Fuse Types 101: A Comprehensive Guide to Circuit Protection

Circuit protection is an essential component of any electrical system ensuring that the flow of electricity is safe and controlled. One of the most common forms of circuit protection is the use of fuses which act as a safety valve by breaking the circuit when too much current flows through it. In this comprehensive guide, we will explore the different types of fuses available and their specific uses helping you to make informed decisions about the best type of fuse for your particular application.

Types of Fuses

Types of fuses are the following:

1. Small cartridge fuses

Cartridge fuses, which form a very important and certainly the most numerous group, may be designed for high-voltage, low-voltage, or miniature applications. For low-voltage applications, the replaceable fuse link is often fitted into a fuse holder that consists of a fuse carrier and a fuse base.

Small cartridge fuses for appliances and electronic equipment are available in different sizes. Except for the 4.5mm diameter fuse (a European addition), these sizes were originally measured in inches; today, they are often described only with the equivalent metric measurement. Any cartridge fuse is usually available with the option of a lead attached to it at each end so that it can be used as a through-hole component.

Fuses may be fast-acting, medium-acting, or slow-blowing, the last of which may alternatively be referred to as delay fuses. Extra-fast–acting fuses are available from some manufacturers. The term Slo-Blo is often used but is actually a trademark of Littelfuse. None of the terms describing the speed of action of a fuse has been standardized with a specific time or time range.

Some cartridge fuses are available in a ceramic format as an alternative to the more common glass cylinder. If the accidental application of extremely high current is possible (for example, in a multimeter that can be set to measure amps, and can be accidentally connected across a powerful battery), a ceramic cartridge is preferable because it contains a filler that will help to stop an arc from forming. Also, if a fuse is physically destroyed by the application of a very high current, ceramic fragments may be preferable to glass fragments.

2. Expulsion fuses

The expulsion fuse, which belongs to the high voltage category, contains a mechanism to move the fuse link away from one of its contacts when the element melts and as a result, a long gap is introduced in the air in series with the fuse. The arcs in the air and within the fuse are extinguished by the expulsion effects of the gases produced by the arc, provided that the current level is not too high. In practice, this type of fuse is only able to interrupt currents up to about 8000A.

‘Rewirable’ semi-enclosed fuses can still be found in the low voltage category.

3. Automotive fuses

In recent years, the amount of electrical and electronic equipment installed in motor vehicles has increased greatly, resulting in more complex 12 V systems and load requirements ranging from 1000 W to more than 2000 W in highly appointed vehicles. For circuit protection requirements, a modern middle-of-the-range model can have about 30 fuses in it, and there may be 75 or more in a luxury car.

Automotive fuses are identifiable by their use of blades designed for insertion in flat sockets where the fuse is unlikely to loosen as a result of vibration or temperature changes. The fuses come in various sizes and are uniformly color-coded for easy identification.

Usually, automotive fuses are mounted together in a block, but if aftermarket accessory equipment is added, it may be protected by an inline fuse in a holder that terminates in two wires. Similar inline fuse holders are manufactured for other types of fuses.

4. Through-hole fuses

Small fuses with radial leads, which seem appropriate for through-hole insertion in printed circuit boards, are actually often used in conjunction with appropriate sockets so that they can be easily replaced. They are described in catalogs as “subminiature fuses” and are typically found in laptop computers and their power supplies, also televisions, battery chargers, and air conditioners. All have slow-blowing characteristics.

These fuse links are produced in 125 and 250 V ratings with cylindrical polymer bodies and two leads protruding radially from them. They are also produced with axial leads, the voltage rating for these fuse-links being 125 V. Such fuse-links are relatively expensive and, in many cases, it is economic to design electronic circuits to operate at lower voltages, such as 125 V, so that they can be protected by simpler fuse-links.

5. Resettable fuses

Properly known as a polymeric positive temperature coefficient fuse (often abbreviated PTC or PPTC), a resettable fuse is a solid-state, encapsulated component that greatly increases its resistance in response to a current overload, but gradually returns to its original condition when the flow of current is discontinued. It can be thought of as a thermistor that has a nonlinear response. While different sizes of cartridge fuses may share the same ratings, differently rated resettable fuses may be identical in size.

When more than the maximum current passes through the fuse, its internal resistance increases suddenly from a few ohms to hundreds of thousands of ohms. This is known as tripping the fuse.

This inevitably entails a small delay but is comparable to the time taken for a slow-blowing fuse to respond.

A resettable fuse contains a polymer whose crystalline structure is loaded with graphite particles that conduct electricity. As the current flowing through the fuse induces heat, the polymer transitions to an amorphous state, separating the graphite particles and interrupting the conductive pathways. A small current still passes through the component, sufficient to maintain its amorphous state until power is disconnected.

