The Complete Guide Introduction to Fuse

Introduction to Fuse , A fuse protects an electrical circuit or device from excessive current when a metal element inside it melts to build an open circuit. With the exclusion of resettable fuses (discussed separately in Resettable Fuses), a fuse must be cut and replaced after it has fulfilled its function.

When a high current flows a fuse, it is said to blow or trip the fuse. (In the case of a resettable fuse, only the word trip is used.) A fuse can run with either AC or DC voltage and can be designed for almost any current.

In domestic and industrial buildings, circuit breakers have become common, but a large cartridge fuse may still be used to protect the whole system from short-circuits or from overcurrent caused by lightning strikes on exposed power lines. In electronic devices, the power supply is almost always fused. Schematic symbols for a fuse are shown in Figure 1.

Alternate schematic symbols for a fuse. See text for explanation.
Alternate schematic symbols for a fuse. See text for explanation.

Those at the right and second from right are most commonly used. The one in the center is recognized by ANSI, IEC, and IEEE but is seldom seen. To the left of that is the fuse symbol known by electrical contractors in architectural plans. The symbol at the far left applied to be simple but has fallen into disuse.

≡ How It Works

Introduction to Fuse ,The element in a fuse is normally a wire or thin metal strip mounted between two terminals. In a cartridge fuse, it is included in a glass or ceramic cylinder with a contact at each end, or in a small metallic can.  The common glass cartridge allows visual inquiry to verify that the fuse has blown.

A fuse reacts only to current, not to voltage. When picking a fuse that will be reliable in conditions of steady current consumption, a safe rule is to figure the most amperage when all components are functioning and add 50%. But, if current surges or spikes are likely, their term will be relevant.

If I am the current wave in amps and t is its duration in seconds, the surge sensitivity of a fuse—which is usually referred to verbally or in printed format as I2t—is given by the formula:

I2t = I2 * t

Some semiconductors also have an I2t rating and should be guarded with a similarly rated fuse. Any fuse will present some resistance to the current flowing through it. Otherwise, the current would not make the heat that blows the fuse. Manufacturer datasheets list the voltage drop that the internal resistance of a fuse is likely to enter into a circuit.

≡ Values

Introduction to Fuse The current rating or rated current of a fuse is normally printed or stamped on its casing and is the highest flow that it should withstand on a continuous basis, at the ambient temperature specified by the manufacturer. The ambient temperature refers to the immediate setting of the fuse, not the larger area in which it may be established.

Note that in an enclosure containing other components, the temperature is normally significantly higher than outside the enclosure. Ideally, a fuse should function probably and indefinitely at its rated maximum amperage but should blow just as probably if the current rises by approximately 20% beyond the maximum.

In reality, manufacturers suggest that continuous loading of a fuse should not exceed 75% of its rating at 25 degrees Centigrade. The voltage rating or rated voltage of a fuse is the maximum voltage at which its part can be counted on to melt in a safe and predictable manner when it is overloaded by excess current.

This is seldom known as the breaking capacity. Above that rating, the remaining pieces of the fuse element may form an arc that sustains some electrical conduction. A fuse can always be applied at a lower voltage than its rating. If it has a breaking capacity of 250V, it will still give the same protection if it is used at 5V. Four differently rated glass cartridge fuses are shown in Figure 2.

Four glass cartridge fuses. See text for details.
Figure 2. Four glass cartridge fuses. See text for details.

The one at the top is a slow-blowing type, rated at 15A. Its element is created to absorb heat before melting. Below it is a 0.5A fuse with a correspondingly thinner element. The two smaller fuses are rated at 5A each.

The center two fuses have the most voltage rating of 250V, while the one at the top is rated at 32V and the one at the bottom is rated at 350V. Clearly, the size of a fuse should never be used as a guide to its ratings.


Introduction to Fuse ,Early power fuses in domestic buildings consisted of bare nichrome wire wrapped around a porcelain holder. In the 1890s, Edison formed plug fuses in which the fuse was contained in a porcelain module with a screw thread, compatible with the base of an incandescent bulb. This design continued in some U.S. urban areas for more than 70 years, is still found in old buildings, and is still being manufactured.

