All About Fuses

Tiny package, big impact

A fuse serves as the overcurrent protection device in an electrical system. A “fusible” link (or links) encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse. Electrical resistance of the link is so low that it simply acts as a conductor.

However, when destructive currents occur, the combustible material inside the fuse burns and the connection of the circuit breaks, thereby protecting the entire system from damage. Further, the fuse protects systems’ “upstream,” meaning that harmful electrical current will not reach out beyond the system to destroy other components, such as power lines, substations, or other homes, for example. Fuses come in an almost endless variety of configurations.

Overcurrents

Given the great deal of energy consumed by the average home every day, it comes as no surprise that home electrical systems can be quite complicated and remarkably sensitive. All circuits (including household electrical systems) experience potentially dangerous overcurrents, a current that contains more energy than the circuit can safely conduct. Harsh environmental conditions, routine deterioration, accidents, extreme weather, corrosion, and heating or cooling can all promote conditions that overload an electrical system.

Reliable protective devices prevent or minimize costly damage to transformers, conductors, motors, and the many other components and loads that make up the complete distribution system. Severe losses can result from power blackouts and prolonged downtime of facilities; fuses provide a simple means of minimizing this risk to life and property.

An overcurrent happens in two common situations: an overload or a short-circuit. An overload is an excessive current relative to normal operating parameters, but one that travels through the normal paths of the electrical circuit. As the name implies, a short-circuit current is one in which electricity flows outside the normal conducting paths.

Overloads

Overloads flood the system with up to six times the energy of a normal current. Usually, they are caused by harmless temporary surges, such as a motor starting. Since they pass very quickly, any increase in temperature inside the fuse does not reach the level necessary to burn the material inside. Continuous overloads can result from defective motors, overloaded equipment, or an excessive number of devices running off one circuit. Such sustained overloads are destructive and must be cut off by protective devices before they damage the distribution system. Because they involve relatively little energy compared to short-circuit currents, removal of the overload current will generally prevent equipment damage.

Short-circuits

Whereas overload currents occur at rather modest levels, the short-circuit (or fault current) can channel energy up to several hundred times the amount of a normal operating current. So much energy builds in the system that the energy jumps out of its predetermined path, creating a host of devastating complications. If not cut off within a matter of a few thousandths of a second, insulation damage, melting of conductors, vaporization of metal, ionization of gases, arcing, or fires can occur.

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