Why is both RTD temperature and thermal overload protection needed?

Although both methods give protection against overload and thermal rise, both have their own pros and cons. Especially with the important objects, both methods are used because they complement each other.

RTD basics

We know metal has the property of varying resistivity with respect to temperature. This relationship is linear for some metals, over wide temperature ranges made them a suitable choice for measuring temperatures. A Resistance Temperature Detector (RTD) is essentially a resistor designed to measure temperature by metals’ng the already known resistance vs. temperature relatimetals. Which made the RTD element as the de facto temperature-sensing unit.

RTD’s are used to measure temperatures of protected device windings or shaft bearings. A temperature rise may indicate overloading of the protected device or the beginning of a fault.

Thermal overload protection basics

Thermal overload protection is based on a mathematical model of the thermal behavior of the protected device (motor etc.) The protection relay continuously measures the phase current amplitudes and calculates the thermal image. The thermal model might consist of one or more time-constant, separate protection for stator and rotor, etc.

Why both are needed?

Although both methods give protection against overload and thermal rise, both have their own pros and cons. Especially with the important objects, both methods are used because they complement each other.