How thermal overload relays protect motors: trip classes 10/20/30, dial setting to motor FLC, phase loss, reset modes and contactor pairing, with a checklist.
A thermal overload relay is the part of a motor circuit that watches for one specific failure: a sustained overcurrent that is not a short circuit. It will not clear a fault, it will not switch the motor, and on its own it will not stop a stall from burning out the winding — but it is the cheapest, most reliable way to take a motor offline before heat destroys the insulation. This guide covers how it works, how to set it, and the mistakes that show up in panel audits.
The heart of the relay is a bimetal strip per phase, heated by the motor current itself. Heat bends the strip; at a calibrated deflection the mechanism trips a latch that opens the contactor coil circuit through the relay's 95-96 (NC) contact. The contactor then drops the motor. The response is deliberately inverse-time: a small overcurrent takes minutes, a large one takes seconds, and a genuine short circuit is left to the upstream breaker.
| Fault | Cleared by | Thermal relay's role |
|---|---|---|
| Short circuit | MCB / MCCB / fuse upstream | None — it must not be asked to break fault current |
| Sustained overload (1.2–7× FLC) | Thermal overload relay + contactor | Trips the contactor coil circuit |
| Phase loss / single-phasing | Thermal relay (differential type) or MPCB | Trips on the current imbalance |
| Stall / locked rotor | Motor protector (MPCB) preferred | Trips, but slowly — stall current behaves like an overload |
| Undervoltage / control failure | Contactor coil / control circuit | None |
Because the relay only signals, it must always be wired in series with the contactor coil — the classic error is landing the relay contacts across the coil, which turns a protection device into a fire risk.
The trip class is the maximum time, in seconds, the relay takes to trip when the motor draws 7.2× its full-load current. It exists to match the relay to the motor's run-up time: trip faster than the start ramp and every start becomes a nuisance trip; trip far slower and a real overload cooks the winding.
| Class | Trip time at 7.2× FLC | Typical loads |
|---|---|---|
| 10 | < 10 s | Light-inertia loads that reach speed quickly: compressors with unloaders, some pumps |
| 20 | < 20 s | The default for general industrial duty: conveyors, mixers, standard pumps |
| 30 | < 30 s | High-inertia or long run-up: fans, crushers, centrifuges, submersible pumps |
Choose from the load's acceleration time, not from habit. If the drive train needs 15 seconds to reach speed under full load, a class 10 relay will trip on every start — the fault is the relay, not the motor.
Every thermal relay has a setting dial spanning a range (for example 13–18 A on a mid-size frame). The setting is the motor's nameplate full-load current (FLC) — nothing else. Three rules:
Record the setting on the panel schedule. During audits, an unrecorded or contactor-sized setting is the single most common finding on motor circuits.
When one phase opens on a running three-phase motor, the remaining two phases carry roughly 1.7–2× normal current while the motor keeps turning (single-phasing). A three-element bimetal relay sees the imbalance directly and trips; a two-element relay with a differential mechanism uses the vector difference of the two heater currents to detect it. Either way the trip is slower than an overload trip — which is exactly why unattended motors are better served by a motor protector (MPCB) with explicit phase-failure protection, and why a thermal relay on a critical pump is a compromise, not a solution.
Most thermal relays offer both reset modes, selected by a small button or jumper. The rule is simple: any motor that must not restart on its own after a trip gets manual reset. Pump rooms, conveyors, crushers and anything a person could be standing next to when the relay trips all qualify. Automatic reset belongs only where a supervised, safe restart is acceptable — a fan in an occupied building with local control, for example. When in doubt, specify manual; it costs nothing and removes the restart question from the safety case.
A thermal overload relay is usually not a standalone DIN-rail device — it clips directly under its matching contactor, and the pair shares one part-number system. The classic pairings are LR2 under LC1-D contactors, 3UA under 3TB/3TF, and the equivalent family combinations in the same way. Mixing families is possible with standalone mounts and adapter kits, but price and reliability both suffer; where a datasheet lists a contactor range, treat it as binding.
Two mounting details matter at the panel-building stage: the relay must sit on the same DIN rail immediately below the contactor with the mechanical interlock engaged, and its main terminals must land on the load side of the contactor — never upstream, where the relay would be protecting the cable instead of the motor.
A motor protection circuit breaker combines overload and short-circuit protection in one adjustable device and adds explicit phase-failure, stall and imbalance detection. It is the right choice for small motors (up to roughly 25 A) where panel space and wiring matter. The thermal relay plus contactor combination remains the better choice for larger motors, higher switching cycles, and any board where the switching device must also serve as the local isolator. The two are not interchangeable on the same drawing — see when an MPCB beats an MCB for the full comparison.
For each motor, write down: motor FLC from the nameplate; trip class from the load's run-up time; reset mode (manual unless a supervised restart is acceptable); the contactor family the relay must match; and the setting you recorded. Five lines. A supplier quoting from that list returns the matched relay and contactor in one pass — ELEHPD stocks the thermal relay families (LR1, LR2, 3UA, LRD, TH and T) alongside the AC contactors they mount to, plus motor protectors where an MPCB is the better fit. For the general selection logic, see our relay selection guide.
Send the motor schedule — power, FLC, run-up time, environment — and we will confirm the matching thermal relay and contactor combination, the trip class, and the documents available for your market.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.