Fuse (electrical)

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 electric current, opening the circuit of which it is a part, and so 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.

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 device should normally be selected with a rating just over the normal operating current of the downstream wiring or equipment which it is to protect.

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. Where the fuses in a system are of similar types, simple ratios between ratings of the fuse closest to the load and the next fuse towards the source can be used, so that only the affected circuit is interrupted after a fault. 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.

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 (such as the high starting current of an electric motor), but will open if the overcurrent condition is sustained.

A fuse also has a rated 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 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 300,000 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.

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 rated 32 volts 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. Under extremely high current or voltage, such fuses can arc over and therefore continue to supply a current. Fuses used in higher energy circuits (for example building wiring installations) have a strong ceramic body which prevents arc over, and are filled with sand to quench any arcs. Small fuses may be held by metal clips on their end ferrules, but larger fuses (100 amperes and larger) are usually 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.

Plug-in type

Plug-in fuses (also called blade or spade fuses), with a plastic body and two prongs that fit into sockets, are used in automobiles. These types of fuses come in three different physical dimensions: mini (or minifuse), ATO® (or ATC) and maxi (or maxifuse).
The physical dimensions, including the connector, of the fuses are as follows (LxWxH) (ampere ratings in the parenthesis):

mini: 10.9x3.6x16.3 mm (1A, 2A, 3A, 4A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A)
ATO: 19.1x5.1x18.5 mm (2A, 3A, 4A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A, 40A)
maxi: 29.2x8.5x34.3 mm (20A, 30A, 40A, 50A, 60A, 70A, 80A)

Replacement circuit breaker

It is possible to replacehttp://www.elfa.se/en/ an ATO-type plug-in fuse with a circuit breaker that has been designed to fit in the socket of a ATO-sized fuse holder. These circuit protectors are more expensive than a regular fuse.

Bosch type

Bosch type fuses are used in older (often European) automobiles, and can also be used instead of glass type fuses in inline fuse holders (but not in ganged fuse holders). The physical dimension of this type of fuse is 6x25 mm.

Color coding of Bosch type fuses

Most fuses of the Bosch type usually use the same color coding for the rated current.

Color	Ampere
yellow	5A
white	8A
red	16A
blue	25A

Sub-miniature

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 power transformers where the expense of a circuit breaker is not warranted. For example, in distribution systems, a power fuse may be used to protect a transformer serving 1-3 houses. A circuit breaker at 115 kV may cost up to five times as much as a set of power fuses, so the resulting saving can be tens of thousands of dollars.

Large power fuses use fusible elements made of silver or copper 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 of 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.

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 the circuit first (most building wiring designs do not provide individual isolation switches for each fuse) can be dangerous in itself, particularly if the fault is a short circuit.

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.

In a multi-phase power circuit, if only one of the fuses opens, the remaining phases will have higher than normal currents, and unbalanced voltages, 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 flexibility 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 with 3, 5 and 13 being the most common) 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.

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.

References

BEV components | Power components | Safety equipment

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