Everything about Thermal Overload Relays: Operation, Selection, and Settings
⚠️ SAFETY: All operations on a thermal relay must be performed POWER OFF. Always verify the absence of voltage (VAT) before intervening.
The thermal relay is the most common safety device in the industry. However, it is often misunderstood or even bypassed out of frustration. A well-chosen and correctly adjusted thermal relay ensures a motor’s longevity is increased tenfold.
[IMAGE: A Schneider Electric thermal relay mounted on a contactor in an electrical cabinet – Alt: Industrial motor protection thermal relay]
How does the bimetal strip of a thermal relay work?
The heart of the relay consists of three bimetal strips (two metals with different expansion coefficients). In case of overload, the current intensity increases, heat rises due to the Joule effect, and the bimetal strips bend. This mechanical deformation releases a spring that switches the control contacts (95-96 / 97-98).
Expert Tip: Modern relays are known as “compensated”. This means they don’t trip faster if it’s 40°C in the cabinet, thanks to a fourth ambient temperature compensation bimetal strip.
What are the differences between a thermal relay and a motor circuit breaker?
Many beginners think that the thermal relay can cut the power current. This is false! If you open a thermal relay, you will see that the power contacts never move. It is the contactor that does the switching work. The thermal relay simply “tells” the contactor to open via the control circuit.
The motor circuit breaker (type GV2) has internal breaking capacity and also protects against short circuits (magnetic protection).
[IMAGE: Comparative diagram between a thermal relay and a motor circuit breaker – Alt: Difference between thermal relay and motor circuit breaker]
How to adjust the nominal current (Ir)?
Check the motor’s nameplate. Identify the nominal intensity (In) corresponding to your coupling voltage (e.g., 4.5A at 400V). Turn the adjustment potentiometer (Ir) exactly to this value.
Why not over-adjust?
If your motor consumes 4.5A and you set the relay to 6A to “be safe”, you are allowing the motor to heat up far beyond its thermal limit. The varnish insulation will bake, crumble, and the motor will eventually short-circuit. The setting is not a suggestion; it is a vital limit.
How to test and maintain a thermal relay?
- The TEST button: Activate it with a small screwdriver. It should simulate a fault and open contact 95-96. If nothing happens, the relay is mechanically seized.
- Manual vs Auto Reset: In “Manual” mode, you must physically press the blue button after a fault. In “Auto” mode, the relay resets itself after cooling. In industry, Manual mode is preferred to force the technician to inspect the cause of the overload before restarting.
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Learn more: To master the basics of electronics, don’t miss our guide on Ohm’s Law.
