Monitoring and Measuring Relay Engineering Guide

What is a monitoring relay? A monitoring relay, also called a measuring and control relay, continuously measures one electrical or physical quantity — voltage, current, phase sequence, liquid level, or temperature — compares it to an adjustable threshold with hysteresis per IEC 60255, and switches a change-over output contact when the quantity leaves the set window, without interrupting the main power path itself. Skip the hysteresis setting or the trip delay and a marginal supply chatters the output relay thousands of times a day, wearing out the contactor coil it drives and nuisance-tripping a line that was never actually in danger. This guide covers the core parameters (threshold, hysteresis, trip delay), the main types by measured quantity (three-phase supply, voltage, current, level, temperature, insulation), output logic and wiring, how a monitoring relay differs from a protective relay or a PLC doing the same job in software, and how the Schneider Zelio Control and ABB CM ranges compare on the shelf.

What a Monitoring Relay Does and Where It Sits in the Panel

A monitoring relay does not switch the load. It watches a quantity and drives a small output contact — usually one or two change-over (SPDT/DPDT) contacts — that feeds a contactor coil, a PLC digital input, an alarm horn, or a trip circuit. The relay itself is rated for control-circuit current, not motor current; the contactor or breaker downstream does the actual switching. That single distinction explains most of the confusion between monitoring relays and protection devices covered further down.

Physically these are DIN-rail modules, typically 17.5 to 22.5 mm wide, mounted next to the contactors and terminal blocks they serve. Power comes either from a separate auxiliary supply (24 V AC/DC or 110-240 V AC depending on the model) or, on some three-phase supply monitors, directly from the lines being measured — no auxiliary wiring needed, which simplifies retrofits into an existing panel with no spare 24 V terminal free.

What we see in the field: panel builders often treat the monitoring relay as an afterthought item added at final wiring, and then discover the DIN-rail space wasn't planned for it. On a retrofit job, checking whether the phase-sequence relay needs an auxiliary terminal or draws from the measured lines directly decides whether you need to pull a new 24 V feed or not.

Core Parameters: Threshold, Hysteresis and Trip Delay

Three settings define how a monitoring relay behaves, and getting them wrong is the single biggest cause of nuisance trips on site.

The threshold is the value at which the relay switches — an overvoltage relay set to trip at 253 V, for example. The hysteresis (also called the reset band or dead band) is the gap between the trip point and the reset point, and it exists specifically to stop the output chattering when the measured value sits right at the threshold. Without hysteresis, a supply that hovers at 253.0-253.2 V would cycle the output relay on and off continuously. The trip delay (on-delay) is a timer that must elapse before the fault condition triggers the output, so a 200 ms voltage dip caused by a nearby motor starting does not trip a relay protecting an unrelated circuit.

Tight hysteresis gives fast, sensitive protection but risks chattering on a noisy supply; wide hysteresis is stable but tolerates a bigger excursion before the relay resets. There is no universal correct setting — it depends on how clean the incoming supply actually is, which is something you only really know after watching the installation for a few days, not from the datasheet alone.

Types of Monitoring Relays by Measured Quantity

Under one product family — Zelio Control on the Schneider side, CM on the ABB side — the function actually monitored varies widely. Matching the relay to the quantity it is meant to catch is the first selection decision.

Voltage Monitoring

Single- or three-phase, with over, under, or window detection (both limits active at once) and adjustable threshold and hysteresis on each limit independently. A window relay catches both a brownout and an unexpected overvoltage spike with one device, which is common on generator or unstable-grid sites. See our dedicated three-phase voltage monitoring article for the asymmetry and sequence side of this.

Current Monitoring

Over- or undercurrent, sensed through a built-in shunt on smaller current ranges or an external CT on larger ones. Undercurrent monitoring is the one people forget: it catches a broken conveyor belt, a pump running dry with the impeller spinning in air, or a lost mechanical load, none of which trip an overcurrent device because the current actually drops, not rises. Overcurrent monitoring on the same relay family catches a jam or a stalled shaft.

