A thermal overload relay is the small protective device that watches the current your motor draws and disconnects it before sustained overload cooks the windings. It does this with no electronics: motor current heats three bimetallic strips, and when they bend far enough, the relay opens a normally closed contact wired into the AC contactor coil circuit, dropping the starter out. This guide explains the working principle, the 95-96 and 97-98 contacts, trip classes, and the settings that keep the relay accurate.
A thermal overload relay is an inverse-time motor-protection device used with a contactor to build a direct-on-line (DOL) motor starter. The contactor switches power to the motor; the overload relay decides when the motor has been drawing too much current for too long and trips the starter.
It protects against the damage pattern of an overloaded motor, which is heat: a jammed conveyor, a failing bearing, or an over-tight process keeps current moderately above the motor’s full-load current for seconds or minutes, and the motor winding temperature climbs until the insulation fails. The relay mimics that heating and trips before the motor reaches its damage point.
An overload is not a short circuit. Short-circuit current is extremely high and must be cleared in milliseconds by a fuse or breaker upstream; a thermal overload relay has no arc-breaking capacity and must never be expected to interrupt fault current. If the contactor side of the starter is new to you, read what a contactor is and how it works first.
The working principle is thermal expansion. Inside the relay, the three motor phases each pass through a heater element paired with a bimetallic strip — two different metals bonded together that expand at different rates when heated. The strip bends toward the metal with the lower expansion coefficient.
A motor draws five to eight times its full-load current for a few seconds during starting. That burst heats the strips only slightly because the delay is short, so the relay does not nuisance-trip on a healthy start. A genuine overload, however, holds the current high for much longer, the strips keep absorbing heat, and the relay trips. This “thermal memory” is an advantage: after repeated starts or an earlier overload, the relay remembers the accumulated heat and trips faster, just as the motor itself is closer to its thermal limit.
Although manufacturers package relays differently, the functional parts are consistent across IEC-style units.
| Часть | Функция |
|---|---|
| Heater elements | One per phase; convert motor current into heat that acts on the bimetallic strips |
| Bimetallic strips | Bend with temperature; provide the inverse-time trip characteristic |
| Differential mechanism | Inner and outer pushrods that amplify strip movement for fast phase-loss tripping |
| Trip bar and snap-action mechanism | Converts slow strip deflection into a decisive contact change |
| Auxiliary contacts 95-96 / 97-98 | NC trip contact for the coil circuit; NO contact for alarm or PLC signalling |
| Current-setting dial | Calibrates the trip point to the motor nameplate current |
| Test, STOP, and reset controls | Simulate a trip, open the NC contact manually, and reclose after cooldown |
| Ambient-compensation bimetal | Reacts to enclosure temperature only, cancelling its effect on the trip point |
IEC overload relays use a dedicated terminal block for the trip contacts: 95-96 is normally closed и 97-98 is normally open. They are small auxiliary contacts rated for control current only — never for the motor main circuit.
| Terminals | Normal state | After trip | Typical job |
|---|---|---|---|
| 95-96 | Closed | Open | Wired in series with the contactor coil to stop the motor |
| 97-98 | Open | Closed | Signals an overload trip to a PLC input or alarm |
The 95-96 contact sits in series with the contactor coil. In a typical control circuit the power path runs: STOP button (NC) → overload relay 95-96 (NC) → START button (NO) → contactor coil A1-A2. When the relay trips, 95-96 opens, the coil loses power, and the main contacts open regardless of what the START button or PLC output is doing. This is the fail-safe arrangement: the overload contact must be NC so that a tripped relay always de-energizes the starter. For the rest of the coil and auxiliary circuit, see how to wire a contactor: power, coil, auxiliary and overload wiring.
The 97-98 contact closes when the relay trips. Wire it to an alarm lamp, a PLC digital input, or a remote monitoring system so operators learn why the motor stopped and can block an automatic restart until the cause is cleared.
A three-phase motor running on two phases draws a dangerously high current in the remaining phases and overheats quickly. Simple single-bimetal relays react slowly to this condition because the two healthy phases heat only moderately. The differential mechanism solves this: when one phase is lost, its strip cools and moves one pushrod, while the overloaded phases bend the other pushrod further. The relative movement between the two pushrods trips the relay far faster than the absolute heating alone would. Not every model offers the same phase-loss sensitivity, so check the datasheet when the motor is critical.
