How to Test a Monitoring Relay: Bench and In-Panel Checks
How do you test a monitoring relay? You apply a controlled input to the sensing terminals with a calibrated source, watch the output contact and the front LED change state at the set threshold, and confirm the reset point falls where the threshold minus the hysteresis band predicts. A test that only checks the trip point and skips the reset band and the trip delay passes a relay that trips at the right value but chatters around it in service, and a chattering output burns contactor coils and floods a PLC input with noise. This covers bench testing with a variac or calibration source, in-panel checks that do not require breaking wiring, phase-sequence-specific tests, and the failure signatures that separate a bad relay from a wiring fault or a marginal load.
Tools and Isolation Before You Start
A bench test needs three things beyond the relay itself: a true-RMS multimeter (average-responding meters misread on a non-sinusoidal test source), a variable voltage source or three-phase test set for the measured input, and a stopwatch or storage scope for the trip-delay check. Most Zelio Control RM and ABB CM units also carry a front LED or mechanical flag that changes with the output relay, useful when the load contactor is not wired yet.
Isolate the output contact from the contactor coil or PLC input before forcing the measured input up and down. The relay does not know the difference between a real fault and a bench test; it drives the same output circuit either way, and cycling a contactor coil dozens of times during a threshold sweep shortens its life for no reason. On three-phase supply monitors, the auxiliary supply terminals and the measured L1-L2-L3 terminals are usually separate blocks. Powering the aux side does not put voltage on the sensing side.
Verifying the Trip Threshold and the Reset Point
Connect the calibrated source to the measured input and raise it slowly (or lower it, for undercurrent or undervoltage functions) while reading the output contact state with a continuity tester or the LED. Note the exact value where the output switches; that is the trip threshold. Then reverse direction and note the value where it switches back. That is the reset threshold, and the gap between the two is the hysteresis band set on the front dial.
Formula: Reset threshold — Source: general monitoring relay hysteresis principle (IEC 60947-5-1 control-circuit devices)
Vreset = Vtrip x (1 - Hysteresis%)
| Symbol | Description | Unit |
|---|---|---|
| Vtrip | Measured trip threshold recorded on the sweep-up test | V, A, or measured unit |
| Hysteresis% | Reset band set on the front dial, expressed as a fraction of the threshold | % |
| Vreset | Expected value at which the output resets on the sweep-down test | V, A, or measured unit |
If the measured reset point does not match the dial setting within a few percent, either the dial is miscalibrated or the relay's internal reference has drifted. Replace it rather than chase the number with the threshold dial, because a threshold adjustment shifts both the trip point and the reset point together.
Verifying the Trip Delay
Most three-phase and voltage monitors carry an adjustable trip delay, typically 0.1-10 s, so a brief sag or a motor inrush does not cause a nuisance trip. Force a step change past the threshold and time how long the output takes to switch, either with a stopwatch against the dial marking or with a scope on the output contact for better resolution than a hand can time. A relay set for a 3 s delay that trips in 0.3 s has a failed timing stage. The threshold detection is fine; the delay circuit is not, and no dial adjustment fixes that.
Confirm the reset side of the delay too, if the relay has one. Some ranges reset instantly once the measured value comes back inside the window; others hold the trip for a fixed reset delay to prevent immediate re-closing into a fault that has not fully cleared. Check the datasheet behavior for the specific series before calling a slow reset a fault.
In-Panel Functional Test Without Breaking Wiring
Once the relay is installed and wired, pulling terminals to bench-test it again is rarely worth it. Two options work without disturbing the wiring. First, many Zelio Control and CM-range units have a front test button that forces the output to switch; use it to confirm the contactor or PLC input responds, which tells you the downstream wiring is sound even though it says nothing about the actual threshold. Second, for a real functional check, simulate the fault condition at the source: open one fuse to simulate phase loss, pull a probe out of the liquid to simulate low level, or lift the aux supply momentarily on a fail-safe unit and confirm the output drops.
What we see in the field: a relay that passes the front test button but does not trip on the real simulated fault usually has a wiring problem on the sensing side, not a bad relay. The test button forces the output stage directly and bypasses the measurement stage entirely.
