In engineering, the basic role of a fuse is to prevent dangerously excessive current from flowing through a conductor. Fuses were originally developed to prevent electrical fires, but have taken on the additional role of protecting electronic circuitry.
In the most basic form, a fuse is an enclosed conductor that will carry the rated current, but will break or melt, interrupting the circuit, if any additional current flows. The enclosure both protects the immediate area from sparks or hot metal, but also, in a wide range of form factors, provides the mechanical means for mounting the fuse in a cabinet or other larger enclosure. The general assumption is that the fuse sacrifices itself to protect the system, and, once the condition that caused the overcurrent condition is corrected, the fuse will be replaced by another fuse, equally ready to give its all.
Refinements in design
While the first fuses were no more than a simple conductor, there are a wide range of electrical and mechanical properties beyond current capacity. For example, when starting an inductive load such as a motor, there may be a brief and safe overcurrent. Time delay fuses, through mechanisms such as a spring-loaded connector that will flex briefly before rupturing, will not break during that initial surge, but will still open the circuit if the overcurrent continues beyond a predefined time. Other fuses, to protect sensitive components, may be designed to open very quickly.
In many applications, one-time-use fuses have been replaced by circuit breakers, a fuse-like but reusable device. In a circuit breaker, the overcurrent still causes a conductor to heat, but, rather than melting or exploding, it flexes and breaks the electrical path, but can then be reset by being moved back into the path.
The form factor of many high-current fuses is a cylinder, which is held in spring clips, covered to avoid electrical shock. Originally introduced in naval gunnery, the practice of a "battle short" has spread into a wide range of electrical applications where, under well-defined conditions, the dangers of an electrical fire are less than the dangers of the fuse-protected component losing power.
In the original application, when going into battle, naval electricians would replace the fuses on naval gun motors with a rod of solid copper. The tradeoff involved was that in a gunnery battle, a gun that stopped firing because its motor lost power could lead to the destruction of a now-defenseless ship. Damage control personnel could deal with an electrical fire, or accept the eventual loss of the motor in return for it continuing during a critical time.
While electrical circuits ideally should be designed so that they always have adequate current, the battle short concept has extended to other situations where a power loss is more catastrophic than a fire. This has even been applied in medicine, where loss of power to a surgical device would kill the patient.