Level Monitoring

Conductive sensing through two or more electrodes measures whether liquid bridges the gap between probes, used for pump up/down control and dry-run protection on submersible pumps. Sensitivity, expressed in kOhm, is adjustable to match the conductivity of the liquid — tap water and lightly contaminated process water need different settings, and getting it wrong means the relay either never detects "empty" or falsely reads foam as liquid. Full detail in our liquid level monitoring relay guide.

Temperature Monitoring

PTC thermistor relays trip when a sensor embedded directly in the motor winding reaches its reference resistance, per IEC 60947-8 — a nominally 3.3 kOhm point, not a temperature reading in the usual sense, because the PTC's resistance curve is deliberately steep at the trip point rather than linear. PT100/PT1000 relays instead read an actual resistance-to-temperature curve and switch at a configured degree value, useful where you need the real temperature for logging, not just a go/no-go trip. Our PTC and PT100 monitoring relay article covers wiring differences between the two sensor types.

The full breakdown, including which relay families cover which combination of these functions in one device, is in types of monitoring relays.

Three-Phase Supply and Phase-Sequence Monitoring

This is the highest-volume application of the whole category, because almost every three-phase motor start benefits from it. A phase-sequence relay checks that L1-L2-L3 arrive in the correct rotation before allowing a motor contactor to close; wrong rotation runs a three-phase motor backwards, which on a pump or fan reverses flow and on some machinery is a safety issue, not just an inconvenience. The relay blocks the start rather than letting the motor run the wrong way and then tripping on overload downstream.

The same device family typically bundles phase loss, phase asymmetry, and over/undervoltage detection in one 22.5 mm module. Phase loss on a running motor means it tries to run single-phase — drawing far higher current on the remaining two lines and overheating fast. Asymmetry detection catches an unbalanced supply before it gets that bad.

Many phase-sequence and three-phase supply monitors, including Schneider's RM17TE-type devices and ABB's CM-MPS, need no separate auxiliary supply — they draw operating power from the measured three-phase lines themselves. That single fact removes one wiring step and one potential point of failure compared with a relay that needs its own 24 V feed. Full detail on sequence and loss detection specifically is in phase sequence and phase failure monitoring relays.

Insulation Monitoring Devices for Unearthed (IT) Systems

In a TN or TT earthing system a first insulation fault to earth is caught immediately by the protective device, because it creates a large fault current. An IT (unearthed or high-impedance earthed) system is built deliberately so that a first fault does not trip anything — the system keeps running, which is the whole point in a hospital operating theatre or a process line that cannot tolerate an unplanned shutdown. But that only works if someone is actually watching for that first fault, because a second fault on a different phase, while the first is still present, does create a large fault current and can be dangerous.

IMDs are standard in medical IT systems (operating theatres, intensive care), because interrupting power mid-procedure is not an option; in marine electrical systems, where the hull itself is the earth reference and a hull fault is a corrosion and safety risk; and in industrial IT systems chosen specifically for uptime. This is a different measurement principle from every other relay in this guide — it measures resistance to earth on an ungrounded system, not a threshold on a live quantity — and it is worth not confusing with a standard ground-fault or residual-current device, which needs the system to be earthed to work at all.

Output Logic: Fail-Safe Wiring, Latching and Reset

Two design choices determine how a monitoring relay behaves when something goes wrong upstream of the relay itself, not just the quantity it measures.

Normally-energized (fail-safe) vs normally-de-energized: a fail-safe relay keeps its output contact energized under normal conditions and de-energizes on fault — and critically, also de-energizes if it loses its own auxiliary supply. That means a blown fuse on the relay's control circuit produces the same safe state as an actual fault, which is the behavior most panel builders want on a protective function. A normally-de-energized relay does the opposite and needs the fault condition to actively energize the output; it is used where a de-energized default state on power-up would itself cause a problem, such as blocking a start every time the panel is powered on.

Automatic vs manual (latched) reset: automatic reset clears the output as soon as the measured quantity returns inside the hysteresis band, appropriate where a transient condition should self-clear. Latched (manual) reset holds the trip until someone acknowledges it at the relay or via a remote reset input, which is the right choice any time an unattended automatic restart would be unsafe — a pump restarting itself after a dry-run trip, for instance, before anyone has checked why it ran dry in the first place.

