Thermal Overload Relay

What is a Thermal Overload Relay?

A thermal overload relay is an economical electromechanical device designed to provide secondary protection for motors and other electrical equipment against overloads and phase failures. It works by detecting excessive current draw that leads to overheating, ensuring the circuit is safeguarded from damage.

Delayed Response: How Thermal Overload Relays Prevent Unnecessary Shutdowns for Motor Protection

When used with contactors, thermal overload relays offer reliable motor protection while allowing for customized motor-starting solutions. Unlike fuses and circuit breakers, they operate with a slight delay, permitting harmless, temporary overloads without unnecessary interruptions.

However, if the motor draws dangerously high currents, the relay’s electrical coil heats up, triggering the contacts to cut off electricity flow. Once the relay cools down, these contacts can be reset manually or automatically, ensuring safe and efficient motor operation.

Features: How Does a Thermal Overload Relay Work?

The thermal overload relay utilizes a bi-metal trip mechanism consisting of several strips made of two metals with different linear expansion rates. Below is the process behind how this mechanism works:

1. When the temperature increases, one metal expands more than the other.
2. The resulting bending action operates a lever to open contacts, tripping the circuit.
3. Once the fault has cleared (either on its own or by the user), the bimetallic strips cool and return to their original, equal positions.
4. This closes the contacts, and the circuit can be powered again, either by an automated or a manual reset.

Benefits of a Thermal Overload Relay

A thermal overload relay offers several key advantages for engineers and technicians:

• An easy but effective operating principle that ensures greater accuracy and support to connected motors.
• Reliable protection for motors in the event of overload or internal phase losses.
• Easy installation that allows for quickly setting up customized motor starting solutions.
• Comes with both automatic and manual reset functions.
• Active over a wide and adjustable current range.
• Temperature compensation features for accurate functioning.
• Cost-effective and can be readily used anywhere.

What Are the Symbols for a Thermal Overload Relay

The thermal overload relay symbol is used in schematic drawings to indicate the presence and position of the relay. There are two standardized types of relay symbols used by engineers and electricians:

Joint Industrial Council (JIC) / National Fire Protection Association (NFPA)

The NEMA standard symbol is usually drawn in two different ways: as two opposing question marks or as a pulse waveform. This symbol is used to represent the heating element.

International Electrotechnical Commission (IEC)

The IEC standard symbol generally resembles the NEMA symbol, being a pulse waveform inside a rectangle representing the device’s heating element.

Make Drawing Electrical Motor Circuits Easier with Capital X Panel Designer!

Manually drawing out a thermal overload relay symbol can be difficult and time-consuming. That is why you should use Capital X Panel Designer instead! This CAD software comes with a library of pre-made symbols that you can simply drag and drop into your schematic, significantly boosting your production speed.

Learn more about how to generate electrical symbols automatically.