Thermal Overload Relays Selection Guide
A thermal overload relay senses the motor's overload current and drops out the contactor; selection looks at bimetal or electronic construction, scale range and contacts.
This guide explains what to check when selecting products in the Thermal Overload Relays category.
Thermal overload relay construction and contacts
In a bimetal thermal overload relay, the motor current indirectly heats a bimetal strip in each phase; the strips bend and trigger the tripping mechanism, and the position of the auxiliary contacts changes. The contacts are marked with standard numbers: 95-96 is the normally closed trip contact and 97-98 is the normally open signal contact. The value on the scale is the motor's rated current, not the trip current.
Many models have a manual or automatic reset selection and test and stop functions on the front. In the product data of the ABB relays in this category, phase loss sensitivity and ambient temperature compensation are stated; a phase-loss-sensitive relay trips earlier on a motor left on two phases than it would for a balanced overload.
Thermal overload relay types and series
- Bimetal thermal overload relays: Siemens 3RU2 and 3MU7, Schneider Electric TeSys LRD, TeSys K (LR2K) and EasyPact TVS (LRE), ABB TF42, TF65, TF96, TF140DU and TA200DU.
- Electronic overload relays: ABB EF and Schneider Electric LR97D and LR9D. They use an electronic circuit that measures the motor current instead of a bimetal. The product data of the ABB EF relays lists trip classes 10E, 20E and 30E together. In LR97D product names, a voltage such as 24 V AC/DC, 110 V AC or 220 V AC is written next to the setting range; the order code varies with this voltage.
- Mounting style: types that mount directly under the contactor and types that are mounted separately. On ABB, separate mounting is done with single mounting kits such as DB42 and DB65; these kits also appear in the same listing.
What trip class means
Trip class states, according to IEC 60947-4-1, the longest trip time in seconds at 7.2 times the relay's setting current; common classes are 10A, 10, 20 and 30. In ABB's thermal overload relay datasheet, the trip time at this current is given as 4-10 seconds for Class 10, 6-20 seconds for Class 20 and 9-30 seconds for Class 30. As the class gets larger, the relay allows a longer start without tripping.
Class 10 and 10A are used for loads that accelerate quickly, such as pumps and compressors; in this category the Schneider Electric LRD and TeSys K relays are listed as Class 10A, and the ABB TF42, TF65 and TF96 relays as Class 10. Loads with high inertia and long starts, such as large fans and centrifuges, need Class 20 or 30. In this case the contactor and cable are also selected from the manufacturer tables according to the long start.
Thermal overload relay selection steps
- Read the rated current on the motor nameplate and select the relay whose setting range contains this value; for example, a relay with a 9-13 A range for an 11 A motor. In star-delta starting, the setting value changes according to the leg the relay is connected to; see the manufacturer's diagram.
- Check contactor compatibility. On ABB, TF42 relays are for AF09-AF38, TF65 relays for AF40-AF65, TF96 relays for AF80 and AF96, and TF140DU relays for AF116 and AF140 contactors. For direct mounting of the Siemens 3RU2, the frame size (S00, S0, S2, S3) must be the same as the contactor's size; 3MU7 relays fit 3MT7 contactors. At Schneider Electric, LRD matches TeSys Deca, LR2K matches TeSys K, and LRE matches EasyPact TVS contactors.
- Determine the trip class according to the starting time.
- Choose the connection and mounting style. In a Siemens 3RU2 order code, if the second-to-last character is B the connection is screw, if C it is spring; the last digit 0 indicates direct mounting on the contactor and 1 indicates separate mounting.
- Complete the short-circuit protection: a fuse or a switch with short-circuit protection must be placed ahead of the thermal overload relay.
The difference between a thermal overload relay and a motor protection switch
A thermal overload relay only senses overload and stops the motor through the contactor. A motor protection switch combines an adjustable thermal trip with a magnetic short-circuit trip, interrupts the circuit with its own contacts and is operated manually; when this type of switch is used with a contactor, a separate thermal overload relay is not needed. Only on magnetic-trip-only types (for example ABB MO132) does a thermal overload relay still provide overload protection. For relays that monitor the current digitally and also offer low-current protection, see the Current Protection Relays category.
