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.
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.
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.
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.
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.
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.
≡ 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.
≡ 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.
≡ 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.
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.