Types of Monitoring Relays: Voltage, Current, Phase, Level, Temperature
What types of monitoring relays exist? Monitoring relays split by the quantity they measure — phase sequence and phase loss, voltage, current, liquid level, and temperature — each comparing that quantity to an adjustable threshold per IEC 60255 and switching an SPDT or DPDT output when the value leaves the set window. Pick the wrong type for the application and the panel either misses the fault it was meant to catch or nuisance-trips on transients that should ride through. This guide covers the five measurement types, how their sensing methods differ (shunt vs. CT, conductive probes, PTC vs. PT100), and where single-function and multifunction versions fit in a panel.
How Monitoring Relays Are Classified
Three things separate one monitoring relay from another: what it measures, how the output behaves on a fault, and how many functions live in one housing. The measured quantity is the first split — phase, voltage, current, level, or temperature, as covered below. Output logic is the second: normally-energized (fail-safe) relays de-energize their output on a fault or on loss of the auxiliary supply, so a blown fuse in the control circuit also drops the load out safely; normally-de-energized types only switch on when a fault is present. Reset mode is the third variable — automatic reset clears the moment the measured value returns inside the window, manual (latched) reset holds the trip until an operator acknowledges it, which matters when a nuisance restart would be worse than a shutdown. For the underlying mechanics of threshold, hysteresis, and delay, see how a monitoring relay works.
Phase Monitoring Relays: Sequence, Loss, and Asymmetry
Three-phase supply relays check phase presence, rotation (L1-L2-L3 sequence), phase loss, and usually voltage asymmetry in one device. Wrong rotation on a motor means it runs backwards — a pump loses prime, a fan reverses airflow, a conveyor runs into the wrong end of the line. These relays sit ahead of the starter and block the contactor coil until sequence and presence check out. Series such as ABB's CM-MPS and Schneider's RM17TE handle this function; some, like CM-MPS, draw their own power from the three-phase lines and need no separate auxiliary supply, which simplifies wiring in a retrofit panel. See phase sequence and phase failure relays for the trip logic in detail, and three-phase voltage monitoring for how asymmetry is measured alongside sequence.
Voltage Monitoring Relays: Over, Under, and Window Detection
Voltage types monitor single- or three-phase supply for overvoltage, undervoltage, or both at once (window detection), with an adjustable threshold and hysteresis on each limit. Undervoltage protects motors from running with reduced torque and overheating at the same load; overvoltage protects sensitive control electronics and can indicate a lost neutral on a three-phase distribution board. The trip delay is what keeps the relay from tripping on a voltage sag caused by a large motor starting elsewhere on the same feeder — set it too short and the panel nuisance-trips weekly; set it too long and it stops catching real sustained faults. What we see in the field: most callouts for a "faulty" voltage relay turn out to be a delay set for lab conditions, not the actual supply noise on that site.
Formula: Reset Threshold — Source: IEC 60255-1, hysteresis definition
Vreset = Vtrip × (1 − H)
| Symbol | Description | Unit |
|---|---|---|
| Vtrip | Set trip threshold | V |
| Vreset | Reset (dropout) threshold | V |
| H | Hysteresis, expressed as a fraction (e.g. 0.05 for 5%) | ratio |
For step-by-step setpoint procedure, see how to set a voltage monitoring relay.
Current Monitoring Relays: Overcurrent and Undercurrent
Current types measure via a built-in shunt for small loads or an external CT for larger ones, and trip on overcurrent, undercurrent, or both. Undercurrent catches a broken conveyor belt, a dry-running pump, or a load that has simply come loose — the motor keeps spinning but does no work. Overcurrent catches a jam: the load stalls but the winding keeps drawing current until something fails thermally. Neither function replaces a thermal overload relay sized to the motor's nameplate current; a current monitoring relay adds fault-specific detection on top, not instead of. Browse the full monitoring and control relays range for shunt and CT variants stocked by function.
Level Monitoring Relays: Conductive Sensing
Level relays measure liquid presence between two or more probe electrodes lowered into a tank or wet well. When the liquid bridges a probe pair, the relay's input impedance drops below an adjustable sensitivity threshold (set in kOhm for the liquid's conductivity) and the output switches — typically wired for pump-up, pump-down, or dry-run protection on a submersible pump. Conductive sensing has no moving parts, which matters in tanks with turbulence, foam, or solids that would jam a float switch; it does not work in non-conductive liquids such as oils. See liquid level monitoring relays for probe placement and sensitivity setting.