Monitoring Relay vs Protection Relay vs PLC Monitoring

These three terms get used loosely and interchangeably on site, but they are different classes of device with different standards behind them.

A monitoring relay, as covered throughout this guide, falls under IEC 60255 as a measuring relay and IEC 60947-5-1 as a control-circuit device; it measures one or a few quantities, compares to a threshold, and switches an output contact. A protective relay (numeric feeder or motor protection relay) also falls under the IEC 60255 protection class but is a considerably more capable device — multi-function protection curves, communication protocols, event logging, coordination with upstream and downstream devices — aimed at medium-voltage switchgear and large motor feeders rather than a DIN-rail panel. A thermal overload relay, by contrast, is a simpler bimetallic or electronic device sized to a specific motor's full-load current; our thermal overload relay engineering guide and overload relay phase loss and single-phasing article both cover that category in depth.

A PLC doing the same monitoring job in software reads an analog input or a digital status bit and evaluates the threshold in ladder logic or a function block. That approach is flexible — the threshold is a variable, not a dial — but it depends on the PLC being powered, scanning, and correctly programmed; a dedicated monitoring relay keeps working as a stand-alone device even if the PLC is down for a firmware update. On motor circuits specifically, the same phase-loss and thermistor logic also shows up built into a motor protection circuit breaker rather than as a separate relay; see MPCB phase loss and single-phasing protection for how that overlaps with a standalone monitoring relay, and browse the motor protection circuit breakers collection for stocked units.

Schneider Zelio Control vs ABB CM: Range Comparison

Both manufacturers cover the same functional territory — three-phase supply, voltage, current, level, temperature — with adjustable threshold, hysteresis and delay, and change-over outputs. Where they differ is module width, how many functions combine in a single part number, whether an auxiliary supply is required, fault-memory and latching options, and true-RMS vs average measurement on the higher models.

Neither range is categorically ahead of the other on function coverage — the practical decision usually comes down to what is already standardized in the panel builder's parts library, and what is in stock. Browse the full monitoring and control relays collection to compare specific part numbers side by side. Siemens SIRIUS (3UG4/3RR2 monitoring, 3RS temperature) and Finder 70/71/72-series units are functional equivalents that slot into the same panel layouts as they come into stock.

How to Select and Set a Monitoring Relay for Your Application

Selection starts with the quantity, not the brand. Identify what actually needs watching — phase sequence on a new motor start, undercurrent on a belt-driven load, level in a wet well — before comparing part numbers; our what is a monitoring relay and how it works primer is a good starting point if the function itself is still unclear. From there, four questions narrow the field fast: single-phase or three-phase supply, auxiliary supply available or not, single function or combined, and automatic or latched reset needed. The step-by-step version of that process is in how to select a phase and voltage monitoring relay.

Once the part is chosen, setting it correctly on site matters as much as picking the right model. Start with the manufacturer's nominal threshold, then adjust hysteresis upward if you see chattering on the supply and adjust trip delay upward if you see nuisance trips on motor starts elsewhere in the panel — both symptoms of settings that are too tight for the actual site conditions rather than a defective relay. Full setting procedure, including where the trip and reset dials are on common Zelio and CM parts, is in how to set a voltage monitoring relay. On motor circuits, pairing a thermistor monitoring relay with the motor's actual PTC sensor is covered in monitoring relays for motor protection and thermistor sensing, and if the downstream device is a contactor rather than a breaker, our contactor selection checklist and the contactors collection cover sizing the coil the monitoring relay's output will actually be driving. For motor circuits that also need thermal overload protection alongside the monitoring function, check the thermal overload relays collection.

Conclusion

A monitoring relay is a small device with an outsized effect on reliability: get the threshold, hysteresis and trip delay right for the actual quantity being watched, choose fail-safe output logic on protective functions, and match the part to whether an auxiliary supply is available. Schneider Zelio Control and ABB CM cover essentially the same functional ground, so the deciding factor is usually what fits the existing panel standard and what is on the shelf. The sibling articles linked throughout this guide go deeper on each function, selection process, and setting procedure — start with whichever quantity your application actually needs to watch.