In electronics and electrical engineering a fuse (short for fusible link) is a type of overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows, which breaks the circuit in which it is connected, thus protecting the circuit's other components from damage due to excessive current.
Fuses (and other overcurrent devices) are an essential part of a power distribution system to prevent fire or damage. When too much current flows through a wire, it may overheat and be damaged, or even start a fire. Wiring regulations give the maximum rating of a fuse for protection of a particular circuit. Local authorities will incorporate national wiring regulations as part of law. Fuses are selected to allow passage of normal currents, but to quickly interrupt a short circuit or overload condition.
Rated current IN This is the maximum current that the fuse can continuously pass without interruption to the circuit, or harmful effects on its surroundings.
The I2t value This is a measure of the energy required to blow the fuse element and is an important characteristic of the fuse. It is an indication of the "let-through" energy passed by the fuse which downstream circuit elements must withstand before the fuse opens the circuit.
Voltage drop The values of the voltage drop across a fuse are usually given by the manufacturer. A fuse may become hot due to the energy dissipation in the fuse element at rated current conditions. The voltage drop should be taken into account particularly when using a fuse in low-voltage applications.
Breaking capacity The breaking capacity is the maximum current that can safely be interrupted by the fuse. Some fuses are designated High Rupture Capacity (HRC) and are usually filled with sand or a similar material.
Voltage rating The voltage rating of a fuse should always be greater than or equal to the circuit voltage. Low-voltage fuses can generally be used at any voltage up to their rating. Some medium-voltage and high-voltage fuses used in electric power distribution will not function properly at lower voltages.
The speed at which a fuse operates depends on how much current flows through it and the material of which the fuse is made. In addition, temperature influences the resistance of the fuse. Manufacturers of fuses plot a time-current characteristic curve, which shows the time required to melt the fuse and the time required to clear the circuit for any given level of overload current.
Where several fuses are connected in series at the various levels of a power distribution system, it is very desirable to clear only the fuse (or other overcurrent devices) electrically closest to the fault. This process is called "coordination" and may require the time-current characteristics of two fuses to be plotted on a common current basis. Fuses are then selected so that the minor, branch, fuse clears its circuit well before the supplying, major, fuse starts to melt. In this way only the faulty circuits are interrupted and minimal disturbance occurs to other circuits fed by the supplying fuse.
Where the fuses in a system are of similar types, simple rule-of-thumb ratios between ratings of the fuse closest to the load and the next fuse towards the source can be used.
Fuses are often characterized as "fast-blow", "slow-blow" or "time-delay", according to the time they take to respond to an overcurrent condition. The selection of the characteristic depends on what equipment is being protected. Semiconductor devices may need a fast or ultrafast fuse for protection since semiconductors may have little capacity to withstand even a momentary overload. Fuses applied on motor circuits may have a time-delay characteristic, since the surge of current required at motor start soon decreases and is harmless to wiring and the motor.
When replacing a fuse, it is important to interpret these markings correctly as fuses that may look the same, could be designed for very different applications. Fuse markings will generally convey the following information;
Cartridge (ferrule) fuses have a cylindrical body terminated with metal end caps. Some cartridge fuses are manufactured with end caps of different sizes to prevent accidental insertion of the wrong fuse rating in a holder. An example of such a fuse range is the 'bottle fuse', which in appearance resembles the shape of a bottle.
Fuses used in circuits rated 200-600 volts and between about 10 and several thousand amperes, as used for industrial applications such as protection of electric motors, commonly have metal blades located on each end of the fuse. Fuses may be held by a spring loaded clip or the blades may be held by screws. Blade type fuses often require the use of a special purpose extractor tool to remove them from the fuse holder.
Other fuse packages can require a variety of measurements such as;
Fuses fitted with tags require the fixing centre measurement. This measurement is the distance between the tag mounting holes on either end of the fuse as measured from the centre of each mounting hole.
Automotive fuses protect the wiring and electrical equipment for vehicles. They are generally rated for circuits no higher than 24 volts direct current, but there exists brands of mini and maxi fuses that are modifed to be able to work in the 42 volt environment.
It is possible to replace an ATO-type plug-in fuse with a circuit breaker that has been designed to fit in the socket of an ATO-sized fuse holder. These circuit protectors are more expensive than a regular fuse.
