What Is a Thermal Overload Relay and How Does It Work

What is a thermal overload relay? It is a protective device wired between a contactor and a motor that measures the motor's running current and opens its auxiliary contact when that current stays above the motor's full-load current (FLC) long enough to overheat the windings, per IEC 60947-4-1. Left uncovered, that condition burns motor insulation in minutes rather than the milliseconds a short circuit takes, which is why the relay's job is separate from a fuse or breaker's job. This article covers what the relay does, where it sits in the starter, how the bimetal element trips, how the dial is set, what trip class means, and the difference between hand and auto reset.

What the Relay Actually Protects Against

A motor overload is not a short circuit. It is current running higher than FLC for a sustained period: a jammed conveyor, a pump running against a closed valve, a bearing seizing up, or simply an undersized motor for the load. None of these trip a fuse or an MPCB's magnetic element fast, because the current involved is often only 1.2-3x FLC, well below a short-circuit fault current. Left alone, that current cooks the winding insulation over minutes. The thermal overload relay is built to catch exactly that band of current and that time scale.

Where It Sits in the Motor Starter

Three devices make up a classic direct-on-line starter: a short-circuit protective device (SCPD — a fuse or an MPCB), a contactor, and the overload relay. The SCPD clears short circuits and ground faults fast, in milliseconds. The contactor switches the motor on and off, thousands of times over its life, but has no trip logic of its own. The overload relay clips or bolts directly onto the contactor's output side, carries the same motor current through its three current paths, and its NC auxiliary contact (terminals 95-96) wires into the contactor coil circuit. Trip the relay, and the coil drops out, the contactor opens, and the motor stops — the relay itself switches no load current. This three-part arrangement is documented in the thermal overload relay engineering guide alongside the coordination tables that link SCPD size, contactor rating, and relay range.

What we see in the field: panel builders sometimes treat the overload relay as an afterthought once the contactor and motor protection circuit breaker are picked, but the relay's setting range has to bracket the actual motor FLC, not just fit the frame. A relay rated 9-13A holding a 14A motor at max dial does not protect it — it just delays the eventual trip.

How the Bimetal Element Trips

Inside a bimetallic overload relay sit three bimetal strips, one per phase, each wrapped with a heater winding or carrying the motor current directly on larger frames. Current through the strip heats it; the two bonded metals expand at different rates and the strip bends toward the low-expansion side. Bend far enough, and a trip bar releases the NC contact. All three strips feed the same bar, so an imbalance between phases — one strip bending faster than the others — trips the relay before any single strip reaches full deflection. That differential action is what gives a well-built bimetal relay its phase-loss sensitivity, without any electronics involved.

The bend-and-release behavior follows an inverse-time thermal curve: a small overload, say 1.2x FLC, takes minutes to trip, while a large one, 6x FLC or more, trips in seconds. That curve is deliberately shaped to track the motor's own heating curve, and it is also why the relay tolerates a motor's inrush current (typically 6-8x FLC for a few seconds at start) without nuisance tripping. Electronic overload relays reach the same inverse-time behavior differently — a current transformer or shunt feeds a microcontroller that runs a thermal model of the motor in software rather than in metal — and add extras like stall detection, ground-fault sensing, and thermal memory that survives a power loss. That comparison is covered separately in thermal vs electronic overload relays.

Setting the Dial to the Motor

The set dial on a bimetallic relay is calibrated directly in amps, and the rule is simple: set it to the motor's nameplate FLC, not to the contactor rating or the SCPD size. Set it below FLC and the relay nuisance-trips during normal running or at every start. Set it above FLC and the winding gets less protection margin than the standard assumes. Service factor and duty cycle can justify a small margin either side, but the FLC number from the nameplate is the anchor.

One exception matters enough to flag here: in a star-delta starter, if the overload relay sits in the delta leg, it only sees phase current, which is line FLC divided by the square root of 3 — about 0.58x the line value. Set it to the line FLC in that arrangement and it never trips on a real fault. The full sizing walkthrough for that case is in overload relay sizing for star-delta starters. For everything else, the step-by-step selection process is in how to select and set an overload relay for a motor.

Trip Class: Why It Is Not One Setting Fits All

Trip class describes how long the relay tolerates a stalled or slow-starting motor before it trips, tested at 7.2x the current setting from cold, per IEC 60947-4-1. Class 10A trips in 2-10 seconds, Class 10 in 4-10 seconds, Class 20 in 6-20 seconds, and Class 30 in 9-30 seconds. Standard pumps and fans run up in a few seconds and suit Class 10 or 10A. High-inertia loads — large fans, centrifuges, crushers — take longer to reach speed, and a Class 10 relay reads that extended run-up as a stall and trips before the motor is even up to speed. Pick a class whose curve sits above the motor's actual run-up current for the full run-up time, or accept nuisance trips on every start. Full detail on matching class to load is in overload relay trip classes 10A, 10, 20 and 30 explained.

Reset: Hand, Auto, and the Stop/Test Button

Once tripped, a bimetallic relay needs the bimetal strips to cool before it can reset — that takes a couple of minutes, not seconds. Hand (manual) reset forces someone to press a button and, implicitly, to check why the motor tripped before it restarts. That is the default on most industrial motors, because auto-restarting a motor into the same jam or seized bearing rarely ends well. Auto reset re-closes on its own once the bimetal cools, and it has a place — some pump and sump applications where an unattended restart is genuinely safe and desired. Most relays also carry a STOP/TEST button for manual tripping during commissioning, plus the standard NC (95-96) and a separate NO (97-98) contact pair for signaling. The reset decision, and when auto reset is appropriate, is covered in more depth in manual vs automatic reset on overload relays.

Ambient temperature plays into this too: a compensating bimetal inside the relay keeps the trip point roughly stable between about -5 and +55/60°C ambient, so the relay is meant to sit at the same temperature as the motor's control gear. Mount it against a hot cable duct inside a poorly ventilated panel, and an uncompensated or poorly-mounted unit drifts off its nameplate trip curve — this depends on how much the panel interior runs above the relay's rated ambient, which varies more than most bills of materials account for.

Where Overload Relays Come From

Schneider Electric's TeSys LRD is the bimetallic line that clips directly under LC1D contactors, spanning roughly 0.10A to 630A across the LRD and LR9F family, Class 10A, phase-loss sensitive, temperature compensated. ABB's TA line (TA25DU up through TA200DU and larger) mounts under A/AF contactors on the same principle, and its E-series electronic units add selectable Class 10/20/30 and a wider 1:3 to 1:4 setting ratio. Siemens' SIRIUS 3RU21 mounts onto 3RT2 contactors in frame sizes S00 through S3, with the 3RB30/3RB31 electronic line adding ground-fault detection on the 3RB31. All three brands, plus a full range of thermal overload relays, are stocked for export from Stoklink; the standards each one is built to, and how coordination tables tie the relay to its contactor and SCPD, are laid out in IEC 60947-4-1 contactors and motor starters standards.

Conclusion

A thermal overload relay's job is narrow and specific: catch sustained current above the motor's FLC before the windings overheat, and leave short-circuit clearing to the SCPD. Get the dial set to nameplate FLC, the trip class matched to the load's actual run-up time, and the reset mode matched to whether an unattended restart is safe, and the relay does its job without nuisance trips or blind spots. Everything downstream of that — coordination tables, star-delta sizing, brand-specific ranges — builds on those three decisions.