM-ESS vs RM35/CM-MPN
Schneider's RM17 series and ABB's CM-ESS/CM-SRS cover one measured quantity per part — voltage only, current only. Schneider's RM35 series and ABB's higher CM-MPN models add adjustable asymmetry percentage, selectable fault memory, and sometimes true-RMS measurement instead of average. The multifunction models cost more per unit but replace two or three single-function relays plus the wiring between them.
What we see in the field: multifunction relays win on new panel designs where DIN-rail space is planned from scratch. Single-function relays win on retrofits, where an existing panel has a spare 17.5 mm slot and adding a wider multifunction unit means re-laying the whole rail. Single-function relays (RM17, CM-ESS) and compact multifunction relays (RM22, entry CM-MPN variants) run 17.5-22.5 mm wide on DIN rail; full multifunction units (RM35, higher CM-MPN) can run wider. Measure the actual remaining DIN-rail length in the panel before specifying — a relay that covers every function on the fact sheet is not a fit if it does not fit.
Auxiliary Supply or Self-Powered From the Measured Line
Some three-phase monitors, including ABB's CM-MPS, measure and power themselves directly from the three-phase lines they monitor — no auxiliary 24 VDC or 230 VAC feed required. Others, including most RM35 multifunction models and any relay that also needs to drive a latching output after the monitored supply has dropped, need a separate auxiliary supply. If the auxiliary supply shares a breaker with the load it monitors, a supply-side fault takes down the monitoring relay along with the fault indication it exists to raise.
For safety-relevant interlocks — blocking a motor start on wrong rotation, for example — specify fail-safe (normally-energized) output logic and, where the relay needs an auxiliary supply, route that supply from a point that cannot fail independently of the monitored circuit.
Setting Range: Threshold, Hysteresis and Trip Delay
Three numbers decide whether the relay protects the load or annoys the operator. The threshold is the setpoint itself. The hysteresis is the reset band — how far the measured value has to recover past the threshold before the output re-energizes, which stops the relay chattering when the supply sits right at the trip point. The trip delay is how long the fault condition has to persist before the output actually switches, which rides through momentary sags and starts-inrush without masking a real fault.
Formula: Voltage asymmetry — Source: IEC 60947-5-1, general measuring-relay convention
Asymmetry % = (max phase deviation from average / average phase voltage) x 100
| Symbol | Description | Unit |
|---|---|---|
| Max deviation | Largest difference between one phase voltage and the three-phase average | V |
| Average phase voltage | Mean of the three measured phase voltages | V |
| Asymmetry % | Result compared against the relay's adjustable asymmetry threshold | % |
A typical starting point for a general motor panel is a 5-10% asymmetry threshold with a 1-3 second trip delay — tight enough to catch a degrading connection before it cooks a winding, loose enough not to trip on normal supply imbalance from single-phase loads sharing the same feeder. Confirm against the specific relay's adjustable range rather than assuming a fixed number; ranges differ between the RM17, RM35, and CM families.
Output Contacts and Reset Mode
Most monitoring relays ship with one or two change-over (SPDT/DPDT) contacts. One contact typically drives the contactor coil or a PLC digital input directly; a second, where fitted, feeds an alarm horn or a SCADA point independent of the load-switching path. Decide reset mode at the same time as output logic: automatic reset re-energizes as soon as the measured value recovers past the hysteresis band, while manual (latched) reset requires an operator to acknowledge the fault before the output switches back.
Latched reset suits unattended equipment where an intermittent fault should not silently restart a motor — a borderline connection that trips, recovers, and re-trips every few minutes on automatic reset is a symptom worth investigating, not a feature to tune around.
Choosing Between Schneider Zelio Control and ABB CM
| Criteria | Schneider RM17 (single-function) | Schneider RM35 (multifunction) | ABB CM-MPS (three-phase, self-powered) |
|---|---|---|---|
| Measured functions | One per part (voltage or current) | Sequence, loss, asymmetry, over/under in one unit | Sequence, loss, asymmetry, over/under in one unit |
| Auxiliary supply | Usually required | Usually required | Not required — self-powered from monitored lines |
| Typical width | 17.5 mm | 22.5 mm or wider | 22.5 mm |
| Measurement method | Average | True-RMS on higher models | True/apparent on higher models |
| Best fit | Retrofit, spare slot, one function missing | New panel, full three-phase supply protection plus asymmetry | New panel where an auxiliary supply feed is inconvenient to route |
Both families cover the same measured quantities to a comparable standard; the decision usually comes down to whether the panel already has a clean auxiliary supply routed to the DIN rail (favors either), and whether the design is a retrofit into limited space (favors single-function) or a new build with room for one combined unit (favors multifunction).
Cross-Check Against Adjacent Protection
A phase/voltage monitoring relay is not a substitute for a thermal overload relay or a motor protection circuit breaker — it catches supply-side faults (wrong rotation, missing phase, sagging voltage) while the overload device catches load-side faults (locked rotor, sustained overcurrent). On motor circuits, specify both: see our guide on overload relay phase loss and single-phasing protection and on MPCB phase loss and single-phasing protection for where those overlap and where they do not. If the application also needs current or level monitoring rather than voltage, see our breakdown of phase sequence and phase failure relays and the wider three-phase voltage monitoring relay guide for the asymmetry and sequence detail behind the numbers above. Once the relay is selected, our companion article on how to set a voltage monitoring relay covers commissioning the threshold, hysteresis, and delay in the panel.
For the full function taxonomy across voltage, current, phase, level, and temperature, start from the monitoring relay engineering guide. Stock covers both families across the monitoring and control relays collection, alongside the contactors they typically drive.
Frequently Asked Questions
Do I need a phase-sequence relay if my motor already has a phase-loss overload relay?
Yes, if wrong rotation direction on power-up is a risk — a phase-loss overload relay catches a missing phase during run but does not check rotation order before start. A combined three-phase supply relay (RM17TE, CM-MPS) checks both in one device.
Can a monitoring relay replace a motor protection circuit breaker?
No. A monitoring relay's output contact switches a control circuit, typically a contactor coil; it does not interrupt fault current itself. An MPCB or contactor plus overload relay still handles the actual power switching and short-circuit protection.
What trip delay should I set for voltage sag ride-through?
There is no universal number — it depends on how the connected load reacts to a sag and how noisy the local supply is. Start in the 1-3 second range for general motor panels and lengthen it only if nuisance trips persist after confirming the hysteresis is set correctly.
Why does my three-phase relay not need an auxiliary supply but my single-phase one does?
Some three-phase relays, like ABB's CM-MPS, derive their own operating power from the three-phase lines they measure. Single-phase and current-sensing relays generally cannot do this reliably at low voltages or low currents, so they draw from a separate auxiliary feed.
Is a wider multifunction relay always the better choice?
Not if DIN-rail space is the constraint. A multifunction relay saves wiring and setup time on a new panel, but retrofitting one into a panel with only a 17.5 mm gap left often costs more in re-layout than adding one single-function relay.
Should the output be fail-safe (normally-energized) or normally-de-energized?
For anything safety-relevant — blocking a start on wrong rotation, for example — use fail-safe (normally-energized) so a lost auxiliary supply also produces a trip signal instead of silently disabling protection.
Conclusion
Selecting a phase and voltage monitoring relay comes down to five decisions in order: identify the actual fault mode, decide single-function versus multifunction, confirm auxiliary supply availability, set threshold/hysteresis/delay to the application's real tolerance for transients, and check the output logic and module width fit the panel. Get the first two decisions right and the setting range is a tuning exercise, not a redesign.
