How to Select PTC thermistors for Over Current Protection

Items to be checked
Selection criteria
How much (in volts) is the power supply voltage of the circuit in which PTC thermistor is used?
(Preventing PTC thermistor from being damaged)
-> Power supply voltage ≤ Maximum voltage of PTC thermistor
When the circuit is operating normally, how much (in amperes) is the current that flow through PTC thermistor?
(Preventing PTC thermistor from malfunctioning)
-> Normal current ≤ Non-operating current of PTC thermistor
When something abnormal such as a short circuit has occurred, how much (in amperes) is the current that flows through PTC thermistor?
(Ensuring reliable protection)
-> Abnormal current ≧ Operating current of  PTC thermistor
PTC thermistor application environment condition, esp. ambient temperature -> PTC thermistor reference temperature (also called Switch temperature or Curie temperature) >maximum operating ambient temperature