The trip class defines how long the relay may take to trip at 7.2 times its current setting (roughly locked-rotor current) with a cold start. Under IEC 60947-4-1, the common classes are:
| Trip class | Typical trip time at 7.2× setting (cold) | Best for |
|---|---|---|
| Class 10A | 2 to 10 seconds | Normal motors that accelerate quickly (pumps, fans, compressors) |
| Class 10 | 4 to 10 seconds | General industrial motors with normal starting duty |
| Class 20 | 6 to 20 seconds | High-inertia loads with long starting times (crushers, mixers, flywheels) |
| Class 30 | 9 to 30 seconds | Heavy starts under extreme load where start current stays high |
Select the class from the motor’s starting characteristic, not from habit. A Class 10 relay on a high-inertia load nuisance-trips during every start; a Class 20 or 30 relay on a quick-starting motor lets it run too long into an overload.
The current-setting dial calibrates the relay to the motor. Set it from the motor nameplate full-load current (FLC), which must fall inside the relay’s adjustable range. Turning the dial up to “make nuisance trips go away” simply removes the protection — the relay is calibrated to the motor, not to convenience. Account for the motor service factor, ambient temperature, and duty cycle where the manufacturer instructs.
Press TEST to simulate an overload trip and confirm the starter drops out and the alarm fires. Press STOP to open the NC contact manually without waiting for the strips to cool. A regular test is the only way to prove the protection path really works — verify that the contactor actually opens when the relay trips.
Modern starters increasingly use electronic overload relays, but thermal relays remain the economical standard for most fixed-speed motor applications.
| Характеристика | Thermal overload relay | Electronic overload relay |
|---|---|---|
| Датчик | Bimetallic strips heated by motor current | Current transformers feeding a microprocessor thermal model |
| Accuracy and repeatability | Good but affected by ambient temperature | High and consistent |
| Thermal memory | Inherent (mechanical) | Modelled digitally |
| Diagnostics | Minimal | Phase loss, current imbalance, ground fault, communications |
| Стоимость | Low | Выше |
Choose a thermal relay for simple, cost-sensitive, standard-duty motor starters; choose an electronic relay when the motor is critical, starting duty is severe, or you need remote diagnostics.
A practical selection sequence is: read the motor nameplate FLC, pick a relay whose adjustable range brackets that FLC, confirm the trip class matches the starting duty, check that the frame fits your contactor (direct mounting saves wiring), and size the upstream fuse or breaker for coordinated short-circuit protection. Many contactor ranges have matching overload frames — the CJX2-D (LC1-D) series contactors are designed to be paired this way.
Because the relay is only half of the starter, our guide to how to size and select an AC contactor and our walkthrough of choosing a contactor for a motor starter cover the same selection logic from the contactor side.
A thermal overload relay protects the motor from the one fault it cannot survive for long: sustained overcurrent. Bimetallic strips heated by motor current give it a natural inverse-time delay that ignores brief starting inrush, trips on genuine overload and phase loss, and remembers heat between events. Wire the 95-96 NC contact in series with the contactor coil, use 97-98 for alarms, set the dial from the nameplate FLC, and test the trip regularly. Do that, and the relay quietly does its job for years inside a motor starter.
Motor current heats bimetallic strips inside the relay. Because the two bonded metals expand at different rates, the strip bends as temperature rises; at the calibrated trip point a snap-action mechanism opens the 95-96 NC contact, de-energizing the contactor coil and stopping the motor.
95-96 is the normally closed trip contact, wired in series with the contactor coil so the starter drops out on overload. 97-98 is a normally open contact that closes on trip for an alarm lamp, PLC input, or remote indication.
An overload relay protects the motor from sustained overcurrent and has no short-circuit breaking capacity. A circuit breaker (or fuse) clears high fault current fast. Motor circuits normally need both: overload protection plus upstream short-circuit protection.
Usually the trip class is too fast for the starting duty, the current setting is too low for the actual starting current, or the relay is still hot from previous starts. Match the class and setting to the motor’s starting characteristic, and let the strips cool before restarting.
Many models offer automatic reset that re-closes the contacts once the bimetallic strips cool. It is convenient for remote installations but should not be used where an unexpected restart could endanger personnel — manual reset is the safer default.
No. It protects only against sustained overload and (on suitable models) phase loss. Short-circuit protection must be provided by a fuse, MCB/MCCB, or MPCB upstream of the starter.
