How to Test and Calibrate a Thermal Overload Relay
What is overload relay testing and calibration? Testing and calibration means verifying a thermal or electronic overload relay trips within the tolerance IEC 60947-4-1 sets for its class — a Class 10A relay, for example, must trip in 2-10 s at 7.2x the current setting from cold — and confirming the dial reading matches the current the sensing element actually responds to. Skip it and a relay marked "20 A" may trip at 17 A or let 24 A pass all day, either nuisance-tripping a healthy motor or letting an overloaded one cook its windings. This article covers the STOP/TEST button, primary current injection, secondary injection on electronic units, trip-class verification, calibration drift, and a field acceptance checklist before a starter goes into service.
Why Test an Overload Relay Before It Goes Into Service
A starter is three devices working together: the SCPD (fuse or MPCB) for short circuits, the contactor for switching, and the overload relay for sustained overload. If the relay's dial is set correctly but the internal mechanism has drifted or was never calibrated tight to begin with, the other two devices are still doing their job — the motor is not. A dial reading has no relationship to trip accuracy unless someone has verified it against a known current.
New relays from Schneider, ABB, and Siemens ship within the tolerance IEC 60947-4-1 requires, so blanket testing every unit out of the box is rarely worth the labor. Testing earns its keep at three points: commissioning a new contactor and relay combination before it's buried in a panel, after any relay has tripped on a genuine fault and been reset, and on a periodic interval for motors where an undetected miscalibration is expensive — large pumps, compressors, and anything in a motor control center with dozens of starters and no one watching each one individually.
The STOP/TEST Button: What It Actually Checks
Every bimetallic relay from the major brands carries a STOP/TEST button on the face. Pressing it pushes the internal trip bar mechanically, dropping out the 95-96 NC contact and — if wired into the contactor coil circuit — opening the starter. It proves the trip bar moves freely, the NC contact makes and breaks, and the field wiring from that contact to the coil is intact.
It proves nothing about current sensing. The bimetal strips never see current during a STOP/TEST press; the button bypasses them and moves the trip bar by hand. A relay whose bimetal has drifted so far it never trips at 10x rated current will still pass a STOP/TEST check every time, because the check doesn't route current through the sensing element at all.
What we see in the field: many panel builders press STOP/TEST once during final assembly test and log the relay as verified. It confirms the mechanical trip path and the wiring to the coil. It says nothing about whether 20 A on the dial actually trips at anything near 20 A.
Primary Current Injection Testing on Bimetallic Relays
Real calibration verification routes actual current through the relay's power poles, the same path the motor current takes. A variable-current injection test set — single-phase is enough for a bimetal element, since each phase strip responds independently — clamps onto the poles, and the dial is set to a known value before current is applied.
IEC 60947-4-1 defines the test currents: at 1.05x the dial setting the relay must not trip within 2 hours from a cold start; at 1.20x it must trip within 2 hours. Those two points bracket the relay's accuracy band around the set point. The 7.2x test, covered below, checks the trip-time curve rather than the trip threshold. A relay that trips early at 1.05x is set too tight for real motor tolerances; one that never trips at 1.20x has drifted loose and will let a genuine overload run.
This test takes real time — the 1.05x check alone can run close to two hours if the relay is borderline — so it gets reserved for commissioning of critical starters and for units under suspicion after a nuisance trip, not for routine spot checks on every relay in a panel.
Secondary Injection Testing for Electronic Overload Relays
Electronic units — ABB's E-series, Siemens 3RB30/3RB31, Schneider's LR9 line under TeSys T — sense current through internal CTs feeding a microcontroller, not a bimetal strip. Driving full rated primary current through a 100 A frame to test it means a bulky, heavy-current injection set; most test benches can't source that comfortably.
Secondary injection sidesteps the problem: a signal is fed into the relay's electronics at the CT secondary or through a dedicated test port, checking the trip algorithm and setpoint logic without the primary current path. It confirms the microcontroller trips at the programmed threshold and class. It does not confirm the CT ratio and burden are correct for that specific frame — an error there shows up only under real primary current, so secondary injection is a commissioning shortcut, not a substitute for primary testing on a sample of units from a new batch.
Phase-loss, ground-fault, and stall protection on electronic units — features standard on Siemens 3RB31 and ABB's larger E-series — usually have a dedicated self-test LED or push-button sequence separate from the main trip-current test. Check the datasheet for the specific sequence; it varies by model and isn't worth guessing at.
Verifying Trip Class Against IEC 60947-4-1
Trip class is the number that separates a relay tuned for a centrifugal pump from one tuned for a crusher. IEC 60947-4-1 defines it by a single test: apply 7.2x the current setting from a cold start and measure the trip time.
