Main Components of an Overload Relay: Bimetal, Trip Bar, Reset
What are the main components of a thermal overload relay? A bimetallic thermal overload relay is built from three heater-driven bimetal strips, a differential trip bar, a hand/auto reset mechanism, and an auxiliary contact set (95-96 NC, 97-98 NO), all inside a molded case sized to IEC 60947-4-1. Miss one part's function and the whole protection scheme breaks down: a stuck reset lever leaves a faulted motor with no restart lockout, a worn trip bar delays the trip past the winding's thermal limit, and a mis-set dial trips on every start or never trips at all. This article covers the bimetal strip, the differential trip bar and the trip curve it produces, the NC/NO contact pair, the reset mechanism, ambient compensation, and the setting dial that ties the relay to the motor nameplate.
The Bimetal Strip: Converting Current Into Movement
Three bimetal strips sit inside the relay, one per phase, each carrying — or heated by a winding around — the motor's line current. A bimetal strip bonds two metals with different thermal expansion coefficients back to back; heat the strip and the two layers expand at different rates, so the strip bends toward the metal with the lower coefficient. On small frames the motor current runs directly through the strip. On larger frames, LR9F-class relays above roughly 80-100 A for example, a separate heater winding wraps the strip instead, keeping the strip's own resistance out of the main current path. Bend tracks accumulated heating, not instantaneous current, so the strip follows the motor's thermal state rather than reacting to a single current spike.
What we see in the field: relays that have absorbed repeated single-phase faults show uneven wear across the three strips well before any visible damage shows up, which is one reason the differential trip bar below catches phase loss faster than a single-strip design would.
The Differential Trip Bar
A trip bar links the free ends of all three bimetal strips. As the strips bend, they push against the bar; once the bar travels far enough, it releases a spring-loaded mechanism that snaps the NC contact open. On a balanced three-phase overload, all three strips bend together and the bar trips on the average deflection, matched to the dial setting. On a phase-loss fault only two strips see current — the remaining phases carry roughly 1.7x their prior load — so those two strips bend further than the third. A true differential trip bar reacts to that difference in deflection between phases, not just the average, and trips faster than a design that only sums total bend.
Trip Curve and Trip Class
The trip bar's release point follows an inverse-time curve: small overloads take minutes to trip, large ones take seconds. That curve is what lets the relay ride through a motor's own inrush, typically 6-8x FLC for a few seconds at start, without tripping, while still catching a sustained 1.2-1.5x overload before the windings overheat. IEC 60947-4-1 standardizes four trip classes, tested at 7.2x the current setting applied from cold.
Formula: Trip Class Test Condition — Source: IEC 60947-4-1, Clause 7
I_test = 7.2 × I_set (cold start), t_trip within class band
| Symbol | Description | Unit |
|---|---|---|
| I_test | Test current applied from cold | A |
| I_set | Current dial setting, equal to motor FLC | A |
| t_trip | Trip time: Class 10A 2-10 s, Class 10 4-10 s, Class 20 6-20 s, Class 30 9-30 s | s |
Class 10A and Class 10 cover most pumps, fans, and general machinery. Class 20 and 30 exist for loads that take longer to run up to speed — large fans, centrifuges, crushers — where a Class 10 relay would nuisance-trip on every start before the motor reaches full speed. See our overload relay trip classes article for the full breakdown by class and application.
Auxiliary Contacts: NC (95-96) and NO (97-98)
Every thermal overload relay carries at least one normally-closed contact, numbered 95-96, and one normally-open contact, numbered 97-98. The 95-96 NC contact wires in series with the contactor coil; when the trip bar releases, this contact opens and drops the coil, disconnecting the motor. The 97-98 NO contact closes on the same trip event and typically drives an indicator lamp, a PLC input, or an alarm relay. It does not carry motor current and should never substitute for the 95-96 contact in the coil circuit. A STOP/TEST button on the relay face manually forces a trip for commissioning checks without waiting for an actual overload.
Reset Mechanism: Hand vs Auto
Once the bimetal strips cool, the trip bar is free to reset, but the contact itself stays latched open until the reset mechanism releases it. HAND (manual) reset requires someone to press the reset button before the contactor can re-energize, forcing a person to find and clear the cause of the trip before the motor restarts. AUTO reset re-closes the contact on its own as soon as the strips cool enough, with no human check in between. Most motor circuits default to hand reset for exactly that reason: an auto-reset relay can re-energize a motor straight back into the fault that tripped it, cycling the winding through repeated heating events. Auto reset is acceptable on some pump and compressor applications where an unattended restart is safe and expected, but it stays the exception, not the default.
