[Rick Parker]

Get the Volt/Ohm meter out, and start looking for 12 Volts & direct shorts; first within the wiring circuit from the toggle switch to the relay (remove the wire from the relay to fan). Power the circuit (switch on, with fuse replaced) and check the spade at the relay for 12 Volts. If this checks out ok, next focus on the wire run from the Relay to the fan (without connecting to the relay) disconnect from the fan itself while checking it for shorts to ground. Lastly check the Fan Motor. 30 amps is alot of current, for it to melt the fuseholder (Is the holder rated for 30 amps?) you have something drastically wrong that could cause a real electrical fire. You should be using 10 gauge wire on that circuit too.
Find the problem and verify how the circuit is wired and protected. Blas makes good points.