Could be a stray screw or nut across a wire and socket leg. Sometime you need to isolate by "zone" by cutting a bus line and temporarily installing a smaller (10 or 15 amp) fuse with a jumper wire to figure out where the fault lies.
All part of standard electrical troubleshooting techniques.
A 20 amp breaker, if too "slow" will at least help you get close to the problem. Melting insulation or discolored bus lines will get you close to the problem.