Polyswitch fuse how they work

My polyswitch fuse has a trip current 0f 0.5A Am I correct in saying the fuse will only trip if the fault current is 0.5A or over? If so then what happens if it's only say 0.3A

Yup. It will only trip when current is 0.5A and above. It will continue as closed circuit if current is less than that

thanks john blue

Unlike a conventional fuse, the actual current at which a polyfuse will trip depends on several factors.
The ambient temperature affects the fuse rating, as does the amount of 'extra' current flow, and the time a taken for the fuse to trip will depend on the temperature rise, caused by a specific excess current flow.

In the case of a 'dead short' everything happens pretty quick. In the case of just a small excess current, the fuse can take a while to 'blow'.

The datasheets show the graphs, but it can be difficult to be precise, because of the variable temperature involved.

The other thing with a polyswitch is that the supply voltage has an effect on the reset current. With a low supply voltage, there won't be much heating and the current will be held near the trip current.

With a large supply voltage, the polyswitch will get really hot, so its resistance will increase a lot, and the current will drop to a small fraction of the trip current. The normal load may have to be disconnected, and the polyswitch allowed to cool, before it will reset.

is there a better alternative then? My supply voltage is only 5v since it's an arudino type board am building.

use crowbar circuit