Eutectic (melting alloy) and bimetallic thermal overload relays both sit in the motor starter circuit, sense current as I²R heat, and open a contactor to clear running overcurrent, but they trip on two physically different phase transitions, and that single difference drives the entire selection logic [S1][S4].
Across NEMA and IEC starter lines, eutectic units are still the legacy "heater type" of choice for submersible pumps, heavy machinery, and university plant retrofits, while bimetallic units dominate where auto-reset, ambient compensation, or DIN-rail packaging matters [S3][S6].
How a eutectic (melting alloy) overload relay trips
A eutectic overload relay uses a spring-loaded lever or ratchet held captive by a solid plug of eutectic alloy, a metallic mixture chosen so it changes from solid to liquid at a sharply defined temperature rather than over a soft range [S1][S6]. The motor current flows through replaceable heater elements sized to motor full-load amps; those heaters dump I²R heat into the alloy until the phase change occurs, at which point the spring snaps the contacts open [S1][S2].
The eutectic melt point is unusually low: thermal units that have spun freely on their ratchet can be re-conditioned by heating them to about 140F until the alloy re-fuses, then letting them cool [S5]. Because the alloy has to physically re-solidify before the mechanism can be re-engaged, eutectic units are inherently trip-free in the cool-down sense, and they cannot be force-reset in a way that bypasses the cool-down [S4][S5]. Reset on most legacy NEMA eutectic designs is manual only, so auto-restart after a trip is not a feature of this construction [S8].
How a bimetallic thermal overload relay trips
A bimetallic overload relay is built around two strips of metal with different coefficients of thermal expansion, bonded face-to-face and carrying (or sitting adjacent to) a series heater element fed by the motor current [S1][S4][S7]. As current rises above the design point, the strip deflects toward the lower-expansion side; once the deflection crosses a mechanical trip threshold, it opens the normally-closed contactor coil circuit and drops the motor out [S4].
Because the trip action is a smooth thermal bend rather than a phase change, bimetallic units are far easier to make ambient-compensated: a second "compensating" bimetal deflects the same way with room temperature, so the net trip threshold stays roughly constant from about -20C up to +60C depending on the OEM's calibration range [S1]. Many bimetallic designs also support auto-reset, the bimetal simply springs back as it cools, and the contactor re-energises on its own, which is why they are the default in IEC DIN-rail motor protection [S1][S7].
Side-by-side comparison: eutectic vs bimetallic
Both constructions are inverse-time devices, meaning a higher overcurrent trips faster, and both are typically specified in trip classes 5, 10, 20, and 30 seconds as defined for thermal overload relays [S4]. The differences that matter on a nameplate or a purchase order are listed below.
Reset behaviour: eutectic = manual only on most NEMA designs, alloy must re-solidify; bimetallic = manual or automatic reset, depending on wiring and model [S1][S4][S8].
Ambient sensitivity: eutectic units require the relay and motor to be at the same ambient temperature, or sizing must be re-corrected from the controller-ambient table, otherwise they nuisance-trip; bimetallic units are routinely built as ambient-compensated across a wide band [S1][S3].
Heater selection: eutectic uses discrete, replaceable heater elements ordered by motor FLA and slip into the relay base, so changing motor rating means changing the heater; bimetallic units are usually dial-adjustable over a 1.5:1 to 2:1 current ratio without heater changes [S1][S2].
Failure mode: a eutectic element can be permanently damaged by repeated forced reset before full cool-down, with documented cases of the alloy separating inside the pot, especially on legacy "N" style elements that shared a solder pot across heater sizes; bimetallic strips rarely fail catastrophically and instead drift in trip point with age [S1][S5].
Auto-reset and remote features: neither offers the diagnostics, comms, or phase-loss memory of an electronic (solid-state) overload relay, but eutectic units are also limited on remote reset, while bimetallic units at least support auto-reset wiring out of the box [S6][S7][S8].
For a full primer on the broader thermal relay family including electronic units, the encyclopedia entry covers trip class definitions and inverse-time curves.
