# What is a FUSE and How it Works?

- Link: https://studyelectrical.com/2017/09/what-is-fuse-and-how-it-works.html
- Published: 2017-09-23T13:58:00+00:00
- Author: 

[⌊2017 09 23 17h45 14⌉⌊2017 09 23 17h45 14⌉[**A fuse protects an electrical circuit
or device from excessive current when a metal element inside it melts to create 
an open circuit. **

In this post, we will look into the construction and working of Fuse and the properties
of fuse material. 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](http://www.studyelectrical.com/2014/05/circuit-breaker-operating-principle-and.html)
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.

[⌊Fuse Symbols⌉⌊Fuse Symbols⌉[

Fuse Symbols

Schematic symbols for a fuse are shown in the 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 the 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 I^(2)t 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.

### Fuse Construction and Working

![Construction of Fuse](https://3.bp.blogspot.com/-PX1_Cilwj5Y/W6sd6n4vF3I/AAAAAAAACrM/
HOhdd9fuUpwz9b_2xwisPAJClsqNN1biACLcBGAs/s1600/Fuse%2Bconstruction.png "Construction
of Fuse")

Construction of Fuse

The typical fuse consists of an element which is surrounded by a filler and enclosed
by the fuse body. The element is welded or soldered to the fuse contacts (blades
or ferrules). The element is a **calibrated conductor**. Its configuration, mass
and the materials employed are selected to achieve the desired electrical and thermal
characteristics.

The element provides the current path through the fuse. It generates heat at a rate
dependent on its resistance and the load current. The heat generated by the element
is absorbed by the filler and passed through the fuse body to the surrounding air.

The filler material, such as **quartz sand**, provides effective heat transfer and
allows for the small element cross-section typical in modern fuses. The effective
heat transfer allows the fuse to carry harmless overloads.

The small element cross-section melts quickly under short-circuit conditions. The
filler also aids fuse performance by absorbing arc energy when the fuse clears an
overload or short circuit.

When a sustained overload occurs, the element will generate heat at a faster rate
than the heat can be passed to the filler.

If the overload persists, the element will reach its melting point and open. Increasing
the applied current will heat the element faster and cause the fuse to open sooner.**
Thus, fuses have an inverse time-current characteristic: that is, the greater the
overcurrent, the less time required for the fuse to open the circuit.**

This characteristic is desirable because it parallels the characteristics of conductors,
motors, transformers, and other electrical apparatus. These components can carry
low-level overloads for relatively long periods without damage.

However, under high-current conditions, damage can occur quickly. Because of its
i**nverse time-current ****characteristic**, a properly applied fuse can provide
effective protection over a broad current range, from low-level overloads to high-
level short circuits.

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