In electronics and electrical engineering a fuse, short for 'fusible link', is a type of overcurrent 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 overcurrent 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 overcurrent 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 overcurrents. Slow-blow fuses (often marked 'T') can tolerate a transient overcurrent condition, but will open if the overcurrent 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 overcurrent 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 20 mm, 1 in (25.4 mm) and 1.25 in (31.75 mm). 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 byproducts 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.
Sub-miniature fuses for instruments may be rated as little as 50 milliamperes. 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.
Power circuit fuses
Fuses for power circuits are available in a wide range of ratings. Critical values in the specification of fuses are the normal rated current, the circuit voltage, and the maximum level of current available on a short-circuit. For example, in North America, a so-called "code" fuse may only be safely used in circuits with no more than 10,000 amperes available on a short circuit.
Fuses are used on power systems up to 115,000 volts AC. High-voltage fuses are used to protect instrument transformers used for electricity metering, or for small loads where the expense of a circuit breaker is not warranted. For example, in North American rural distribution systems, a 7200 volt power fuse may be used to protect a consumer's small power transformer.
Large power fuses have fusible elements made of silver or copper to provide stable and predictable performance.
Fuses compared with circuit breakers
Fuses have the advantages of often being less costly and simpler than a circuit breaker for similar ratings. High rupturing capacity fuses can be rated to safely interrupt up to 300,000 amperes at 600 V AC. Fuses can be selected that operate so quickly they limit the "let-through" energy into the circuit, helping to protect downstream equipment from damage. However, fuses are inherently a one-time-only device, requiring replacement after they've served their function. In a three-phase power circuit, if only one of the three fuses operates, the remaining phases will be unbalanced, with possible damage to motors. Fuses only sense overcurrent, or to a degree, overtemperature, and cannot usually be used with protective relaying to provide more advanced protective functions, for example, ground fault detection.
A circuit breaker is a piece of equipment which is designed to protect an electrical apparatus from damage caused by overload or short circuit. Unlike a fuse which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.
Circuit breakers are often implemented with a solenoid (electromagnet) whose strength increases as the current increases and eventually trips the circuit breaker. Alternatively a bimetallic strip may be used which heats and bends with increased current. Some circuit breakers incorporate both techniques. This allows the properties of the circuit breaker to be tailored to suit the application, with the electromagnet generally responding to short, large surges in current (short circuit) and the bimetallic strip responding to smaller but longer-term (overload) overcurrent conditions. Circuit breakers for larger currents are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism.
Under short-circuit conditions a current of many times greater than normal can flow (see maximum prospective short circuit current). When a circuit breaker tries to interrupt this current, an arc may form allowing the flow of current to continue even though the contacts of the circuit breaker are open. Circuit breakers incorporate features to divide and extinquish the arc. In air-insulated and miniature breakers an arc chute structure consisting (often) of metal plates or ceramic ridges cools the arc, and blowout coils deflect the arc into the arc chute. Larger circuit breakers such as those used in electrical power distribution may use vacuum, an inert gas such as sulfur hexafluoride or have contacts immersed in oil to suppress the arc. The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a higher prospective short-circuit current may result in failure of the breaker to safely interrupt a fault.
Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel. Power circuit breakers are built into switchgear cabinets. High-voltage breakers may be free-standing outdoor equipment or a component of a gas-insulated switchgear line-up.