Where the eutectic design still wins
Field experience on heavy industrial and institutional sites keeps coming back to the same conclusion: heater-type units, especially eutectic, have "almost zero points of failure" and outlast electronic overloads on the same motors [S3]. For a 3 hp single-phase submersible pump on a legacy NEMA Size 1 starter, the consensus among electricians is to use a melting-alloy element in a Class 10 trip, not a bimetallic, and not an electronic, because the motor cannot tolerate repeated stalled-rotor heating and the installer wants the simplest possible protective device [S3].
Eutectic relays are also preferred where vibration, dust, or corrosive atmospheres would attack the small bimetal welds and adjustment mechanism of a bimetallic unit, since the eutectic element is a sealed alloy pot with no fine mechanical adjustments to drift [S3][S5]. The trade-off is no auto-reset: a tripped eutectic relay demands a human visit, which is a feature in unattended pump applications where a repeat-trip is a symptom that must not be silently re-started [S3][S5].
Where bimetallic units are the better pick
IEC-style motor protection, where starters are DIN-rail mounted inside climate-controlled cabinets, almost always uses ambient-compensated bimetallic relays because the enclosure and the motor are at very different temperatures, and a non-compensated eutectic would nuisance-trip on cold mornings or under-ventilated panels [S1][S3][S7]. Adjustable trip dials (typically 0.6-1.0 of the heater's nominal current) let one relay cover a range of motor ratings, which is materially cheaper in stocking cost than a shelf of eutectic heater elements [S2][S7].
Bimetallic also fits when auto-reset is wanted for safety or process reasons, for example, a remote pump station where a brief overload should clear itself rather than latching the motor off, or a conveyor where a momentary jam should not stop the line. Auto-reset on a bimetallic relay must still be applied carefully, because repeated thermal cycling will damage the protected motor, exactly the failure mode the relay exists to prevent [S1].
Standards, trip classes, and the comparison to electronic units
Both eutectic and bimetallic thermal overload relays are classified by trip class, the maximum time in seconds within which the relay must trip from a cold start at 7.2x the current setting, with Class 5, 10, 20, and 30 being the common values for motor protection [S4]. IEC 60947-4-1 is the governing product standard for thermal and electronic overload relays used with contactors, and it defines both the trip class and the conventional non-tripping / tripping current boundaries; NEMA motor protection practice in North America aligns with the same trip-class conventions inside the starter [S1][S2][S4].
Compared with electronic (solid-state) overload relays, both heater types share two limitations: no continuous motor-winding temperature modelling from embedded PTC sensors, and no native fieldbus diagnostics; eutectic adds the constraint of manual-only reset, while bimetallic at least covers auto-reset and ambient compensation [S6][S7][S8]. For a more general look at how thermal sensing principles compare across very different form factors, the thermistor vs bimetallic heat detector write-up applies the same deflection-vs-phase-change logic to fire detection, and the broader thermal imaging camera encyclopedia entry covers the non-contact side of the same temperature-sensing family.
Decision rule for the next starter on the bench
Pick eutectic when the motor and relay share the same ambient, the duty cycle is rough (submersible pumps, crushers, conveyors in dirty plants), and you want a relay that physically cannot be re-closed before it has cooled, accepting manual reset as a feature [S3][S5]. Pick bimetallic when the relay lives in a cabinet at a different ambient than the motor, you need adjustability over a current range, or auto-reset is required by the process, accepting that the trip point is less absolute than a phase-change event [S1][S3][S7].
On the next spec sheet, the first item to lock is the trip class (Class 10 is the safe default for standard induction motors; Class 20 or 30 only for high-inertia loads), the second is whether the installation is ambient-compensated, and only then choose the heater technology. For details on how inverse-time curves and trip classes are written into IEC 60947-4-1 and NEMA starter selection tables, the thermal relay reference page links back to the same conventions Schneider, Siemens, and Eaton publish in their 9065CT9701 and Sirius catalogues [S1][S2].
Spec-level background on the components involved: melting furnace.