Mini and ATO Color-coding:
Bosch type fuses are used in old (often European) automobiles. The physical dimension of this type of fuse is 6x25 mm with conical ends. Bosch type fuses usually use the same color coding for the rated current. The DIN standard is 72581/1
|Color||Continuous amps||Instantaneous fusing amps||Continuous fusing amps|
|Red on Yellow||2.5A||6A||5A|
|Red on Green||5A||12A||10A|
|Green on Black||5A||12A||10A|
|Red on Brown||6A||14A||12A|
|Purple on Yellow||25A||60A||50A|
|Yellow on Red||30A||75A||60A|
Large power fuses use fusible elements made of silver, copper or tin to provide stable and predictable performance. High voltage expulsion fuses surround the fusible link with gas-evolving substances, such as boric acid. When the fuse blows, heat from the arc causes the boric acid to evolve large volumes of gases. The associated high pressure (often greater than 100 atmospheres) and cooling gases rapidly extinguish (quench) the resulting arc. The hot gases are then explosively expelled out of the end(s) of the fuse. Other special High Rupturing Capacity (HRC) fuses surround one or more parallel connected fusible links with an energy absorbing material, typically silicon dioxide sand. When the fusible link blows, the sand absorbs energy from the arc, rapidly quenching it, creating an artificial fulgurite in the process.
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.
"Current-limiting" fuses operate so quickly that they limit the total "let-through" energy that passes into the circuit, helping to protect downstream equipment from damage. These 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 and replaced if damaged.
Circuit breakers must be maintained on a regular basis to ensure their mechanical operation during an interruption. This is not the case with fuses, in which no mechanical operation is required for the fuse to operate under fault conditions.
In a multi-phase power circuit, if only one fuse opens, the remaining phases will have higher than normal currents, and unbalanced voltages, with possible damage to motors. Fuses only sense overcurrent, or to a degree, over-temperature, and cannot usually be used independently 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 flexibility of relay protection by adding a pyrotechnic device to the fuse operated by external protection relays.
In the UK, older electrical consumer units (also called fuse boxes) are fitted either with semi-enclosed (rewirable) fuses (BS 3036) or cartridge fuses (BS 1361). Semi-enclosed fuses are fuse wire carriers fitted with replacable lengths of wire (commonly supplied to domestic consumers as short lengths of 5A-, 15A- and 30A-rated wire wound on a piece of cardboard). Modern consumer units usually contain miniature circuit breakers (MCBs) instead of fuses, though cartridge fuses are sometimes still used, as MCBs are rather prone to nuisance tripping.
Renewable fuses (rewirable or cartridge) allow user replacement, but this can be hazardous as it is easy to put a higher-rated or double fuse element (link or wire) into the holder ("overfusing"), or simply fitting it with copper wire or even a totally different type of conducting object (hairpins, paper clips, nails etc.) to the existing carrier. Such tampering will not be visible without full inspection of the fuse. Fuse wire was never used in North America for this reason, although renewable fuses continue to be made for distribution boards.
The box pictured is a "Wylex standard". This type was very popular in the United Kingdom 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 fitting 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.
In North America, fuse boxes were formerly used in buildings wired before about 1950. These used screw-in "plug" type (not to be confused with what the British call plug fuses), in screw-thread holders similar to 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 rejection 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 in the socket, which defeated the overcurrent protection function and resulted in a dangerous condition. Plug fuses are no longer used for branch circuit protection in new residential or industrial construction.
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.
The rating on a BS1362 fuse specifies the maximum current the fuse can pass 'indefinitely' under standard conditions. The fuse will pass higher currents than the rated value for significant periods, depending on how high the overload is. Fuse manufacturers publish tables or graphs of fuse characteristics to allow electrical system designers to specify the correct fuse for the conditions under which it will be expected to operate. One example is the table published by Cooper-Bussmann for their BS1362 fuses. In this table it can be seen that the fuse is specified to be able to carry its rated current for a minimum of 1,000 hours; 1.6 times its rated current for a minimum of 30 minutes; and 1.9 times its rated current for a maximum of 30 minutes. Thus, this BS1362 13A fuse is only rated to break its circuit after carrying 24.7 Amps for 30 minutes.