Temperature Monitoring Relays: PTC Thermistor and PT100/PT1000
Two sensing methods cover motor and process temperature. A PTC thermistor relay reads a sensor embedded in the motor winding and trips when its resistance jumps at the reference value defined in IEC 60947-8 — the response is closer to a threshold switch than a graduated reading. A PT100 or PT1000 relay reads an actual resistance-to-temperature curve, so it can display the temperature and switch at a chosen set point rather than one fixed trip level, and often supports two independent thresholds (alarm and trip). PTC protection reads the winding directly, where an overload relay estimates heating from current draw and ambient compensation — that's the practical difference from thermal overload relay protection, and why some motor circuits use both. For phase loss interactions with thermal protection, see phase loss and single-phasing protection; for the thermistor-specific wiring, see thermistor motor protection.
Single-Function vs. Multifunction: Which Type to Specify
Single-function relays — one quantity, one module, typically 17.5 mm wide — cost less and are simple to swap when only one function is needed, such as a stand-alone phase-sequence relay ahead of a single starter. Compact multifunction types combine two or three related functions (phase sequence plus over/undervoltage plus asymmetry, for instance) in one 22.5 mm module, which saves panel space on a standard motor feeder. Higher multifunction models add selectable fault memory, true-RMS measurement instead of average, and sometimes a fourth or fifth combined function. The trade-off: replacing a multifunction relay for one failed function means replacing all of them, and commissioning has more parameters to set correctly.
| Criteria | Single-Function (e.g. RM17TE, CM-MPS) | Compact Multifunction (e.g. RM22) | Multifunction (RM35, higher CM models) |
|---|---|---|---|
| Functions per module | 1 (phase, voltage, or current) | 2-3 combined | Up to 4-5 combined |
| Module width | 17.5 mm | 22.5 mm | 22.5 mm |
| Auxiliary supply | Often none (self-powered from measured line on some, e.g. CM-MPS) | Typically required | Typically required |
| Measurement method | Average | Average | True-RMS on higher models |
| Fault memory / latching | Rarely offered | Selectable on some models | Selectable |
Both Schneider Zelio Control and ABB CM cover the same measurement types across this range; the choice usually comes down to panel space, whether an auxiliary supply is already available, and how many spare parts the maintenance team wants to stock. For a step-by-step method, see how to select a phase and voltage monitoring relay. In the same motor circuit, monitoring relays typically sit alongside motor protection circuit breakers and contactors — see the contactor selection checklist and MPCB phase loss protection for how these devices divide the protection job between them.
Frequently Asked Questions
What's the difference between a monitoring relay and a protection relay?
A monitoring relay watches one or a few electrical or physical quantities against adjustable thresholds and drives a low-power output contact; it does not interrupt the main current itself. A protection relay, in the IEC 60255 protection class, typically combines several protection functions with time-current curves and is built to signal a breaker trip directly on a fault.
Can a single relay monitor both voltage and current?
Yes. Multifunction models such as the Schneider RM35TF or higher ABB CM units combine phase sequence, voltage, and in some models current in one 22.5 mm housing. Single-function types, one quantity per module, cost less and are simpler to replace individually when only one function is needed.
Do phase-sequence relays need an auxiliary supply?
Some do not. Types such as ABB's CM-MPS draw power from the three-phase lines they measure, so wiring is simpler and the relay keeps working even if a separate control supply fails. Other models in the same family need an auxiliary supply, so check the datasheet before wiring a replacement.
How do I choose between conductive level sensing and a float switch?
Conductive sensing has no moving parts, so it holds up in tanks with turbulence, foam, or solids that would jam a float. It needs a conductive liquid and an adjustable sensitivity setting matched to the fluid's resistivity — non-conductive liquids such as oils rule it out.
What's the difference between PTC and PT100 temperature monitoring?
A PTC thermistor relay reads a near-binary threshold: sensor resistance jumps sharply at the reference temperature defined in IEC 60947-8, so the relay detects "over threshold" rather than a graduated value. A PT100/PT1000 relay reads an actual resistance-to-temperature curve, so it can display and switch at a chosen set point instead of one fixed trip level.
Conclusion
Matching the relay type to the fault you actually need to catch — rotation and phase loss, voltage window, over/undercurrent, level, or winding temperature — matters more than any single feature list. Start from the failure mode, not the catalog page: a motor that runs backwards needs a phase-sequence relay, not a voltage monitor; a dry-running pump needs level or undercurrent sensing, not a thermal overload relay alone. For the underlying threshold, hysteresis, and delay mechanics shared across every type, see the monitoring relay engineering guide.
