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Electric Fuses – Devices to Prevent Circuit Damage and Fire

written by: phoenixwriter • edited by: RC Davison • updated: 5/4/2010

There are a lot of different types of fuses found in electrical equipment today. But do you really know how they work?

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    Fuse – An Overview

    Fuses, also called “fusible links", are small electrical devices, which provide protection for an electrical circuit whenever an uncontrollable amount of current flows. The basic components of most fuses are metal alloys that are designed to easily melt, breaking the circuit when an excessive amount of electric current flows through it.

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    The Mechanism: How It Works

    To make the discussion much simpler, let us concentrate on fuses that are used in most households. There are many types of fuses used in many different types of applications from protecting your computer's power supply to protecting heavy industrial equipment. Although varied in purposes, the same principles are employed.

    The basic mechanism of the fuse is very easy to understand. When a device in your house develops a "short circuit" (meaning the current it normally uses to power it is now being diverted, because of a failure, to ground) it will conduct an excessive amount of current. The fuse that is protecting this outlet will pass that current but in doing so will get very hot and the metal element inside will melt, breaking the path of the current flow. Until this fuse is replaced with a new one, that outlet will not work.

    Usually, fusible links are encased in an enclosure called the “fuse box" where the mains wiring comes in and is distributed to your house's outlets. The main fuse links your house to the power coming from the power company and then smaller individual fuses protect the wiring going to the networks of electric outlets in your home. Many applications today, such as household protection, are being replaced with electromechanical circuit breakers. These devices perform the same function as fuses but use a different mechanism to detect the excessive current flow and interrupt the circuit. They have the feature that once they trip and protect the circuit, they can be reset and used again, so you don't need keep a supply on hand as you do with fuses.

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    Are Fuses Always Reliable?

    Fuses are very reliable devices to protect your equipment, provided that they are properly specified for the application. A fuse that is under-rated will fail when normal currents are flowing, while a fuse that is over-rated will allow more current to flow than the circuit may be designed for, creating a hazardous condition. The time it takes a fuse to interrupt a circuit depends on the current being applied, the fuse construction and the temperature of the fuse. All these factors have to be considered when designing the fuse into a system.

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    Some Terms Used with Fuses

    Rated current - The amount of current the fuse can pass with out interrupting the circuit. This should be selected depending on how much current the device uses under all normal conditions.

    Voltage rating – This rating should be greater than or equal to the amount of voltage being applied to a circuit. This has implications in how much energy the fuse will be able to block.

    Voltage Drop – How much voltage will the fuse develop across it at nominal current flow. This has implications on low voltage circuits.

    Breaking Capacity – The maximum amount of current the fuse can safely interrupt. This is chosen depending on how much energy is available from the source providing the current.