The PTC thermistor mz6 series sensors are temperature sensitive, and positive temperature coefficient thermistors (PTCs) have higher resistance values at higher temperatures. They belong to semiconductor devices and can be used in series with a single or multiple motor protectors.
RTD (Resistance Temperature Detector) is a sensor whose resistance changes as its temperature changes. The resistance increases as the temperature of the sensor increases. The resistance vs temperature relationship is well known and is repeatable over time. An RTD is a passive device. It does not produce an output on its own. External electronic devices are used to measure the resistance of the sensor by passing a small electrical current through the sensor to generate a voltage. Typically 1 mA or less measuring current, 5 mA maximum without the risk of self-heating.