The way the heat carried into the lower wire makes me think there was a bad connection in the fuse holder. Curious tradeoff - high resistance gives low current and little heat, low resistance gives plenty of current but little heat, while somewhere in the middle things go poof. What's that little ditty? Birdy, birdy, in the sky; power equals r squared i. Or was it Twinkle, twinkle, little star; power equals i squared r?