how ptc thermistors are used for over current protection and also for liquid level measurement?
please explain.

 

PTC's are made of materials so that, increase in electrical resistance when temperature increases. As the temperature increases, the resistance increases, so less current flows, there by avoiding inrush of current.

 

If you had two resistors in a voltage divider network; one fixed, one a PTC thermistor, both would be warmed by the current passing through them, making them hotter than the ambient air temperature. If they were then immersed in a liquid, the PTC resistor's being chilled would reduce it's temperature, thus it's resistance. This would be measureable by voltage across the divider.