The History of the Thermal Relay: From Watchmaking to the Connected Industry

By the electrotuto.com technical team – Automation Expert Guide

Did you know that the protection of your 15kW three-phase motor today relies on an invention originally intended… to avoid getting lost at sea? Let’s dive into the fascinating history of the thermal relay, this silent guardian of our industrial installations.

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1. Before the thermal relay: The “All or Nothing” Era

At the beginning of the industrial era, the protection of electric motors was rudimentary. Fuses were mainly used. Although effective against sudden short circuits, fuses had a major flaw: they are unable to “sense” a slight but prolonged overload.

The result? The motor would heat up slowly, the insulation would melt, and the machine would eventually burn up before the fuse even thought of melting. Technicians of the time had to monitor the temperature by touch or by the smell of burnt varnish. An imprecise method, we can agree.

2. The Genesis: The gift of watchmaking (1759)

The history of the thermal relay begins long before modern electricity. In 1759, the British clockmaker John Harrison was looking for a solution to keep his marine chronometers accurate despite temperature changes during sea voyages.

He invented the bimetal strip: two strips of different metals (often steel and brass) welded together. Since these metals expand differently under heat, the strip curves. Harrison used it to compensate for the springs in his watches.

3. Adapting to electricity: The Joule Effect enters the stage

It wasn’t until the 20th century that this mechanical invention met electricity. The principle is brilliantly simple: the motor current is passed through small heating resistors that surround the bimetal strips.

• Slight overload: The bimetal strip heats up, curves slowly, and eventually pushes a tripping mechanism after a few minutes.
• Heavy overload: The curvature is rapid, and tripping occurs in a few seconds (Class 10 or 20).

This was the birth of the differential and compensated thermal relay. “Compensated” means it has a fourth bimetal strip that cancels out the effect of the electrical cabinet’s ambient temperature. “Differential” means it detects if a phase is missing, protecting the motor against destructive single-phase operation.

4. Where are we heading? The Era of Electronic Relays

While the good old electromechanical bimetal relay remains indestructible in small installations, Industry 4.0 is changing the game.

Today, we are moving to electronic protection relays (such as the Schneider TeSys or Siemens Sirius ranges).

• No more bimetal strips: Current sensors (CTs) precisely measure the intensity.
• Embedded intelligence: They calculate the motor’s “thermal image” in real-time via a microprocessor.
• Connectivity: They send an alert to your smartphone or the PLC even before the motor stops. This is predictive maintenance.

Conclusion

The thermal relay is a perfect example of “low voltage” technology that has survived through the centuries. From 1759 marine watchmaking to 2026 artificial intelligence, it remains the life insurance for your motors.

Want to learn how to precisely adjust your relay to avoid nuisance tripping? Check out our complete guide on adjusting the thermal relay.

Learn more: To master the basics of electronics, don’t miss our guide on Ohm’s Law.