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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 characteristics
Each type of fuse ( and all other overcurrent devices) has a time-current characteristic which shows the time required to melt the fuse and the time required to clear the circuit for any given level of overload current. In common fuse selection, simple ratios between the fuse closest to the load and the next fuse to the source can be used to prevent having several circuits interrupted. In power system design, main and branch circuit overcurrent devices can be co-ordinated for best protection by plotting the time-current characteristics on a consistent scale, making sure that the source curve never crosses that of any of the branch circuits. To prevent damage to utilization devices, both "maximum clearing" and "minimum melting" fuse curves are plotted. Simular curves are used for all overcurrent devices: fuses, common circuit breakers and power circuit breakers.
Fuses are often characterized as "fast-blow" or "slow-blow" | "time-delay", 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 (household plug type are often marked 'T') can tolerate a transient overcurrent condition, but will open if the overcurrent condition is sustained.
A fuse also has a interrupting capacity, which is the maximum current the fuse can safely interrupt. Generally this should be higher than the prospective short circuit current though it may be lower if another fuse or breaker upstream can be relied upon to take out extremely high current shorts. Miniature fuses may have an interrupting rating only 10 times their rated current. Fuses for low-voltage residential power systems are commonly rated to interrupt 10,000 amperes, which is a minimum capacity regulated by the electrical code in some jurisdictions. Fuses for larger power systems must have higher interrupting ratings, with some low-voltage current-limiting "high rupturing capacity" (HRC) fuses rated for 200,000 amperes. Fuses for high-voltage equipment, up to 115,000 volts, are rated by the total apparent power (megavoltamperes, MVA) of the fault level on the circuit.
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 the overcurrent device 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 overcurrent device ampere rating for a given conductor, insulation material and installation conditions. Local authorities will incorporate these national codes as part of law. An overcurrent devise should normally be selected with a rating just over the normal operating current of the downstream wiring or equipment which it is to protect.
Overcurrent devices installed inside of enclosures are "derated" at least per the US NEC. This is a hold- over from the first mounting of electrical devices on the surface of slate slabs. The slate was the insulating material between devices mounted in air. So, rather than change the fuse rating, it became common to allow only 80% of the current value of the overcurrent device when the circuit is in operation for 3 hours or more (continuous loading).
As well as a current rating, fuses also carry a voltage rating indicating the maximum circuit voltage in which the fuse can be used. For example, glass tube fuses ( most common of which are 0.25 inch diameter x 1.25 inch long) are rated 32 volts when over 8 amperes, and should never be used in line-operated (mains-operated) equipment even if the fuse physically can fit the fuseholder. Fuses with ceramic cases have higher voltage ratings. Fuses carrying a 250 V rating can be safely used in a 125 V circuit, but the reverse is not true as the fuse may not be capable of safely interrupting the arc in a circuit of a higher voltage.
Fuse packages
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 mm) and 1.25 in (32 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 or voltage, such fuses can arc over and therefore continue to supply a current. So fuses used in such situations (for example building wiring installations) have a stronger ceramic body which better prevents arc over, and are filled with sand to quench any arcs (see maximum prospective short circuit current), so they usually have higher voltage ratings. 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.
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 and pico fuses used in electronic devices may be directly soldered to a printed circuit board. Often these fuses are installed only to protect the external utilization device, not the electronics.
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 5Kv distribution systems, a 7200 volt power fuse may be used to protect a consumer's small power transformer serving 1- 3 houses.
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. The blown fuse must be replaced with a new device which is less convenient than simply resetting a breaker and therefore likely to discourage people from ignoring faults. On the other hand replacing a fuse without isolating (in most builing wiring there are not individual switches before the fuses) can be dangerous in itself particualarlly if the fault is a short cuircuit. High rupturing capacity fuses can be rated to safely interrupt up to 300,000 amperes at 600 V AC. Special current-limiting fuses are applied ahead of some molded-case breakers to protect the breakers in low-voltage power circuits with high short-circuit levels.
Fuses can be selected that operate so quickly they limit the "let-through" energy into the circuit, helping to protect downstream equipment from damage. These "current-limiting" fuses clear the fault in less than one cycle of the AC power frequency. Circuit breakers cannot offer similar rapid protection.
