In electrical engineering a fuse, short for 'fusible link', is a type of over current protection device. It has as its critical component a metal wire or strip that will melt when heated by a prescribed (design) current, opening the circuit of which it is a part, thereby protecting the circuit from an over current condition.
A practical fuse was one of the essential features of Edison's electrical power distribution system. An early fuse was said to have successfully protected an Edison installation from tampering by a rival from a gas-lighting concern.
Fuse time current characteristics
Each type of fuse has a time-current characteristic which shows the time required to melt the fuse for any given level of overload current. In power system design, main and branch circuit fuses can be co-ordinated for best protection by plotting the time-current characteristics on a consistent scale, making sure that the source fuse curve never crosses that of any of the branch circuits. To prevent damage to fuses, both "maximum clearing" and "minimum melting" curves are plotted.
Fuses are often characterized as "fast-blow" or "slow-blow," according to the time they take to respond to an over current condition. Fast-blow fuses (sometimes marked 'F') open quickly when the rated current is reached. Ultrafast fuses (marked 'FF') are used to protect semiconductor devices that can tolerate only very short-lived over currents. Slow-blow fuses (often marked 'T') can tolerate a transient over current condition, but will open if the over current condition is sustained.
A fuse should normally be selected with a rating just over the normal operating current of the downstream wiring or equipment which it is to protect. Properly-selected fuses (or other over current devices) are an essential part of a power distribution system to prevent fire or damage due to overload or short-circuits. Usually the maximum size of fuse for a circuit is regulated by law. For example, the Canadian Electrical Code, the United States National Electrical Code, and the UK Wiring Regulations provide limits for fuse sizes for a given conductor, and local authorities will incorporate these national codes as part of local law.
Fuses are often sold in standardised packages to make them easily interchangeable. Cartridge fuses are cylindrical and are made in standard lengths such as 20mm, 1" and 1.25". Smaller fuses often have a glass body with nothing but air inside so that the fuse wire can be inspected. Unfortunately under extremely high current faults such fuses can arc and therefore continue to supply a current. So fuses used in such situations (for example building wiring installations) have a stronger ceramic body and are filled with sand to quench any arcs (see maximum prospective short circuit current). Small fuses may be held by metal clips on their end ferrules, but larger fuses (100 amperes and larger) are often bolted into the fuse holder.
High-voltage fuses used outdoors may be of the expulsion type, allowing arc by-products to be discharged to the air with considerable noise when they operate.
Blade fuses, with a plastic body and two prongs that fit into sockets, are used in automobiles.
Blade fuse as found in most modern vehicles
Sub-miniature fuses for instruments may be rated as little as 50/1000 of an ampere. These may have wire leads or may be fitted into small two-pin sockets. Sub-miniature fuses used in electronic devices may be directly soldered to a printed circuit board. Often these fuses are installed only to prevent a fire, and not to protect the electronic device.
The BS 1363 13A plug has a BS 1362 cartridge fuse inside. This allows the use of 30A/32A (30A was the original size; 32A is the closest European harmonised size) socket circuits safely. In order to keep cable sizes manageable these are usually wired in ring mains. It also provides better protection for small appliances with thin flex as a variety of fuse ratings (1A 2A 3A 5A 7A 10A 13A common ratings in bold) are available and a suitable fuse should be fitted to allow the normal operating current while protecting the appliance and its cord as well as possible. With some loads it is normal to use a slightly higher rated fuse than the normal operating current. For example on 500W halogen floodlights it is normal to use a 5A fuse even though a 3A would carry the normal operating current. This is because halogen lights draw a significant surge of current at switch on as their cold resistance is far lower than their resistance at operating temperature.
In most other wiring practices the wires in a flexible cord are considered to be protected by the branch circuit over current device, usually rated at around 15 Amperes, so a plug-mounted fuse is not used. Small electronic apparatus often includes a fuse holder on or in the equipment, to protect internal components only.
Other types of fuse
So-called "self-resetting" fuses use a thermoplastic conductive element that opens the circuit on overload, and then restores the circuit when they cool. These are useful in aerospace applications where replacement is difficult.
A "thermal fuse" is often found in consumer heating equipment such as coffee makers or hair dryers; it contains a fusible alloy which opens when the temperature is too high due to reduced air flow or other fault.