"Most of the time when people say a fuse or breaker is operating improperly or blowing when it shouldn't, it's because the device is doing exactly what it's supposed to do. The fault is usually in the circuit, but no one wants to admit it's their error. It's kind of like when you're computer programming. When the output isn't what you wanted, the first thing you want to believe is that there's something wrong with the machine, not that you have a coding error. Using that as a starting point, I would say he or she needs to be absolutely sure that the circuit is not overloaded. If it is not, then the next most likely problem is overheating caused by loose or corroded connections. That should solve 99% of the tripping or blowing faults. If there is still a problem, then it just might be a fuse that has been subjected to repeated overcurrent and heating that was not sufficient to blow the device, but which over a long period of time caused the material in the fuse to degrade. After enough time the fuse might be compromised and will blow before it's maximum current and temperature rating is reached. This is not a bad thing, however. It means the fuse will protect the downstream wiring from overheating and causing a fire. It might be a nuisance, but unless it interrupts the circuit on process equipment (think of a factory or a smelting plant), it's not dangerous. Considering the number of years fuses have been used, they have a pretty good track record for holding up to environmental stresses. The big disadvantage to fuses is that when they do finally blow, it costs money to replace them. And when you're replacing a fuse in a company switch, you're talking about a 200- or 440-amp device. It's physically challenging and dangerous to do this. The term ‘qualified personnel' is no misnomer here. If you don't understand arc flash, PPE (personal protective equipment) and lock out/tag out procedures, you have no business changing a fuse of this size."
-Richard Wolpert, co-owner, Union Connector, quoted by Richard Cadena in "Focus on Fundamentals," PLSN, Jan. 2011