Testing Phase Sequence and Phase Loss Relays Specifically
Phase-sequence and phase-failure units such as the phase sequence and phase failure relays in the RM17TE and CM-MPS families need a different test than a simple voltage sweep, because they check rotation and presence across three lines rather than a single threshold. Swap any two of the three phase leads at the relay's own terminals, not at the motor, and confirm the output blocks; this simulates reverse rotation without touching the load. Then restore the correct sequence and pull one phase fuse to simulate phase loss. The output should drop within the set delay. Because CM-MPS-type units draw their own power from the measured lines, a fully dead three-phase supply should also produce a de-energized, safe output rather than a floating one.
The output of a phase or voltage monitor almost always feeds the coil circuit of a contactor or a starter, so confirm the downstream device actually drops out during the phase-loss test, not just that the relay's own LED changes. A relay that trips correctly but leaves the motor running because of a wiring error downstream is a passed relay test and a failed panel test. Both need checking separately.
Common Failure Modes on Test
No output despite an obvious fault: dead relay, or the aux supply feeding it has failed. Check aux voltage before condemning the relay. Chatter near the threshold instead of a clean switch: hysteresis dial set too narrow for a noisy supply, or a marginal internal reference. Widening the hysteresis a step is a legitimate fix if the application tolerates the wider window; otherwise, replace the unit. Trip delay ignored, output switches instantly no matter the dial position: failed timing stage. Replace it. Relay trips, LED confirms it, but the contactor or PLC does not respond: the fault is downstream, not in the relay. Verify with a continuity check on the output circuit before ordering a replacement part.
For the broader selection and setting logic behind these thresholds, see the monitoring relay engineering guide and the specifics in how to set a voltage monitoring relay; both cover the dial settings referenced in the tests above. The full range of monitoring and control relays stocked at Stoklink covers three-phase, voltage, current, level, and temperature functions from Schneider Zelio Control and ABB CM.
Frequently Asked Questions
Can I test a monitoring relay without a variac or calibration source?
Partially. The front test button confirms the output stage and downstream wiring, and simulating the fault at the source (pulling a fuse, lifting a probe, lifting aux supply) confirms the functional response. Neither one verifies the exact trip threshold value; for that you need a calibrated variable source on the measured input.
Why does my monitoring relay trip immediately when I close the fuse, before the delay elapses?
Check that the delay setting was not left at its minimum after commissioning, and confirm the timing stage with a stopwatch test. If the output switches instantly regardless of where the delay dial is set, the timing circuit itself has failed.
How often should monitoring relays be re-tested after commissioning?
There is no universal interval; it depends on panel duty and environment. Many panel builders fold a functional test, the test button plus one simulated fault, into the same schedule as contactor and overload relay checks during planned maintenance.
What does it mean if the reset threshold does not match the hysteresis dial setting?
It means the relay's internal reference has drifted or the dial itself is miscalibrated. Because a threshold adjustment moves the trip point and the reset point together, a hysteresis mismatch cannot be corrected by re-tuning the threshold; replace the unit.
Can I test a phase-sequence relay by swapping phases at the motor terminals?
Swap the leads at the relay's own terminals instead. Swapping at the motor also reverses the motor, which is the exact condition the relay is meant to prevent, and it is unnecessary risk to the load when the relay terminals give the same test result.
Does a monitoring relay need its own test certificate for panel commissioning?
Most panel builders record the tested trip value, reset value, and delay against the datasheet range as part of the panel's functional test record, rather than a separate certificate for the relay alone. Check the project's inspection and test plan for the specific requirement.
Conclusion
Testing a monitoring relay is three checks, not one: the trip threshold, the reset point implied by the hysteresis setting, and the trip delay. Skipping the reset and delay checks lets a relay pass on the number that matters least in service, the exact trip point, while missing the chattering or nuisance-tripping behavior that actually causes callbacks. Isolate the output from the load before sweeping the input, use the front test button and a simulated fault for in-panel checks that do not require breaking wiring, and separate a relay fault from a downstream wiring fault by checking the relay's own indicator against the load response.
