I must admit I was dubious about the claimed ability of this product to identify a blown fuse but I am now a believer. While replacing a bow light bulb, I noticed the led on the switch panel was dim and the bow light would not operate. When I checked the fuse box the bow light fuse position LED was illuminated, indicating a blown fuse. The unit must contain a high resistance by-pass circuit that powers the indicator. How ever they do it, this is not your grandpa's fuse block. Unit is well made, See full review