What type of relay uses resistive heating to open contacts?

The correct answer is the thermal overload relay. This type of relay operates based on the principles of resistive heating, which occurs when an electrical current passes through a conductive material and generates heat. In a thermal overload relay, the current flowing through the relay heats a resistive element (typically a bimetallic strip) that bends as it gets heated. When the temperature rises to a predetermined threshold, the bending of the strip opens the contacts, thus interrupting the electrical circuit to provide protection for the motor.

This mechanism is effective for preventing overheating conditions that could damage the motor. The time it takes for the relay to trip is also adjusted based on the degree of overload, allowing for different operating characteristics depending on the application. This design ensures that normal starting conditions, which may draw higher current temporarily, do not cause the relay to trip unnecessarily.

The other options may involve related principles of operation but do not specifically use resistive heating to open contacts in the same manner.