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What is a FUSE and How it Works?


A fuse protects an electrical circuit or device from excessive current when a metal element inside it melts to create an open circuit. With the excep­tion of resettable fuses, a fuse must be dis­carded and replaced after it has fulfilled its func­tion.

When high current melts 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 work with either AC or DC voltage, and can be designed for almost any current. In residential and commercial 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. Those at the right and second from right are most frequently used. The one in the center is approved by ANSI, IEC, and IEEE but is seldom seen. To the left of that is the fuse symbol understood by electrical contractors in architec­tural plans. The symbol at far left used to be com­mon but has fallen into disuse.

How a FUSE Works?

The element in a fuse is usually a wire or thin metal strip mounted between two terminals. In a cartridge fuse, it is enclosed in a glass or ceramic cylinder with a contact at each end, or in a small metallic can. (Old-style, large, high-amperage fuses may be packaged in a paper or cardboard tube.) The traditional glass cartridge allows vis­ual inspection to confirm that the fuse has blown.

A fuse responds only to current, not to voltage. When choosing a fuse that will be reliable in conditions of steady current consumption, a safe rule is to figure the maximum amperage when all components are functioning and add 50%. 

How­ever, if current surges or spikes are likely, their duration will be relevant. If I is the current surge in amps and t is its duration in seconds, the surge sensitivity of a fuse—which is often 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 protected with a similarly rated fuse.

Any fuse will present some resistance to the cur­rent flowing through it. Otherwise, the current would not generate the heat that blows the fuse. Manufacturer datasheets list the voltage drop that the internal resistance of a fuse is likely to introduce into a circuit.