While circuit breakers are becoming more prominent in 12-volt system design, the fuse remains a common, and useful, technology. If you're not up on your fuses, here's an overview of where they fit into automotive circuit protection.
Uses And Types
According to "Fuseology" by Littelfuse, the purpose of a fuse is to serve as a sacrificial device. Inside a fuse is a conductive strip of material that will melt if the fuse overloads, also called its "breaking capacity." If a system overloads or shorts, the fuse breaks the circuit, or "opens," to protect the larger system; once a fuse pops it cannot be reused.
In automotive electrical work, the vast majority of fuses are "blade"-type fuses, where the fuse has two prongs that fit into a small plug, however the use of bolt-down fuses and cartridge fuses has gradually increased in popularity over time.
Sizes And Amperages
Blade fuses come in six sizes, which are, from smallest to largest: Micro2, Micro3, low profile mini, mini, ATO®/ATC® (also called "regular") and maxi. Automotive fuses always have their amperage written on the top of the fuse; if you remove the cover of an automotive fusebox, the amps each fuse can handle will be the first thing you see. Each level of amperage also has a specific color associated with it. However, the colors can repeat, so teams should be aware of this issue: violet, in mini and regular fuses sizes, is 3A, while with a maxi fuse it will be 100A.
Counterintuitively, amp rating does not necessarily correspond to fuse size. Regular-sized fuses start at .5 and go all the way up to 40A, while Micro2 fuses range between 5 and 30A. Generally to prevent issues, anybody maintaining vehicles, including drivers who may be forced to make emergency repairs, should pay attention to match both size and color.
To select the right fuse amperage within a range, start by calculating your maximum and minimum fuse amperages and taking a number near the middle of the range. For this, it can be helpful to reference charts such as this one from Littelfuse:
(Click Image to Enlarge)
No fuse lasts forever. Eventually, even a perfect electrical system that never overloads will wear it out. However, fuse life can be driven down by a number of factors. Temperature, for example, can reduce a fuse's life. Generally a fuse's amperage is rated for 25 degrees Celsius, or 77 degrees Fahrenheit. However, fuses will respond faster or slower to a given overload at higher or lower ambient temperatures, respectively, as detailed in page 61 of "Fuseology".
Other environmental factors can also affect fuse life. One common factor is exposure to electrical pulses, as these may weaken fuse material. Another is high, continuous voltage drops; these are often caused by improper fuse and wire gauge pairings. These issues are common, so if a piece of equipment begins going through fuses at a faster rate, but a short or overload isn't found, one or more of these causes may be to blame.
Proper circuit protection is often the difference between machinery that keeps going and machinery that's perpetually on the side of the road. Knowledge of best practices for selecting, installing and replacing fuses helps avoid outages that put your equipment out of commission.
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