Formula: Trip-class verification — Source: IEC 60947-4-1, Table 7
ttrip = f(Itest = 7.2 × Iset, cold start)
| Symbol | Description | Unit |
|---|---|---|
| I_set | Dial current setting (equals motor FLC, or FLC/√3 in a delta leg) | A |
| I_test | Test current applied, 7.2x the dial setting | A |
| t_trip | Measured trip time from a cold start | s |
The measured time has to land inside the class window: Class 10A trips in 2-10 s, Class 10 in 4-10 s, Class 20 in 6-20 s, Class 30 in 9-30 s. A relay sold as Class 20 that trips in 4 s under this test is mislabeled, damaged, or has drifted toward a faster class — any of which matters if it was sized for a load with a long run-up. See our trip class breakdown for how the classes map to load type.
Ambient temperature at the test bench matters here. Bimetallic elements are built with a compensating bimetal so the trip point doesn't drift much between roughly -5 and +55/60 C, but "doesn't drift much" is not "doesn't drift at all." Run the 7.2x test at something close to room temperature and note the actual ambient on the test record — a trip time near the edge of the class window is easier to explain, and to defend later, with that number logged.
Calibration Drift, Dial Accuracy, and When to Replace
Bimetal strips move by physically bending under heat, thousands of times over a relay's service life. Repeated thermal cycling, contact pitting on the NC pair from interrupting coil current, dust ingress in an unsealed enclosure, and vibration all push the trip point away from where the dial says it is. None of this shows up on a visual inspection.
None of the three major brands offers field recalibration of the sensing element. The dial adjusts where the mechanism trips within its designed range; it does not reset a mechanism that has drifted outside its original tolerance. A relay that fails the 1.05x/1.20x band check or the 7.2x trip-class check gets replaced, not adjusted further.
What we see in the field: some panel builders keep a failed relay around to reuse the mechanical hardware — the mounting clip, the auxiliary contact block — while binning the sensing element itself. That's a reasonable call for parts that don't carry current; it is not a substitute for replacing the part that failed the test.
Field Acceptance Testing Checklist
A practical sequence for a new or reworked starter before it's released to production:
1. Confirm the dial is set to the motor's nameplate FLC — or FLC/√3 if the relay sits in the delta leg of a star-delta starter, not the line current. 2. Press STOP/TEST and confirm the contactor drops out, verifying the mechanical trip path and coil wiring. 3. For critical or suspect units, run primary injection at 1.05x and 1.20x the setting to check the accuracy band, or secondary injection on electronic units to confirm the trip logic. 4. Verify auxiliary contact wiring — 95-96 NC into the coil circuit, 97-98 NO into any alarm or PLC input — reads correctly at the terminal, not just at the relay. 5. Log the test current, measured trip or no-trip result, and bench ambient temperature on the commissioning record. 6. On electronic units, run the phase-loss and ground-fault self-test sequence and confirm the status LEDs match the datasheet's expected pattern.
Skipping straight from "dial is set" to "wired and closed up" is how a miscalibrated relay ends up protecting a motor for years without anyone knowing it was never really checked. See our selecting and setting guide for the setup steps that come before this checklist, and our nuisance tripping guide if a unit fails testing intermittently rather than consistently.
Frequently Asked Questions
Does pressing STOP/TEST prove the overload relay is calibrated?
No. STOP/TEST moves the trip bar mechanically and checks the NC contact and its wiring. It never routes current through the bimetal or CT, so it says nothing about whether the dial setting matches the actual trip current.
What current do I inject to verify trip class?
7.2x the dial setting, applied from a cold start, per IEC 60947-4-1 Table 7. The measured trip time must land inside the class window: 2-10 s for Class 10A, 4-10 s for Class 10, 6-20 s for Class 20, 9-30 s for Class 30.
Can a bimetallic overload relay be recalibrated in the field?
No. The dial only adjusts the set point within the mechanism's designed range; it does not correct a bimetal element that has drifted outside tolerance. A relay that fails calibration testing is replaced.
How often should overload relays be tested?
At commissioning of a new starter, after any genuine fault trip and reset, and on a periodic interval set by criticality — more frequently for large motors and MCC starters where an undetected miscalibration is expensive, less frequently for low-consequence loads.
Does test-bench ambient temperature affect the result?
Yes, to a degree. Compensated bimetal elements limit drift across roughly -5 to +55/60 C ambient but don't eliminate it. Run the trip-class test near room temperature and log the actual ambient so a borderline result can be interpreted correctly.
What's different about testing an electronic overload relay versus a bimetallic one?
Electronic units (ABB E-series, Siemens 3RB30/3RB31, Schneider LR9) can be checked with secondary injection at the CT or a test port, avoiding the need to source full primary current. That confirms the trip logic but not the CT ratio and burden, which only real primary current at commissioning of a new frame batch will catch.
Conclusion
Testing an overload relay is two separate questions, not one. STOP/TEST answers "does the mechanical trip path work?" Primary or secondary current injection at the IEC 60947-4-1 test points answers "does it trip at the current the dial says it will?" Treating the first as proof of the second is how a genuinely undersized or drifted relay sits in a panel unnoticed until it either nuisance-trips a healthy motor or lets a real overload run. For the full protection picture this testing sits inside, see our thermal overload relay engineering guide, and browse verified-stock thermal overload relays and motor protection circuit breakers for the rest of the starter.
