[Sid Squid]

If it's only ever the main fuse that pops, check all the other fuses are the correct ones. If they are, then the fault is located in one of a few quite specific areas:
Solenoid
Black-red wire from the solenoid to;
1 regulator (there's two, they're doubled)
2 fuse labeled 'fuel'
3 ignition switch
Ignition switch itself
Orange wire from ignition switch to;
1 left switchgear
2 fuse board

If the fault were further down the system, the relevant sub circuit fuse would pop, the above mentioned areas are the only ones that are protected solely by the main fuse. This of course assumes that no-one has used a screw instead of the correct fuse in one of the sub circuits. All bets are off then.
The wiring in the main loom is well protected, so the fault likely lies in one of the branches that go to the above mentioned areas, check them carefully for damage before looking deeper.