Understanding thermal overload relays: Pros and Cons

15th Nov 2023

Thanks to HVAC systems, we can enjoy a comfortable temperature in our homes and workplaces. But did you know that they also need to maintain a good temperature for them to function safely and efficiently? This

is what thermal overload relays are for.

A thermal overload relay is a device used to protect the system by switching the system off whenever overheating is detected. It acts as the “thermostat” of the “house” which is the electrical system since its role is to regulate the temperature of the circuit. It also protects the system against overload, single phasing, and voltage imbalance.

Working Principle of Thermal Overload Relays

Thermal overload relays operate based on the principle of thermal expansion. They use bi-metal strips that bend and open the contacts when an overload is detected. In short, thermal overload relays detect overheating caused by excessive current and then interrupt the circuit to prevent damage to the system or equipment.

Electronic Thermal Overload Relay

Advantages of Thermal Overload Relays

  • Higher accuracy

Thermal overload relays incorporate bimetallic technology and an electromechanical design, giving them higher accuracy.

  • Cost-effectiveness

Thermal overload relays have a straightforward and simple design which contributes to their cost-effectiveness. They are also cheaper than circuit breakers.

  • Ability to handle higher temperatures

Thermal overload relays are able to deal with higher temperatures due to their use of bimetallic strips.

  • Adjustable safety settings

Thermal overload relays enable adjustment of the current range to match the specific operational needs and characteristics of motors.

  • Ease of operation

It is relatively easy to operate thermal overload relays mainly due to their simple design and adjustable settings.

Disadvantages of Thermal Overload Relays

  • Slow response time

Compared to electronic overload relays, thermal overload relays have slower response times due to their limited sensitivity to small overloads and rapid changes in load.

  • Lack of precision due to environmental factors

Harsh environments and sensitivity to ambient temperature may contribute to lesser precision of overload thermal relays.

  • Requires regular calibration

To ensure proper functioning and accuracy, some overload thermal relays may require regular calibration, adding to the maintenance requirements.

How to Choose the Best Thermal Overload Relay

Here are the main factors to consider in choosing the right thermal overload relay for your system:

  • Current rating

The thermal overload relay’s current rating should match the equipment’s full-load current as seen on the nameplate.

  • Restart type

Thermal overload relays have two restart types: manual and automatic. The manual reset type requires human intervention to reset, while the automatic reset type doesn’t.

  • Adjustability

Look for a relay with customizable features to match the motor’s or equipment’s specific characteristics and requirements.

  • Reliability

Not all thermal overload relays are created equal. Choose a brand with a reputation for high-quality products.

Thermal overload relays are an essential part of a system. To utilize them better, it is also important to be aware of both their strong and weak points.

In choosing a thermal overload relay, reliability should be a top consideration. Having a poorly functioning relay may cost the safety of your home or business.

The TOSUNlux TSR2-D Thermal Overload Relay is among the best overload relays available in the market. It can be used for circuits with a frequency of 50Hz or 60Hz, a rated insulation voltage of 660V, and a rated current of 0.1 A – 93 A. It is effective for protecting motors against phase break and overload.

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