JohdotaA thermal overload relay protects a motor against overload. It is mounted directly under the contactor, and the motor current flows through it. The thermal overload relay does not break the power circuit itself. It stops the motor through the contactor.
How it works
- The motor current heats the bimetal strips in the relay.
- When the current is too high for too long, the strips bend and the relay trips.
- The NC contact 95–96 opens and breaks the contactor control. The NO contact 97–98 closes and can switch on a fault light.
- A tripped relay is reset with the reset button. On many relays you can choose manual or automatic reset.
Setting
The current setting is adjusted to the rated current on the motor rating plate. The trip class tells how fast the relay trips: a class 10 relay trips from cold in 4–10 s when the current is 7.2 times the setting.
Remember
- A thermal overload relay does not protect against short circuits. It needs fuses upstream, for example a switch-fuse. A typical motor feeder is a switch-fuse, a contactor and a thermal overload relay.
- When 95–96 is in series with the control and the control uses self-holding, the motor does not start by itself after a reset.
In your region
Showing notes for
United States
NEC 430.32 allows an overload trip rating of at most 125% of the nameplate current for motors with a service factor of 1.15 or more, or a temperature rise of 40 °C or less, and at most 115% for other motors. If the motor will not start, 430.32(C) allows up to 140% and 130%.
How to set the dial comes from the relay maker. One manufacturer says: set it to the full-load current (FLA) when the service factor is 1.15 or more, and to 0.9 × FLA when it is lower. The relay itself trips at 120% of the setting.
US trip classes are given at 600% of full-load current: a class 10 relay trips the motor within 10 s.
Source: Mike Holt, EC&M: motor calculations, NEC 430.32 (2019) · Rockwell Automation: E100 overload relay instructions 193-IN081 · Siemens: Basics of Control Components, part 2 (STEP 2000 course)
United States, Canada
On ladder diagrams the overload relay is marked OL. Its thermal elements, often called heaters, are drawn in the main circuit, and its contact in the control circuit opens when it trips.
Source: Siemens: Basics of Control Components, part 1 (STEP 2000 course) · NFPA 79 First Draft report, 2024 edition (13.2.3, Annex E Table E.1 device designations) · Square D (Schneider Electric): Wiring Diagram Book 0140CT9201, symbols and examples
Canada
The rules for overload settings in Canada are in CEC Section 28. How to set the dial comes from the relay maker. One manufacturer says: set it to the full-load current (FLA) when the service factor is 1.15 or more, and to 0.9 × FLA when it is lower.
Source: Electrical Industry News Week: motor rules in CEC Section 28 (2021) · Rockwell Automation: E100 overload relay instructions 193-IN081
UK & Ireland, Europe, Australia & New Zealand, Other IEC countries
No regional notes on this page yet. If something is different in your country, please send feedback.
Frequently asked questions
What current should a thermal overload relay be set to?
The rated current on the motor rating plate, for the voltage and connection in use. In star-delta starting the relay is usually after the main contactor, in the winding supply. It then carries the winding current and is set to about 0.58 times the rated current.
Why does a thermal overload relay trip?
The motor has drawn too much current for too long: an overload, a sticking bearing or load, a missing phase or a setting that is too low. Find the cause, and do not set it above the rated current.
How do you reset a thermal overload relay?
Let it cool for a moment and press reset. With automatic reset it resets by itself. A self-holding control still needs a new start press, but a rotary switch control restarts the motor. So use automatic reset only where an automatic restart is not dangerous.
What is the difference between a thermal overload relay and a motor protection circuit breaker?
An overload relay protects only against overload and does not break the motor current itself: it opens the contactor control, and the contactor stops the motor. Fuses are needed for short circuits. A motor protection circuit breaker has both protections, breaks the current itself and can isolate the feeder by hand.
Does a thermal overload relay protect against short circuits?
No, only against overload, so it needs fuses upstream, for example a switch-fuse. A typical motor feeder is a switch-fuse, a contactor and an overload relay.
What does trip class 10 mean?
The trip class tells how fast an overload relay trips. Class 10 trips from cold in 4–10 s at 7.2 times the set current.
Sources
- ABB: TF65 and TF96 thermal overload relays, technical data
- Trip classes in IEC 60947-4-1 (Schneider Electric community, in Indonesian)
- Schneider Electric FAQ: overload relay trip classes 10A, 10, 20 and 30
- Mike Holt, EC&M: motor calculations, NEC 430.32 (2019)
- Rockwell Automation: E100 overload relay instructions 193-IN081
- Siemens: Basics of Control Components, part 2 (STEP 2000 course)
- Siemens: Basics of Control Components, part 1 (STEP 2000 course)
- NFPA 79 First Draft report, 2024 edition (13.2.3, Annex E Table E.1 device designations)
- Square D (Schneider Electric): Wiring Diagram Book 0140CT9201, symbols and examples
- Electrical Industry News Week: motor rules in CEC Section 28 (2021)