I use them for my trailer's battery monitor and they always seemed to blow but they looked intact. I visually inspected them and they looked fine, but there was no continuity when checked with an ohmmeter. The fuse worked fine until I packed up and moved to a new camp and it failed again. Having replaced about 6 of them, one of them has now clocked up a total of 5 weeks and 5,000 miles. It appears that bouncing while driving caused one end of the fuse to internally break, rendering it unusable. I've never seen a fuse do this, so I have to assume it's because of the cheap design.