Order notes
The sales unit is the piece. Siemens and Schneider Electric product names state the setting range; the last number in the ABB type code is the upper value of the setting range (the range of the TF42-5.7 relay is 4.2-5.7 A). For products listed as made-to-order, the lead time is confirmed before ordering. You can reach the relevant relay by typing a manufacturer code such as LRD16 or 3RU2116-1GB0 into the search.
Comparison and selection table
| Selection Criterion | Direct Contactor-Mount Thermal Overload Relay | Separate (Adapter/DIN) Mount Thermal Relay | Electronic Overload Relay |
|---|---|---|---|
| Tripping Principle | Bimetallic strip deflection under overload (Trip Class 10A) | Bimetallic element mounted on standalone terminal base | CT-sensing microprocessor overload & phase-loss protection |
| Setting Range (Ir) | Narrow band matched to motor FLC (e.g. 4–6 A, 9–13 A, 12–18 A) | Fine dial adjustment across rated motor current band | Wide adjustment ratio (e.g. 5:1) + selectable Class 10/20/30 |
| Contacts & Reset | 1 NO (97-98 alarm) + 1 NC (95-96 trip), Manual/Auto reset | 1 NO + 1 NC auxiliary contacts, Test & Stop buttons | Minimal heat dissipation, phase unbalance & jam detection |
| Short-Circuit Note | No magnetic trip — must be backed up by fuses or MCCB/MCB | Requires upstream gG/aM fuse or circuit breaker | Ideal for heavy-duty starting (fans, crushers, centrifuges) |
Frequently asked questions
What amperage is a thermal overload relay set to?
It is set to the rated current on the motor nameplate. The relay scale shows the motor current, not the trip current: according to ABB's thermal overload relay datasheet, the relay does not trip at 1.05 times the setting value and trips within two hours at 1.2 times. Raising the setting above the nameplate current to prevent frequent tripping weakens the protection; the cause of the tripping must be found first. In star-delta starting, the setting value is taken from the manufacturer's diagram according to the leg the relay is connected to.
Why can't a thermal overload relay be reset right after it trips?
Because the heated bimetal strips cause the trip, resetting the relay depends on the strips cooling; this wait also gives the motor windings time to cool. In the manual reset position, the relay is reset by pressing the button after it has cooled. In the automatic reset position, the contact closes by itself; if the control circuit has a maintained signal, the motor can restart on its own, so this position is used only where the project provides for it.
Does a thermal overload relay protect the motor on phase loss?
Phase-loss-sensitive thermal overload relays do. When one phase is lost, the motor draws a higher current from the remaining two phases; a relay with a differential mechanism trips earlier on this imbalance than for a balanced overload. In the product data of the ABB relays in this category, phase loss sensitivity is stated, and the Schneider Electric TeSys K product names include the term differential. Because the relay looks at current, it does not detect a phase sequence error or a missing phase on the network while the motor is stopped; a phase protection relay is used for that.
What is the difference between an electronic overload relay and a bimetal thermal overload relay?
In a bimetal relay, strips heated by the motor current cause the trip; the construction is simple and no separate supply is needed. In an electronic relay the current is measured and an electronic circuit makes the trip decision. The practical result is a wider setting range and several trip classes in a single relay: in this category the ABB EF45-45 appears in the product data with a 15-45 A range and with classes 10E, 20E and 30E. Schneider Electric LR97D product names also include a voltage value; check this value in the order as well.
Where are terminals 95-96 and 97-98 on a thermal overload relay connected?
95-96 is the normally closed trip contact and is connected in series with the contactor's coil circuit; when the relay trips, this contact opens, the coil loses power and the contactor disconnects the motor. 97-98 is the normally open contact; it closes when the relay trips and is used to drive a fault lamp, a siren or a PLC input. On the power side, the motor current passes through the relay's three poles. The manufacturer's circuit diagram is the basis for connection.