The fuse box is often located in a cluster of components, as illustrated here. The box is shown with the cover off.
To guard against this, ancillary circuits have fuses. The most common type of fuse is a short length of thin wire enclosed in a heatproof casing often glass. The size of the fuse wire is the thinnest that can carry the normal current of the circuit without overheating, and it is rated in amps.
In a negative (-) earth-return system, the current flows from the positive (+) terminal to the component being operated. The component is earthed to the car body, which is earthed to the negative (-) terminal of the battery.
Thin wires conduct less easily than thick ones, because there is less room for the electrons to travel through. The energy needed to push current through a resistance is transformed into heat. This can be useful, for example in the very thin filament of a light bulb, which glows white hot.