mystery electrical (burnt fuse) problem

[jwinkler]

Hi all - it's been a while since I've posted. I guess it's time for the car to start challenging me again.

I was driving this weekend, and out of nowhere, I was engulfed in white plasticky-smelling smoke. I pulled over, and noticed the last of the smoke coming out of the main fuse harness inside the firewall (the car is a classic roadsters). Turns out what had burned was a "custom" wire-and-fuse solution I put together years ago to run a heavier gauge wire to the fan power.

The solution was to run a 6-gauge (vs. the original 12-gauge) wire with an inline fuse to provide power to my black magic electric fan. The power is switched with a relay. What I saw when I looked at the problem, the "fire" had happened at the fuse itself (a 30 amp fuse). No other damage to any wires in the area, and everything except the fan works fine. Fortunately, it was cool enough, and without too many stop lights, for me to make it home under the car's own power rather than on the back of a flatbed.

My question is what would make a fuse (blade style) combust like that? Even with the fuse still smoking, the wire itself wasn't hot compared to the normal heat of the engine bay. The fan has been running fine up until this happened, and my "custom" re-wiring has been in place for at least 7 years.

Could water in the fuse housing have caused this problem - the only other unusual thing that happened was that before I took the car out, I washed it (an unfortunately rare occurrence) with a hose, which results in a lot of underhood water on the flat surfaces. I dried what I could see, but didn't check thoroughly.

If I replace the wiring, is there anything I could measure at the fan (amp draw?) that would indicate a problem?

Thanks,

JLW