I'll take an educated guess. I bet you used a little 1/2W or 1W pot for this experiment which is grossly undersized for that test. Next, your claim of applying 28V directly across this device makes no sense. 28V across a 0.105 ohm device would draw 267A. It should have tripped no problem in that test.
I don't think you understand what the PPTC device does or what the ratings mean. Basically, you want the operating current to be < 3A and the trip or fault current to be > 5.1A. If you read the datasheet, it says that that It(A) is the minimum current the device will trip at, meaning it may require more current to trip. The voltage rating is just the maximum voltage that should be applied across the device. It has nothing to do with the device tripping but you are exceeding this rating by expecting it to work on a 28V circuit.
Overall, you seem to be thrashing around in the dark. You started with a 5W resistor and expected <2W to burn it out and now you messing with >5A trip points on a power supply that doesn't seem capable of sourcing that much current. Your data jsut isn't making any sense. You first claimed 24V and 650 ohms gave 48mA which makes no sense. You're now claiming 28V directly across this device which also makes no sense.
This is Engineering Tips, a site for engineering professionals to get help with a specific issue tehy have run into. It's not a site where you should expect users to spoon feed someone who doesn't even seem to understand ohms law.
