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Fuses What You Need To Know

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#1 KernowCooper



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Posted 07 August 2013 - 01:39 PM

The two most common automotive fuses are the glass style  and blade-typeApril_2012_Notes_Common_Automotive_Fuses


(known as Maxi Blade ATO, ATM or ATC). Glass fuses' amperage ratings are imprinted in tiny numbers on their metal ends or in a paper slip inside the glass tube. Blade fuses are color-coded for idiot-resistance (i.e., red is always 10 amps and so on).


There are 2 reasons why a fuse blows

  • Short Circuit: This occurs when the electrical current can't follow its normal path for some reason and flows directly to ground. Because the shorter route offers less resistance, the current can generate heat that degrades wiring and/or damages electrical components.
  • Overload: This occurs when the flow of the electrical current exceeds the wiring and/or equipment's capacity in the circuit. Two causes of overloads are too many components on one circuit or when one of the devices malfunctions. Like circuit breakers, properly functioning fuses protect the circuit by opening, blocking the current flow before it can damage electrical components.


All Fuses have Voltage Drop


A voltage drop across the fuse is usually provided by its manufacturer. There is a direct relationship between a fuse's cold resistance and its voltage drop value. Once current is applied, resistance and voltage drop of a fuse will constantly grow with the rise of its operating temperature until the fuse finally reaches thermal equilibrium or alternatively melts when higher currents than its rated current are administered over sufficiently long periods of time.


What you need to know about the difference between Glass and Blade Fuses



Blades type fuses are generally rated for blow current; i.e 15amp is the maximum current the fuse will take before blowing

Lucas (glass) fuses show continuous rating which is roughly half the blade fuses blow rating,but some are dual marked showing continuous and blow.


So caution is needed if your thinking of making a custom loom incorporating a blade type of fuse box, and taking out a old glass type fuse, fuse box.


Conditions when fuses blow


As listed above there are 2 main reasons why a fuse blows 1. Short Circuit 2. Overload


There is a 3rd reason why a fuse can blow.

Fuses age just like everything else. Over time, they will fail. So a blown fuse is not always an indicator that there is something wrong with the power circuit.

But if the inside of the glass fuse is black color or darkened, and there is no trace of the fuse element (the center connector), you know that there was a major short circuit somewhere in the circuit


But if a fuse is replaced and it blows straight away don't try a 3rd investigate why!


Never replace a Blown Fuse with one of a higher rating, find out the reason it blew. If you replace the fuse with one of a higher rating you risk a wiring loom/component fire


Also worth mentioning is its standard practice to run the fuses at 75% of there load limit

Edited by KernowCooper, 08 August 2013 - 03:11 PM.

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