≡ Small Cartridge Fuses

Introduction to Fuse ,Small cartridge fuses for appliances and electronic equipment—such as those shown in Figure 2–are available in sizes listed in Figure 3. With the exclusion of the 4.5mm diameter fuse, these sizes were basically measured in inches; today, they are often described only with the equivalent metric measurement. Any cartridge fuse is normally available with the option of a lead attached to it at each end so that it can be used as a through-hole component.

The approximate physical sizes of commonly used small glass or ceramic cartridge fuses are shown here with the codes that are often used to identify them.
Figure 3 The approximate physical sizes of a commonly used small glass or ceramic cartridge fuses are shown here with the codes that are often used to identify them.

Fuses may fast-acting, medium acting, or slow-blowing the last of which may alternatively be assigned to as delay fuses. Extra-fast–acting fuses are prepared by some manufacturers. The term Slo-Blo is often used but is really a trademark of Littelfuse.

None of the terms representing 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 under application of extremely high current is possible, a ceramic cartridge is preferable because it contains a filler that will help to stop an arc from forming. Also, if a fuse is actually destroyed by the application of very high current, ceramic fragments may be preferable to glass fragments.

≡ Automotive Fuses

Introduction to Fuse, 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 heat changes. The fuses come in various sizes and are uniformly color-coded for easy identification. A selection of automotive fuses is shown in Figure 4. The type at the top is typically defined as a “maxi-fuse” while the type at bottom-left is a “mini-fuse.” Here again, size is unrelated to function, as all three of those pictured are rated 30A at 32V.

Three automotive fuses. All have the same rating: 30A at 32V.
Figure 4 Three automotive fuses. All have the same rating: 30A at 32V.

Normally automotive fuses are mounted together in a block, but if aftermarket accessory equipment is added, it may be defended by an inline fuse in a holder that terminates in two wires. This is shown with two sample fuses in Figure 6. Similar inline fuse holders are manufactured for other types of fuses.

Two blade-type fuses, commonly used for automotive applications, shown with an inline fuse holder. The plastic cap, at right, is closed over the holder when a fuse has been installed.
Figure 6 Two blade-type fuses, commonly used for automotive applications, shown with an inline fuse holder. The plastic cap, at right, is closed over the holder when a fuse has been installed.

≡ Strip Fuses

Introduction to Fuse, High-amperage fuses for vehicles may be sold in “strip fuse” format, also known as a fusible link, designed to be clamped between two screw-down terminals. Since some jumpers may look very similar, it is necessary to keep them separate. A strip fuse is shown in Figure 7.

This strip fuse is intended for use in diesel vehicles. The example shown is rated 100A at 36V.
Figure 7 This strip fuse is intended for use in diesel vehicles. The example shown is rated 100A at 36V.

≡ Through-Hole Fuses

Introduction to Fuse, Small fuses with radial leads, which seem suitable for through-hole insertion in printed circuit boards, are actually often utilized in conjunction with appropriate sockets, so that they can be easily replaced. They are explained in catalogues as “subminiature fuses” and are typically found in laptop computers and their power supplies, also televisions, battery chargers, and air conditioners. Three examples are shown in Figure 8. All have slow-blowing characteristics.

Three subminiature fuses terminating in wire leads. From left to right: 10A at 250V, 2.5A at 250V, and 5A at 250V.
Three subminiature fuses terminating in wire leads. From left to right: 10A at 250V, 2.5A at 250V, and 5A at 250V.

≡ Resettable Fuses

Introduction to Fuse Properly known as a polymeric positive temperature coefficient fuse (often abbreviated PTC or PPTC), a resettable fuse is a solid-state, encapsulated part that greatly enhances its resistance in response to a current overload, but gradually returns to its primary condition when the flow of current is discontinued.

