I have a number of older (and newer) string inverter based systems where I use an Outback or Midnite Fused Combiner box as well as string fuses inside inverters (like SMA or Fronius). I always make sure the fuses are sized properly per solar panel specs. I have noticed that over time I will have a blown fuse on one of the strings. I replace the fuse and the system works fine for many years (doesn't re-blow the fuse). I notice this especially on older systems where the customer reports lower system performance. What in your experience causes string fuses to intermittently blow over time?