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Discussion Starter · #1 ·
I really don't get fuses and ratings and all that stuff. Was checking a few things today when the the curiosity in me :rolleyes: got thinking . As a test I just tried to see what amount of current circuits actually drew compared to the rating of fuse that's there. For instance I took out the fuse for the reversing lights and put a meter on there and put on the ignition and the meter read about 3 amps, when the fuse in there is 15amp. Done the same with the left headlight and meter read 3.5 amps and again 15 amp fuse. Why is there such a gap between the two figures??
 

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Fuses are designed to protect the wire, not the device. Nothing to do with the normal current draw. Wiring is sized to avoid voltage drop.

Also fuses are rated at continuous current but need about double to blow.
 

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Discussion Starter · #4 ·
Oh I see, but why not have a fuse thats slightly above the normal draw and then if a surge or short circuit occured then fairly instance blow. And why do fuses blow at such a high level above the rating of the fuse? Sorry I sound like a little child :rolleyes: thanks, James.
 

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Well, a device is designed to run on full rated power. What part of a device would a fuse protect? The fuse protects the wiring if the device fails or the wire breaks.

Also the fuses are a deliberate weak point, they do age and you don't want to have them blowing on a normal voltage surge from an alternator, so there's a bit of over design.
 

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At least on cars you can see if fuse has blown or not - how many times have you gone to a domestic pug to check fuse and thought hmmmmm has it or hasn't it! Then you find a fuse in the usual kitchen draw but you don't know if it's good or bad - why don't they change colour when they blow!!!

Sorry I digress as I'm now off for a nice drive through the misty Norfolk countryside to get lost!.

BTW Don't like these bi xenons in fog in the dark!
 

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Just correcting a few minor factual errors...

The rating of a fuse is the current it will nominally carry in steady state without blowing. In addition to the steady state current, there are also the different variants; normal, fast and anti-surge. These latter charateristics determine how quickly the fuse blows in an over-current situation. Low voltages used on cars tend to mean very high inrush currents, particularly on lamps. The fuses don't lend themselves to traditional anti-surge construction so tend to be a little over-sized.

At 40-50% over nominal current, it could take a minute or two to blow.
At twice nominal current, the fuse will blow within 5 seconds max.

Also, fuses aren't necessarily rated correctly for the wiring they are intended to protect. For example, the wiper system has a 30A fuse on it. Whislt the primary feed from it is in 2.5mm wire which is rated at 30A, there are many sub-circuits that are in 1.5mm, which is rated at something more like 18A max.

To answer the domestic fuse question, they have to be in very robust packing which precludes the use of any obvious optical indication.

Alternators don't generally have voltage surges but any device being fed by it can have a current surge.
 
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