After the resettable fuse cools, it gradually re­crystallizes, although its resistance does not fall back completely to its original value for more than an hour.

The maximum safe level of current for a resettable fuse is known as the hold current, while the current that triggers its response is termed the trip current. Resettable fuses are available with trip-current ratings from 20mA to 100A. While conventional appliance and electronics fuses may be rated as high as 600V, resettable fuses are seldom rated above 100V.

Typical cartridge fuses are affected only to a minor extent by temperature, but the current rating of a resettable fuse may diminish to 75% of its normal value at 50 degrees Centigrade and may drop to 50% of its normal value at 80 degrees Centigrade. In other words, a fuse that is rated for 4A at 25 degrees may tolerate a maximum of only 3A when it operates at twice that temperature.

Conventional slow-blowing fuses are temperature-sensitive, but to a lesser degree than resettable fuses.

Resettable fuses are used in computer power supplies, USB power sources, and loudspeaker enclosures, where they protect the speaker coils from being overdriven. They are appropriate in situations where a fuse may be tripped relatively often, or where technically unsophisticated users may feel unable to replace a fuse or reset a circuit breaker.

Brand names for resettable fuses include Poly­Switch, OptiReset, Everfuse, Polyfuse, and Multi­fuse. They are available in surface-mount (SMT) packages or as through-hole components, but not in cartridge format.

6. Surface-mount fuses

Because surface-mount fuses are difficult or impossible to replace after they have been soldered onto the board, they are often resettable. As consumer electronic products become smaller and more portable, printed circuit boards have become more densely populated, forcing fuses located on the board to become even smaller. Additionally, more stringent electrical performance criteria have been imposed on these types of board-level fuses.

7. Blade-contact-type fuses

These are generally referred to as NH fuses, NH being an abbreviation of Niederspannungs Hochleitungs which is German for low-voltage high-breaking capacity. They are for use by authorized persons, mainly for industrial applications, and are used in factory distribution systems and also in the distribution cabinets of the ESI in power-distribution networks.,

The fuse elements are generally made from copper strips. The body is usually made of ceramic but high-temperature thermosetting plastic materials have been used. Bodies often have a rectangular outside cross-section with a circular longitudinal hole through them, and end plates, complete with the blade contacts, are attached to the body with screws.

NH fuse links are generally available for applications up to and including 1250A, for AC circuits operating at levels up to 500 V and DC circuits of voltages up to 440 V. In some installations, only the front of the fuse link is visible and there is, therefore, a requirement for an indicator at the center of the front of the fuse body.

8. End-contact or screw-type fuses

This is a very old fuse system, often referred to as the Diazed type or in some countries by the abbreviated form of this: ‘Zed’ type. Diazed was derived from ‘diametral abgestruft’, that is ‘diametral steps’. The term ‘bottle’ type is also used, this name stems from the characteristic shape of the fuse links.

Production of these fuses is perpetuated by the continuous need to supply replacements for some of the very large numbers which have been installed over many years. They are mainly produced with ratings up to 63Afor use in AC circuits operating at levels up to 500 V. The limitation in the current rating is largely caused by the difficulty of producing satisfactory contact with the holders, rather than by shortcomings in the fuse-link design. Higher ratings, up to 200A, have been produced but they have not proved popular.

9. Semiconductor fuses

The majority of these fuses are used to protect power semiconductors and are usually rated at 690, 1000, or 1250 V AC, but other voltage ratings will be found. Square-ceramic-body designs are very popular, the body lengths being shorter than those of fuse links used for industrial applications. End terminations suitable for bolted connections with fixing centers at 80 or 110 mm are widely used, but alternative versions with tapped holes in the ends are available. The latter design has the advantage of being more compact and is often used for ratings above 1000A.

10. Liquid fuses

In the earliest non-current-limiting fuses, the arcs were quenched in a liquid and this principle has been used for many years to produce fuse links. Many of them are in use in the UK. They have a body consisting of a glass tube with metal ferrules at each end and within it is the element. This is normally of silver strip or wire, with a straining wire across it except for low current ratings, say 10A and below, in which a wire is used as the element and this is made strong enough to make a separate strain wire unnecessary. In all designs, the element is positioned near the top of the tube so that it is shielded from corona discharge by the upper ferrule. The element is held in tension by a spring anchored to the lower ferrule and the tube is filled with an arc-extinguishing liquid, usually a hydrocarbon. When the element melts during operation, the spring collapses and the arc is extinguished in the liquid. To relieve the tube of excessive pressure, a diaphragm on the upper ferrule is ejected, except under very moderate conditions. Provision is usually made to allow the user to recharge by inserting a new fuse-element assembly and new liquid.