Thermal protection relays

How does a thermal overload relay work?

The thermal relay is a protection device against electrical overloads. It does not protect against short circuits and has no breaking capacity. This is the reason why thermal relays are often used with contactors. The thermal relay opens the control circuit of the contactor, which then cuts off the current.

Inside a thermal relay, there are 3 bimetallic strips which bend when heated and switch on the cut-off device when the set threshold is exceeded. 

After tripping, you have to wait for the bimetallic strips to cool before resetting the thermal relay.

What is the difference between a thermal overload relay and a motor circuit breaker?

Thermal relays only provide thermal protection (against overloads). Motor circuit breakers provide magnetic protection (against short circuits) and have a breaking capacity.

Why install a thermal overload relay?

In the industrial sector, operating losses can reach thousands or even millions of euros, and a single motor fault can stop an entire production line.

That's why electrical industry professionals recommend all motors should be protected against overloads, which can lead to a gradual loss of motor power. The increase in heat inside the motor can then damage the magnetic circuit needed to generate the motor torque.

If the electrical overload is too great, it can also destroy the motor by melting the insulation. The rotor jams due to the accumulation of molten material in the air gap, causing a short circuit. 

Additionally, starting and/or speed variations can cause parasitic electrical currents in the installation. With a three-phase AC supply, the sum of all the parasitic currents will accumulate in the Neutral wire and cause very high currents, even if the installation is balanced.

What are the different thermal overload relays?

There are 2 types of thermal relays:

Bimetallic overload relays, known as electromechanical thermal relays, detect abnormal motor overheating caused by prolonged current absorption. 

Electronic thermal relays, where bimetallic strips have been replaced by a more powerful electronic device that greatly improves tripping threshold accuracy.

How do I choose a thermal overload relay?

There are 3 essential criteria for choosing a thermal relay:

• The rated current of the thermal relay, which must correspond to the rated current of the motor connected to it
• The setting current of the thermal relay should be lower than the current rating of the motor
• The tripping time of the thermal relay, which allows the motor to start without tripping the protection devices

Finally, the choice of thermal overload relay also depends on the model of the contactor already in the installation (to ensure compatibility) and the price you want to pay for it.

Remember also to choose the right class of thermal overload relay, class 10 to be placed upstream of a motor with normal starting (low inertia) and class 20 for a motor with high inertia, i.e. driving heavy loads and therefore taking a long time to start. These classes correspond to different tripping times (standard IEC 60947-4):

• Class 10 = tripping time at 7.2 rated current between 4 and 10 seconds
• Class 20 = tripping time at 7.2 rated current between 6 and 20 seconds

Thermal overload relays for TeSys D contactors

To meet the needs of the industrial sector, One-Elec offers TeSys LRD relays from Schneider Electric that are compatible with TeSys D contactors. Class 10 or 20, depending on the type of motor to be protected, these thermal relays provide overload protection for motors up to 630 A.

The thermal protection setting range is between 0.1 and 630 A.

These relays are available in electromechanical and electronic versions, depending on the setting accuracy required.