That would seem to be an over current condition. The change of a few degrees shouldnt be enough to melt the plastic on the fuse unless you notice that the case is getting really hot. Then you should check that the fan is running. Dogie has some info about reversing the fan for better air flow but i would suspect that the fan is not running and thats what is causing the build up of heat, the other option would be that the socket for the fuse is loose and causing high resistance. Was there a good amount of resistance when you replaced the fuse?
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Fan was running when I found the fuse like that. Could have stopped at some point and started again? I had trouble pulling the fuse out and had to use pliers to get it out. I had all the power disconnected at the time. It was also difficult to push the new fuse in.
I thought this was weird though. I plugged the miner back in without the fuse and the fan kicked on. Didn't mine though. I turned it back off and put the fuse in. It started mining. Is that normal?
The key part is: "I had trouble pulling the fuse out and had to use pliers". Best bet is that there was a bad connection between the fuse and the fuse connector causing heat. The heat "welded"the fuse to the fuse connector. When a fuse gets hot the fuse link melts--at a low temperature, that is how they work. If there is heat it is the fuse connection.
In my case I noticed that the fuse was to hot to hold my finger on. That was caused by a bad connection between the fuse and fuse connector. Later the fuse was cool but the fuse was "welded: to the fuse connector--could not bee removed with my fingers.
Insure that the fuse has a good/tight mechanical connection to the fuse connector and is not hot.
Don't blame the dust, it is your friend protecting your equipment.
I've been checking it all day on and off and last night since it happened. Everything seems to be running fine. Looks like it's happened to a few people out of the blue as well from the comments above. I'm gonna chalk it up as a fluke or that maybe the fuse over time vibrated itself loose.
I've worked around car audio for years and have never seen a fuse melt like that while not blowing.
I'd like to thank everyone for your help and information, much appreciated.