Main Components of a Measuring Relay: Sensing, Threshold, Output
What are the main components of a measuring relay? A measuring relay is built from four functional blocks: a sensing input that reads voltage, current, temperature, level, or three-phase data; a comparator that checks the reading against an adjustable threshold with hysteresis, per IEC 60255; a timing stage that applies a trip delay; and an output relay, usually one or two SPDT/DPDT change-over contacts, that switches the load circuit. Skip the hysteresis or delay stage and the output chatters on every transient, which wears out the contactor coil it drives faster than the motor it protects. This guide walks through the sensing input, the threshold and comparator stage, hysteresis, trip delay and fault memory, the output contact logic, and the power supply arrangement that ties the four blocks together.
The Sensing Input Stage: What the Relay Actually Reads
Every measuring relay starts with an input transducer matched to the quantity it watches. Voltage relays sample the line directly, single- or three-phase, through internal resistive dividers rated for the supply class. Current relays read through a built-in shunt for small full-load currents or through an external CT for anything above a few amps — undercurrent catches a broken belt or a dry-running pump, overcurrent catches a jam. Temperature relays take one of two forms: a PTC thermistor embedded in the motor winding that presents a step-change resistance at its reference point (per IEC 60947-8), or a PT100/PT1000 that reports a continuous resistance-to-temperature curve. Level relays use conductive sensing — electrodes at different heights measure whether the liquid bridges the circuit, and the sensitivity in kOhm is adjustable for the liquid's conductivity. Three-phase supply relays are a special case: they sample all three lines simultaneously to derive phase presence, sequence, and asymmetry in one pass.
The input stage is where the biggest selection mistake happens: choosing a relay whose current range or thermistor count doesn't match the load. A monitoring relay type overview is the fastest way to confirm which input a given application actually needs before ordering.
The Threshold and Comparator: Setting the Trip Point
Once the input is digitized or conditioned, a comparator checks it against a set threshold. Three modes cover nearly every application: overvalue (trip above the set point), undervalue (trip below it), and window (trip outside a band around a nominal value, used for asymmetry and voltage-band monitoring). The threshold itself is adjustable, usually through a front-panel potentiometer or a rotary switch with a printed scale, and the adjustment range is a spec line worth reading closely — a relay rated 10-100% of nominal current is not interchangeable with one rated 20-120%.
What we see in the field: installers set the threshold to the load's rated value and stop there, without checking the tolerance the load actually needs. A pump motor with 8% normal current ripple set against a 5% window trips on start-up, not on a fault.
Hysteresis: Why the Reset Point Isn't the Trip Point
A comparator with a single threshold and no hysteresis will chatter the moment the measured value sits close to the trip point — every small ripple crosses the line and re-crosses it. Hysteresis solves this by setting the reset point at a percentage offset from the trip point, so the output stays tripped until the value clears a wider margin. On most Schneider Zelio Control and ABB CM-range relays this is a fixed or adjustable percentage of the threshold, typically in the 1-20% band depending on the model.
Formula: Reset Threshold from Hysteresis — Source: IEC 60255, general measuring relay practice
Vreset = Vtrip × (1 − H%)
| Symbol | Description | Unit |
|---|---|---|
| Vtrip | Set trip threshold | V, A, or engineering unit of the measured quantity |
| H% | Hysteresis, expressed as a percentage of the trip threshold | % |
| Vreset | Value the relay must return to before the output resets | same unit as Vtrip |
An undervoltage relay set to trip at 340 V with 5% hysteresis will not reset until the supply climbs back to 357 V — not 340 V. Confuse the two and a commissioning engineer assumes the relay is faulty when it simply hasn't cleared the reset band yet. For the full breakdown of hysteresis alongside trip delay and latching, see the dedicated hysteresis, trip delay and latching guide.
Trip Delay and Fault Memory
The third stage sits between the comparator and the output: a timer. The trip delay (also called on-delay or response delay) holds the fault decision for a set period before energizing or de-energizing the output, so a brief dip or inrush transient doesn't trip the relay. Typical adjustable ranges run from a fraction of a second to tens of seconds; three-phase supply relays commonly also carry a separate start-up delay to ride through motor inrush.
Fault memory determines what happens after the delay expires and the fault clears. Automatic reset relays re-energize the output the moment the measured value re-enters the reset band. Manual (latched) reset relays stay tripped until an operator or a PLC issues a reset command, even after the fault condition is gone — the standard choice where a fault must be acknowledged before restart, such as dry-run protection on a pump.
