Fuses 101: What You Need to Know
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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.
In automotive electrical applications, the vast majority of fuses are "blade"-type fuses, constructed with two prongs that fit into a small plug. However the use of bolt-down fuses has increased as more vehicles come equipped with accessories requiring higher current carrying capabilities, and cartridge fuses have grown popular in situations where smaller footprints are required for optimal use of space.
ATO/ATC blade type fuses
Blade fuses (shown at right) come in six sizes, which are, from smallest to largest: Micro2®, Micro3®, Low Profile MINI®, MINI®, ATO®/ATC® (also called "regular") and MAXI® or MAX. Blade fuses provide protection for amperage ratings up to 80A.
Bolt down fuses (shown at left) provide protection from 30A to 600A and are ideal for battery and alternator circuit protection. They are called “bolt down” because they need to be secured to a fuse holder using a screw, nut, or bolt.
Cartridge fuses (shown at right) offer increased time delay and low voltage drop protection and are typically designed in compact sizes to save on space and weight. They provide protection from 15A to 70A.
All automotive fuses have their amperage written on the top of the fuse. If you remove the cover of an automotive fuse box, it will be the first thing you see. Each level of amperage also has a specific color associated with it. Blade type fuses use a universal color schema across manufacturers. However, for bolt down and cartridge fuses, the color schema can vary by manufacturer. For example, a clear-colored MINI fuse signifies 25A, while clear in a MAXI fuse signifies 80A.
Counterintuitively, amp rating does not necessarily correspond to fuse size. Regular-sized fuses start at .5A and go all the way up to 40A, while Micro2 fuses range between 5A and 30A. To avoid issues, anybody maintaining vehicles—including drivers who may be forced to make emergency repairs—should take care 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. Fuse selection charts such as the ones shown below from Littelfuse and Eaton can also help.
The current rating of a fuse is typically derated 25% for operation at 25°C (77°F) to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25°C ambient environment. Keep in mind that as your operating temperature rises above 25°C (77°F), fuses will react more quickly, and as temperatures go lower, fuses will react more slowly, as detailed in page 61 of "Fuseology".
Installing and Replacing Fuses
Once a fuse element breaks, the fuse cannot be reused. A fuse break can happen for various reasons such a short in the electrical system, frayed wiring, a conductor exposed to the elements or an electrical device malfunctioning and overloading the circuit. If a fuse break becomes recurrent, it’s a sign you may need to check for a short or an overload.
The most important thing to remember when plugging in an automotive fuse is to make sure the vehicle is turned off, otherwise arcing could occur. Also keep in mind that fuses come in a wide variety of materials and amperages. If you are unsure which fuse is required, consult your owner’s manual or an automotive professional.
Not all fuses are created equal, but all are affected by overload current and their length of time in operation. No fuse lasts forever. Even a perfect electrical system that never overloads will eventually wear out a fuse. Other factors include:
- High or low temperature environments.
- Pulse: Electrical substations, for example, produce low-level electrical pulses that can weaken fuse elements in vehicles routinely parked nearby.
- Inrush current that exceeds the value for which the wires or equipment are rated.
- Continuous voltage drops, which are often caused by improper fuse and wire gauge pairings (for proper pairing consult a wire gauge chart when choosing a fuse).
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 factors may be to blame.
Proper circuit protection is often the difference between machinery that keeps going and machinery that's perpetually out of service. Knowledge of best practices for selecting, installing and replacing fuses helps avoid outages that put your equipment out of commission.