Well, an 8A Polyfuse will take quite a while to trip. Also, is that an 8A hold or an 8A trip? Here's an 8A hold PTC, it's trip current is 16 amps!
If you look at the Bel Fuse 0ZRB0800FF1A Datasheet, you'll see that it takes quite a while at 16A for it to trip.