The Output Stage: Contacts and Switching Logic
The output is what most panel builders actually wire, and it's worth separating two independent choices: contact configuration and switching logic. Configuration is almost always one or two change-over (SPDT/DPDT) contacts rated for a contactor coil, a PLC digital input, or an alarm circuit — the relay does not switch the main power itself. Switching logic is the safety-relevant choice: normally-energized (the output relay is energized in the healthy state and de-energizes on fault or on loss of auxiliary supply) versus normally-de-energized. Normally-energized wiring is the fail-safe default, because a blown fuse or a disconnected relay produces the same signal as a real fault, and the downstream contactor drops out instead of staying latched in.
This is also the interface point to the rest of the protection scheme. The relay's output contact typically feeds a contactor coil circuit or, on motor circuits, coordinates with a motor protection circuit breaker upstream. Wiring details for auxiliary supply and output terminals are covered separately in the how to wire a monitoring relay guide.
Power Supply: Auxiliary vs Self-Powered Designs
Not every measuring relay needs a separate power feed. Three-phase supply relays such as ABB's CM-MPS or the equivalent Schneider Zelio units in the phase-sequence family draw their own operating power directly from the three lines they measure — one less terminal to wire, and the relay keeps monitoring even if a control-transformer fuse blows elsewhere in the panel. Single-quantity relays, meaning most voltage, current, level, and temperature types, need a dedicated auxiliary supply, commonly 24 V DC or 24-240 V AC/DC universal input on the newer ranges.
This depends on how the panel's control power is distributed: on a small machine with one 24 V DC supply, a self-powered three-phase relay is one fewer load on that rail; on a large switchboard with several control transformers, auxiliary-powered relays let you tie monitoring to a specific sub-circuit's health. Both approaches show up across the monitoring and control relays range from Schneider and ABB, and the choice is rarely about cost — it's about which supply the relay needs to still be watching when something else fails.
How the Four Blocks Work Together
Trace a single fault through the relay and the four blocks stop being abstract. An undervoltage event on one phase reaches the sensing input as a dropping RMS value. The comparator flags it the instant the value crosses the set threshold. The trip-delay timer holds that flag for its set period — long enough that a half-cycle sag from a nearby motor starting doesn't trigger a nuisance trip, short enough that a real sustained dip still trips before downstream equipment is damaged. If the delay expires with the fault still present, the output relay switches: contacts change state, the contactor coil it feeds drops out, and the motor stops. If the relay is set to manual reset, it stays in that state until acknowledged, even after voltage recovers.
Cross-brand, Schneider Zelio Control and ABB CM-range relays implement this same four-block architecture; what differs is how many functions are combined in one 17.5-22.5 mm housing, whether the aux supply is needed, and whether the higher-end models measure true-RMS or average value. For phase-specific behavior, the phase sequence and phase failure relay article covers the three-phase input case in more detail.
Frequently Asked Questions
What are the four main components of a measuring relay?
A sensing input (voltage, current, temperature, level, or phase), a comparator with an adjustable threshold and hysteresis, a trip-delay timer, and an output relay with one or two change-over contacts. Some models add fault memory (latching) as a fifth block.
What is the difference between the threshold and the hysteresis setting?
The threshold is the value at which the relay trips. Hysteresis is the offset percentage that sets a separate, lower (or higher, for undervalue functions) reset point, so the output doesn't chatter when the measured value sits near the threshold.
Why does a monitoring relay have a trip delay if it's already measuring continuously?
Continuous measurement without a delay would trip on every brief transient — a motor starting nearby, a switching surge, a momentary sag. The trip delay filters these out by requiring the fault condition to persist for a set time before the output actually switches.
Does a monitoring relay switch the main power circuit directly?
No. Its output contact is a low-power change-over signal that drives a contactor coil, a PLC input, or an alarm circuit. The contactor or breaker actually interrupts the main power.
Why do some three-phase monitoring relays not need an auxiliary supply?
Models like ABB's CM-MPS or comparable Schneider phase-sequence units draw their own operating power directly from the three-phase lines they're measuring, which removes one wiring point and keeps the relay working even if a separate control-power fuse fails.
What's the difference between automatic and manual reset on the output stage?
Automatic reset re-energizes the output as soon as the measured value clears the hysteresis band. Manual (latched) reset holds the tripped state until an operator or PLC clears it, even after the fault condition disappears — used where restart needs to be a deliberate decision.
Conclusion
The sensing input decides what a relay can detect; the comparator and hysteresis decide when it decides; the trip delay decides how patient it is; the output stage decides what happens next and whether that decision is safe by default. Get the four blocks matched to the actual load — input range, hysteresis band, delay timing, and fail-safe logic — and nuisance trips drop out of the maintenance log. For the broader picture across relay types and selection criteria, the monitoring relay engineering guide ties these components back to specific application choices.
