2005 Ford Exped. Blown Starter Fuse?

Every time I star the ignition , the starter fuse blows.  Have already checked the starter relay..it's ok.  Have replaced the starter and still fuse blows.  Have no idea what the problem is.  Currently have an aircraft grade resetable fuse in place of the 30 amp maxi-fuse that came as the original equipment.  Aircraft fuse has a small delay time, but it will now start and has not tripped the aircraft fuse yet.  Any suggestions on what the underlying problem is????

Make sure the signal wire to the starter is not touching anything. if it grounds it will blow the fuse.