It can be thought of as a thermistor that has a nonlinear response. Three through-hole examples are shown in Figure 9. While different sizes of cartridge fuse may share the same ratings, variously rated resettable fuses may be identical in size. The one on the left is rated 40A at 30V, while the one on the right is rated 2.5A at 30V. (Note that the codes printed on the fuses are not the same as their manufacturer part numbers.) The fuse at the top is rated 1A at 135V.

Some through-hole resettable fuses. See text for details.
Figure 9 Some through-hole resettable fuses. See text for details.

When more than the highestbcurrent 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 includes a polymer whose crystalline structure is filled with graphite particles that conduct electricity. As current flowing through the fuse produces 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 highest 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. See Figure 10.

Figure 10. The three curves give a very approximate idea of the temperature sensitivity of three types of fuses commonly used to protect electronic equipment. The left-hand scale provides an approximate value for the amperage which will trip the fuse.


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

≡ How to Use it

Introduction to Fuse Any equipment that may be plugged into a wall outlet should be fused, not only to protect its components but also to protect users who may open the box and start studying with a screwdriver. Equipment that includes powerful motors, pumps, or other inductive loads should be protected with slow-blowing fuses, as the initial surge of current when the equipment is switched on is likely to rise well above the rating of the fuse.

A slow-blowing fuse will allow a surge for a couple of seconds. Other fuses will not. Conversely, fast-acting fuses should be applied with electronic equipment, especially integrated circuits that are quickly and easily damaged. Any device using substantial battery power should be fused because of the unpredictable and generally bad behavior of batteries when they are short-circuited.

Parallel links between multiple large batteries should be fused to avoid the possibility that a highly charged battery may attempt to recharge its neighbor(s). Large “J size” fuses rated from 125A to 450A have become common in the solar power community, where banks of lead-acid batteries are often used.

These fuses have a thick brass tab at each end, drilled so that they can be bolted into place. Alternatively, they will push-fit into an appropriate fuseholder. For cartridge fuses up to 1/4″ in diameter that don’t have leads attached, appropriately sized fuseholders are available in several formats: Panel mounted fuse enclosure is probably the most common, consisting of a plastic tube with a spring-contact at the bottom, and a plastic cap with a second contact inside. The cap either screws onto the tube of the fuse, or is pushed down and turned to hold it in place.

A nut is provided to secure the fuseholder after it has been inserted into a hole drilled in the panel. The fuse is dropped into the tube, and the cap is applied. This type of holder is available in full-length or shorter, “low profile” formats. A low-profile holder is shown in Figure 12. It is shown assembled at right, with its component parts disassembled alongside.

Figure 12. A low-profile panel-mounted fuse holder shown disassembled (left) and assembled (right).

Circuit board mounted fuse enclosure is fundamentally the same as the panel-mounted version, but with through-hole solder pins attached. Fuse block is a small plastic block with two clips on its upper surface for insertion of a cartridge fuse. Fuse clips can be bought individually, with solder pins for through-hole mounting. Inline fuse holder is designed to be inserted in a length of wire. Usually made of plastic, it will either terminate it, wires or will have metal contacts to crimp or solder at each end. See Figure 4-6. Through-hole fuse holders are available for subminiature fuses.

≡ What Can Go Wrong

Repeated Failure

When a fuse in a circuit blows frequently, this is known as nuisance opening. Often it can result from failure to take into account all the aspects of the circuit, such as a large filtering capacitor in a power supply that draws a major surge of current when the power supply is switched on.

The formally correct procedure to address this problem is to measure the power surge, properly known as peak inrush current, with an oscilloscope, calculate the I2 * t of the wave form, and select a fuse with a rating at least 5 times that value. A fuse should never be replaced with an equivalent length of wire or any other conductor.

Soldering Damage

When a through-hole or surface-mount fuse is soldered into place, heat from the soldering process can cause the soft metal element inside the fuse to melt partially and reflow. This is likely to change the rating of the fuse. Generally, fuses should be treated with the same caution as semiconductors when they are fixed in place with solder.


A fuse should be placed close to the power source or power input point in a circuit, so that it protects as much of the circuit as possible.

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