I was under the impression fuses either worked, or had blown. Over the last few days my understanding has changed. Problem : 12 volt dc bow thruster, does not work. Check battery, battery good, run over system with volt meter... The motor turns both directions, operated by two relays, one one way, one the other.... Fairly straightforward... Electricity getting to the relays, but not coming out. 24 hours later, replacement relay box. Exactly the same issue. Phone a colleague. He tells me it's the fuse. I tell him I've checked it and it seems ok. He tells me to get a jump lead and bypass the fuse. He's right. It is the fuse. What's going on here? Here's the fuse in question. It's going to have to get opened.