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# simple fuse question

#1
06-25-06, 09:25 AM
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simple fuse question

I have a simple motor device which is running off a 12v D.C. battery which I'd like to add a fuse to. I know that the main rating for fuses are in terms of Amperes but what about voltage? Does it matter whether the fuse is for A.C. or D.C.?

I have lots of those glass tube fuses and holders around the house which I'd like to use but i know they are rated for 110V A.C. Could they be used for this particular DC application too?

#2
06-25-06, 05:28 PM
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volts does matter for fusses. However inyour case at 12 volts you dont' need to worry. The fuse voltage needs to be at least the voltage of the system. It can be higher but not lower.

#3
06-25-06, 05:54 PM
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Although I agree that volts does matter, I think it's backwards from what you state.

Of course, I am no electrical engineer, but here is my thinking.

A fuse blows not because the amprage has been exceeded, but because the wattage (heat) has been exceeded.

A 5a 250v fuse will allow 1250 watts before it blows.
A 5a 12v fuse will allow only 60 watts before it blows.

I could be way wrong, but I would say you need to determine what your actual load is at 12 vots, and use a fuse rated at 12v and 120% of actual amp draw. If you can convert your load to watts you could then calculate what amp fuse you need for a fuse rated at 250v.

#4
06-25-06, 06:28 PM
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Actually joed has it right.

Here is a passage from a bussman manual.

Voltage rating
Most low voltage power distribution fused have 250 or 600 volt ratings (other ratings are 125 volts and 300 volts).The voltage rating of a fuse must be at least equal or greater than the circuit voltage. It can be higher but never lower. For instance, a 600 volt fuse can be used in a 208 volt circuit. The voltage rating of a fuse is its capability to open a circuit under an overcurrent condition. Specifically the voltage rating determines the ability of the fuse to supress the internal arcing that occurs after a fuse link melts and an arc is produced. If a fuse is used with a voltage rating lower than the circuit voltage, arc supression will be impaired and, under some fault conditions, the fuse may not clear the overcurrent safely. Special consideration is necesary for semiconductor fuse application, where a fuse of a certain voltage rating is used on a lower voltage circuit.
So basically it is a function of safety and correct function. If you use a 125 volt fuse in a 600 volt system, when the fuse pops, it may really POP!! An amp is an amp regardless of the amount of voltage behind it. The current is a product of the voltage and the impedance of the system. When you are dealing with 12 volts, there is not enough pressure to be overly concerned with. There is not enough power there to make most any fuse blow up.

Think of it this way. If you were right md, all current ratings on any wire would have to be at a specified voltage. They obviously aren't.

and if you look at the older glass tube auto fuses, they are usually rated 125 or 250 volts.

#5
06-25-06, 07:34 PM
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Originally Posted by mdtaylor
A 5a 250v fuse will allow 1250 watts before it blows.
A 5a 12v fuse will allow only 60 watts before it blows.

.
These wattage calculations do not apply to the fuse itself. Indeed if 5 amps is flowing from a 250 volt circuit, then the LOAD is consuming 1250 watts. If the fuse were consuming 1250 watts, it would be glowing cherry red and be VERY hot, and fuses dont do this. The watts ( heat ) developed by a fuse is deterined by its internal resistance, which is very low compared to the circuit load.