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Fuses are used in electrical systems and electronic devices to prevent system overload. Alternatively, fuses also refer to simple triggers for incendiary devices, as well as mechanisms used to prevent fluid loss in hydraulic systems.

Electrical fuses normally function as conductors, allowing current to pass through them unhindered. However, when the current surpasses the fuse’s capacity, the fuse will open, stopping current flow. In a basic fuse, excessive current causes an internal conductive metal strip to melt, thereby opening the circuit. Such fuses are rendered useless once they have served their purpose, and therefore require replacement. This may lead to confusion for those who regularly visit the fuse box in their homes to reset switches; most modern “fuse boxes” actually contain circuit breakers rather than fuses. Circuit breakers allow users to reset the circuit without replacing any components, and are the norm in most homes.

Fuses are manufactured in an extensive range of sizes, capacities and styles. They can be designed to limit or interrupt current, either immediately or after a preset period of time. Such time delay fuses open only after they have received prolonged current overloads, not short power surges. Of the many fuse types available, perhaps the most well-known is the plug-in, or blade fuse. These small fuses involve a pair of blades partially encased in a colored plastic housing, where they are connected to one another by a thin strip of conductive metal. The blades plug in to the circuit, and when current in the circuit exceeds their rating, the metal between them melts, stopping the flow of electricity. Other types of fuses include glass- and ceramic-tube fuses, cartridge fuses, Bosch type fuses and milk-bottle fuses.

Most simple fuses are only capable of interrupting currents less than ten times their current rating. Higher currents will typically cause a short circuit, which is when the current finds a route other than that intended. Short circuits allow current to continue flowing through a circuit even after the fuse has melted, can cause major system damage. Fuses intended for high-voltage operations typically have a higher interrupt-current rating.