Most vehicle electrical circuits are designed by the manufacturer to use a specific type and size of fuse. It is difficult to put the wrong style of fuse in place, but it is much easier to put a fuse with the wrong amperage rating in the circuit. If the fuse is rated for too low an amperage, it may not be able to handle the circuit's power load and will blow when power is sent through the circuit. If the rating of the fuse is correct for the circuit, there is likely a short in the circuit causing the fuse to blow. On the other hand, using a fuse with a higher amperage rating may keep the fuse from blowing, but electrical issues could still cause damage to the downstream components, wiring, or even result in a fire inside the vehicle.
An electrical surge happens when the diodes or regulating circuits in the alternator or voltage regulator fail and the alternator creates too much electricity for the system. If this happens, you may notice a check engine light on the dash. You may also notice a loud hum through your radio or all the lights on the vehicle may appear brighter than normal, or increase in brightness when accelerating. Fuses will blow once the flow of electrical current exceeds the fuse's rating to prevent overloading the system.
A short-circuit happens when the resistance in a circuit drops to zero. This can be caused by failure of an electrical component, a bad ground, or a grounded power wire, often the result of damaged insulation. When any of these scenarios occurs, large amounts of power are able to flow unrestricted through the circuit, which causes the fuse to blow. If there is no fuse in the circuit, if the fuse has been bypassed, or if a fuse rated for too high an amperage for the circuit is used, it can cause the components or wiring to overheat or catch fire.
Every electronic component on your vehicle, like your radio, headlights, horn, and windshield wipers, requires a certain amount of electricity to operate. Each system includes a fuse in the circuit to protect it from damage due to an electrical issue, such as a short-circuit or power surge. Electrical issues may be a one-time occurrence, but if a fuse fails, or "blows", after being replaced, there is likely a larger issue that needs to be diagnosed.
Fuses are safety components designed to limit the amperage running through a circuit. Each fuse contains a metal element inside the plastic or glass casing which will melt or "blow" when an electrical issue causes more amperage to flow through the circuit than the circuit and fuse are rated to carry. This interrupts the flow of power so that electricity does not continue to pass through the circuit, and protects the electrical circuit and accessories from damage that can be caused by overloading or overheating.
Each circuit in a vehicle can handle a specific amount of current flow, or amperage, so every circuit needs a fuse which can handle the appropriate amount of current. Since different circuits require different amounts of power, fuses of a variety of different ratings may be used in your vehicle's fuse box. Fuse ratings are determined by how many amps a circuit can safely handle. As the amp rating of a fuse increases, the size of the fuse element will increase to handle more current flow.
Many vehicles also use a cartridge-style fuse in the power distribution boxes located under the hood of the vehicle. Cartridge-style fuses are designed to handle the higher heat of the engine compartment as well as higher current loads.
Automotive relays should not be confused with the fuses in the power distribution boxes under the hood. Relays are electronically-operated switches which complete or break an electrical circuit. This makes them similar to fuses, but relays are able to open and close as needed to connect and disconnect the circuit. Relays normally look like small black cubes, which differ from the color-coded fuses. If a relay fails, it can cause components in the circuit to not turn on or shut off as needed. Faulty relays can usually be replaced similarly to the method used to replace a fuse, by simply pulling the relay out of the socket and replacing it with a new one.
If an electrical component fails, it is recommended you start diagnosing the problem at the fuse box. It may help to have the owner's manual or a repair manual to locate and identify the location of the fuse which may be blown.
We hope this has helped you better understand the role fuses play in the power system of your vehicle and why they may fail. If you are unable to replace or diagnose a fuse or electrical issue, we can help you find a local repair shop.