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What Is The Fuse?

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 Schematic symbols for a fuse are shown in the figure. 

How Does it Work?

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 a 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 — given by the formula:


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.

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