Circuit breakers which have interrupted a severe fault should be removed from service and inspected by a qualified technician -- or replaced with a new breaker!
In a multi-phase power circuit, if one contact of a device such as of a breaker or one fuse opens, the remaining phases will have higher than normal currents, with possible damage to the coils of motors or solenoids. Fuses only sense overcurrent, or to a degree, over-temperature, and cannot usually be used with protective relaying to provide more advanced protective functions, for example, ground fault detection.
Some manufacturers of medium-voltage distribution fuses combine the overcurrent protection characteristics of the fusible element with the convenenice of relay protection by adding a pyrotechnic device to the fuse operated by external protection relays.
Fuse boxes
Old electrical consumer units (also called fuse boxes) were fitted with fuse wire that could be replaced from a supply of spare wire that was wound on a piece of cardboard. Modern consumer units contain magnetic circuit breakers instead of fuses. Cartridge fuses were also used in consumer units and sometimes still are as miniature circuit breakers (MCBs) are rather prone to nuisance tripping. (In North America, fuse wire was never used in this way, although so-called "renewable" fuses were made that allowed replacement of the fuse link. It was impossible to prevent putting a higher-rated or double links into the holder ("overfusing") and so this type must be replaced.)
The box pictured is a "Wylex standard". This type was very popular in the British Isles up until recently when the wiring regulations started demanding Residual-Current Devices (RCDs) for sockets that could feasibly supply equipment outside the equipotential zone. The design does not allow for fitment of RCDs (there were a few wylex standard models made with an RCD instead of the main switch but that isn't generally considered acceptable nowadays either because it means you lose lighting in the event of almost any fault) or residual-current circuit breakers with overload (RCBOs) (an RCBO is the combination of an RCD and an MCB in a single unit). The one pictured is fitted with rewirable fuses but they can also be fitted with cartridge fuses and MCBs. There are two styles of fuse base that can be screwed into these units—one designed for the rewirable fusewire carriers and one designed for cartridge fuse carriers. Over the years MCBs have been made for both styles of base. With both styles of base higher rated carriers had wider pins so a carrier couldn't be changed for a higher rated one without also changing the base. Of course with rewirable carriers a user could just fit fatter fusewire or even a totally different type of wire object (hairpins, paper clips, nails etc.) to the existing carrier.
In North America, fuse boxes were also often used, especially in homes wired before about 1950. Fuses for these panels were screw-in "plug" type (not to be confused with what the British refer to as plug fuses), in holders with the same threads as Edison-base incandescent lamps, with ratings of 5, 10, 15, 20, 25, and 30 amperes. To prevent installation of fuses with too high a current rating for the circuit, later fuse boxes included restrictor features in the fuseholder socket. Some installations have resettable miniature thermal circuit breakers which screw into the fuse socket. One form of abuse of the fuse box was to put a penny (coin) in the socket, which defeated the overcurrent protection function and resulted in a dangerous condition. Plug fuses are no longer used in new residential or industrial construction, but are often found as branch circuit protection for electric cooking units (ranges) & dryers.
Nevertheless, these plug fuses have a remarkable service record. Many have even protected branch circuits and appliances against lightning caused voltage surges.
British plug fuse
The BS 1363 13 A plug has a BS 1362 cartridge fuse inside. This allows the use of 30 A/32 A (30 A was the original size; 32 A 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 (1 A 2 A 3 A 5 A 7 A 10 A 13 A 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 500 W halogen floodlights it is normal to use a 5 A fuse even though a 3 A 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 overcurrent device, usually rated at around 15 amperes, so a plug-mounted fuse is not used. Small electronic apparatus often includes a fuseholder 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, then restores the circuit when they cool. These are useful in aerospace applications where replacement is difficult. Common kind is the Polyswitch self-repairing fuses.
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.
See also
- Antifuses are devices which are electrically open until a large voltage "blows" them shorted.
- Earth leakage circuit breakers are somewhat related.
- Semiconductor fuses are very fast types of electrical fuses.
- Circuit breakers are an alternative to fuses for overcurrent and other protection
- Fuse for the other meanings of the word.
External links
- for the Bussman manual of fuse selection. Heavily oriented to United States NEC but has a good explanation of the principles of fuse selection.
- Free IC DataSheet Search Site : http://www.Datasheet4U.com