Electrical Problem With 2001 Chevy Express

I am driving a 2001 Chevy Express. It keeps blowing the main light fuse (40 amp). When it blows the engine will not start. The first time it blew I could start the engine.

When I replace the fuse there is a large spark and sometimes the new fuse blows before it is in place. When the weather is hot the fuse blows more often.

I am going thru too many fuses.

Any ideas as to where the problem may be.