A fuse (a shortened term for fusible link) is a type of protection device in electrical applications. Fuses are designed specifically to be the "weakest" link in the electrical chain. The two basic parts of a fuse are the fuse element, the metal filament that conducts electricity and allows it to pass through the fuse, and the housing which is non-conductive. The use of fuses apply theoretically to any application where electricity is used to power a device or circuit. Because power fluctuations are relatively commonplace in our modern world, fire can be the result of a large swing in amperage, we need something to sense these situations and protect the expensive electrical equipment installed. In a "non-fused" system, the amperage is allowed to vary greatly without any interruption. IN a "fused" system, the peak electrical input for the system is designed into the "FUSE". Therefore the fuse limits all use of power behind it.
Fuses are made for several facets of industry, each category of fuse is constructed of different materials to suit the application and cost parameters. The fuse element is always conductive and typically is comprised of zinc, copper, silver, aluminum or similar alloy. The fuse housing does not conduct electricity and thus is usually made using ceramic, glass, plastic, fiber-glass, laminates or combinations of the former(s).
Fuses are a necessary safety precaution in nearly all electrical devices and wired circuitry. If current running through the system spikes (is higher than the rating on the fuse) the fuse will fail by design. The essential component that will deteriorate in the fuse is a metal wire or strip that melts when an excessive amount of current flows through it. The wire heats up due to the current and fails, thus interrupting the circuit to which it is connected. The fuses are installed so that further damage by overheating or fire is prevented under these adverse conditions, that are outside of specifications. Fuses are typically a cheap yet effective way to protect the expensive or difficult to re-install components in an electrical systems. For instance a fuse is far cheaper than a wiring harness, batteries, transformers, and switchgear.
So we know how a fuse blows, we know why a fuse is there, what make it blow in the first place?
Some common explanations for current fluctuations that result in blown fuses:
system overload (ie too much volume from the amp)
other device failures
What makes fuses different?
A helpful tool in beginning to understand the need for different types of fuses is to imagine all the different types of electrical components found in your home and garage. From your stereo to a hair dryer, from the TV to the surge protectors feeding from your walls, each of these electrical circuits, no matter the application has a minimum of one fuse installed for safety. Furthermore, each one of these examples may have varying power requirements. So for each voltage and amperage range, we have a different type of fuse. Next we need to add in operating conditions, like waterproofing, high temperature, low temperature, and so on to cover all the applications required in both AC and DC. You can probably now start to see that, for example, a 15 amp fuse at 12 volts for your car stereo will not work to protect your 10 amp refrigerator at 110 volts in the USA.
Based on the variety of fuses available, let's start to break it down. First decide what kind of power you are using, AC or DC. The difference is a very big deal. Please notice that ac fuses are rated for higher voltages and this usually keeps the hobbyist using small voltages using only DC fuses. However, if you plan to upsize a DC installation by connecting batteries, solar panels, or wind generators in series, you need to consider very large voltage DC fuses. BE SURE TO CHECK!!! Now that you know what kind of power the fuse is designed for, let's move on to pick the right type, and size.
The next 2 important distinctions are voltage and amperage. Let's start with the voltage range. The voltage range is important but not the limiting factor. Choose a fuse that has your system voltage in the middle. If the middle of the voltage range isn't feasible, choose the closest you can. For instance a 24 volt system will work fine with a fuse rated for 8-44 volts. Not quite the middle mathematically, but we are well within the limits.
Fuses are designed to sense a amperage above their rated listing. So a 100 amp fuse should blow with a 100 amp charge applied to it almost instantaneously. However a sustained 88 amp load will do so as well. WHY? Well, fuses are designed to sense a pulse of power, and as such are able to be heated up over time to the same result. So a fuse running over 85% for a sustained period of time will "Slow Blow" the fuse.
How long until it "SLOW BLOWS"? That is a million dollar question right there, I say it depends on temperature...
Finally the fuse must be designed to sustain in the installed environment. Since electrical circuitry is nearly everywhere now, that means the fuses must learn to do so as well. The main factors here are temperature, weatherproofing, and code compliance. However, each still does the same thing, protect the circuit behind the fuse from a spike of power greater than the rated amount of amperage or current on the fuse. If more current tries to go through, the fuse breaks.
So in conclusion, the basics behind a fuse are current limitation. A fuse, when appropriately chosen, limits the current to the circuit, and will not deteriorate in the environment installed. The size of the fuse is a bit bigger (derated properly) and meets the code guidelines if applicable. So now let's go look at how the math for those fuse ratings work...