Some builders set every relay to auto reset to cut nuisance callbacks, but that trades a five-minute site visit for a motor that keeps restarting into an unresolved fault; the trade only makes sense where an unattended restart carries no downstream risk. Our manual vs automatic reset comparison covers the application split in more detail.
Ambient Temperature Compensation
A bimetal strip responds to any heat, not just the heat generated by motor current, including the temperature inside the panel it is mounted in. Without compensation, a relay mounted in a hot enclosure would appear to see more current than it actually does and trip early, while one in a cold enclosure would tolerate a real overload longer than intended. Compensated relays add a second bimetal element that reacts to ambient temperature alone and offsets the main strip's response, holding the trip point roughly stable across about -5 to +55/60 C ambient.
Mount the relay in the same thermal environment as the motor's control gear, not next to a transformer or a bank of resistors. A panel that runs consistently above the compensated range shifts the effective trip point even on a compensated unit, and an uncompensated small-frame relay drifts more than a compensated large-frame one under the same conditions.
Setting Dial, Heater Range, and Terminals
The front dial is calibrated directly in amps and set to the motor's full-load current (FLC) from the nameplate, not to the breaker or fuse rating upstream. Each relay frame covers a current band: Schneider's TeSys LRD spans roughly 0.10 A to 630 A across its LRD and LR9F sub-ranges, ABB's TA line runs from TA25DU up through TA200DU and larger frames, and Siemens SIRIUS 3RU21 covers frame sizes S00 through S3 up to about 100 A. The frame is picked first for the motor's FLC, then the dial is fine-tuned within that frame's band. Terminals accept ring lugs or bare wire sized to the frame's current rating, and the relay body snaps onto the base of its matching contactor so the assembly reads as one unit in the panel.
For the electronic alternative to these bimetal internals — current transformers, a microcontroller, and a wider 1:3 to 1:4 setting ratio instead of a single dial — see our thermal vs electronic overload relays comparison. For the full working principle behind the bimetal and trip-bar assembly described above, see how a thermal overload relay works, and for phase-loss behavior specifically, see phase-loss and single-phasing protection.
Frequently Asked Questions
What are the main components of a thermal overload relay?
Three bimetal strips, one per phase, a differential trip bar, a hand/auto reset mechanism, an NC contact (95-96) wired to the contactor coil, an NO contact (97-98) for indication, a STOP/TEST button, and a setting dial calibrated in amps to the motor FLC.
Why does the relay need three separate bimetal strips instead of one?
Three strips, one per phase, let the trip bar compare deflection between phases. That differential comparison is what makes the relay sensitive to phase loss, where only two phases carry current after a lost conductor.
What is the difference between the 95-96 and 97-98 contacts?
95-96 is normally closed and wires into the contactor coil circuit; it opens on a trip and drops the motor. 97-98 is normally open and closes on a trip for indication or alarm use, and it does not carry motor current.
Why does a hot panel affect the trip point?
The bimetal strips respond to any heat, including ambient enclosure heat, not only heat from motor current. Compensated relays add a second bimetal element that cancels this ambient effect; uncompensated or poorly mounted relays can drift, tripping early in a hot enclosure.
Should I use hand reset or auto reset?
Hand reset is the default for most motors: it forces someone to find and clear the fault before the motor restarts. Auto reset is acceptable only where an unattended restart is safe, such as some pump applications.
Conclusion
Every part of a thermal overload relay maps to one function in the trip decision: the bimetal strips sense heat, the differential trip bar compares that heat across phases and decides when to release, the contacts carry the trip signal to the contactor and to indication, the reset mechanism decides whether a person has to intervene, and the compensation bimetal keeps the whole assembly honest about enclosure temperature. Match the frame and dial to the motor's FLC and the class to its start time, and the rest of the mechanism does its job without intervention. For sizing and setting steps beyond the dial itself, see our how to select and set an overload relay guide, or the full thermal overload relay engineering guide for the complete picture across every relay type covered in thermal overload relays at Stoklink.
