A 30 amp fuse is rated to carry 30 amps continuously. It will blow at a higher instantaneous current, usually +50% of rating. If yours is taking 30 secs to fail the current will probably be only a bit above 30 amps, but then Saab will have rated the fuse to the next size higher than the load, so you've got a current well above normal.
Remember a fuse only protects the wiring it doesn't protect the equipment itself!!
Further notes on overheating. A new fuse (i.e. not 8 years old) has remained intact. fitting a new radiator temp sensor switch has not improved matters. The fan is not cutting in on low speed untill temp gauge needle is almost on red and high speed is reluctant to kick in above that. any further advice available.
beneath the rad on the lefthand side(standing in front of car)there is a connector in the line from the rad thermo switch,check that is not corroded,then bridge the connections with a bit of wire to ensure that the fan does indeed run at both speeds,it sounds as if yours is only running at high speed,if no slow speed,suspect the resistor on the rad shroud,if that is open circuit you will need a new one,unless you have a strong heart do not go to Saab for a new one,they will want £234 I think was the last I heard,A friendly computor shop may find you one,it is a 2.2ohm built into a heat-sink,I bought two @57p each.if you have ac and the fan runs when you use it,then there is nowt wrong with the slow speed.