Thomas Edison invented the fuse more than a hundred years ago. Three days later, it blew and was subsequently bypassed with an aluminum-coated gum wrapper.
That last sentence is pure conjecture and has no basis in fact. Some might say it's an outright lie but I prefer to think of it as poetic license. That's when a writer is allowed to tell blatant lies with impunity. So please don't impugn my poetry.
It's no lie that fuses are probably the lowest of low-tech technology in our industry. But there is more to the seemingly simple devices than meets the eye. Did you know, for example, that a 10-amp fuse may or may not blow at 11 or 12 amps?
Inverse-Time Related
Fuses are inverse-time related devices, meaning that the higher the overcurrent, the faster they blow. A 10A North American miniature fuse (UL/CSA 248-14) can take up to an hour to blow with as much as 135 percent of the rated current. That means, for example, that you could stand in line at the Department of Transportation and get your driver's license renewed in the time it takes for a 10A fuse passing 13.5 amps to blow. And for a European fuse (IEC 60127-2) it can take up to an hour to blow with 150 percent of the rated current.
But because fuses are inverse-time-related devices, they will blow much quicker with a higher overcurrent. A 10A normal blow North American fuse is supposed to blow in less than two minutes if it is carrying 200 percent of the rated current while a 6.3A quick-acting I European fuse is supposed to blow in less than 20 milliseconds (20 thousandths of a second) if the current is 1,000 percent of the rated current.
Someone asked me recently if fuses could become fatigued and blow prematurely. I had never heard of such a thing, and I had to stop and think about how a fuse works. Inside of the fuse cartridge is a metal link, usually made of aluminum, through which the current flows. The combination of the resistance of the metal and the current causes it to heat up, and if there is enough current to build sufficient heat, then the metal link will melt, thus opening the circuit. As the metal link goes through many cycles of heating and cooling (after the circuit is turned off), it expands and contracts, which could possibly fatigue the metal.
Nuisance Trips
But I'm no fuse expert, and that's no lie; so I turned to Richard Wolpert, ETCP Recognized Trainer and one of the owners of Union Connector (manufacturer of power distribution equipment), and I asked him if he thought that fuses could fatigue and start to nuisance trip. Here's what he had to say:
"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 percent 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."
A Vicious Cycle
Richard's point about a loose or corroded connection is a valid one. When connections are not properly torqued, or when socketed devices like lamps aren't properly seated, the poor contact results in a high impedance or high resistance path for current. The combination of resistance and current causes heating, which in turn causes corrosion and even higher resistance and higher heating. It becomes a cycle of escalating proportion and will eventually result in catastrophe. I have heard from a number of people who have witnessed burned components, melting parts, and fires erupting because of loose connections. For example, someone told me last week that they were working a show where the house electrician assured them that the feeder tails were tied in properly, but one of the phase conductors was never tightened properly and it eventually burned where it went into the company switch. It's good to have instruments and tools to help you maintain and troubleshoot your systems, but when it comes to power distribution systems, a bare hand gauging the temperature of working (and sufficiently insulated) components can be your best friend.
There are five ways we protect ourselves from the dangers of electricity: isolation, insulation, overcurrent protection, grounding and GFCIs. Fuses play a very important part in the safety of power distribution systems. And as long as you never bypass them, they will continue to do so.
