This is why you need an RCD. The earth wire of a very long cable has been shorted to the line at a distant point. The R1+R2 was too high to blow the fuse and over heated the earth.
Luckily the earth wire broke and stopped the over-heat and protected the fuse. Cable now no longer functions due to overheating, but until an RCD was added and re-connection of the earth, everything was OK.