The loom was substandard to start with, (They wern't very well designed) the switch like with everything else was just pulled from the parts bin.

The trouble is the span of it, that being some 9 degrees, this closes and thus brings the fans on too early, and then they stay on for far to long!

Being "Juicy", coupled with the time they are on is what causes the fuse to melt, by replacing with the "Uprated" switch you have just a few degrees span and so fan cycles are short, plus rewire and you have solved the problem

Martin