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Fuses. I just recently bought some ato fuses to use in my

furnace they are 3 amp...
Fuses. I just recently bought some ato fuses to use in my furnace they are 3 amp fuses rated to 30 volts. I'm questioning how these fuses work at lower voltages do they still open at 3 amps and why. I have some controllers that R rated for multiple voltages and by voltage the amp limits very so as the voltage going through the controller goes down the amps allowed goes up why doesn't the same concept apply to a fuse or does it?
These ato fuses are rated at 3 amps up to 30 volts wouldn't this be a seven and a half amp fuse at 12 volts?
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Answered in 1 hour by:
10/15/2017
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., .
Category: Home Improvement
Satisfied Customers: 22,767
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Hello,

My name is Billy. I assume you are talking about the circuit board protection 3 amp fuse?

Have you added aftermarket items to the control system?

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Customer reply replied 1 month ago
No,

Ok, the 3 amp fuse is there for one reason...to protect the circuit board.

It does not matter what other loads, volts or amps you add via controllers.

The 3 amp (some are 5 amp) are fixed values solely for the board. ....if 3 amps+ comes through the fuse blows.

Now these atc fuses are only designed for 12dc/24ac normal operation...But the pricicple is the same with any fuse...it is heat protection. Amps is what carries the heat not the voltage.

ALL fuses and breakers are designed for amp trips not volts. Your home electrical panel can use a single 20 amp breaker of which you run 120v through...but gang 2 together like an ac....and you have 240 volts....

I do not know if I am explaining right....but the fuse on the board is designed for protection against shorts....not operating amperage. If the wires get shorted at the ac unit, the fuse trips due to direct short....rarely do they trip do to the system running 3.5 amps...it is in response to a short.

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Customer reply replied 1 month ago
I'm not sure that you can answer my question so can you put this back out for someone else. I don't need to know what fuses are for, I need to know how it works and why. Somethings about ohms law and how it applies or doesn't

Then you need to ask a specific question regarding the fuses and operation.

Resistance (ohms) creates heat( amps)....that is why breakers are listed in amps.

Resistance is the load in the circuit.

Not only do I have 25 years experience in the field, I also teach nights at a community college HVACR class.....so if you have any specific question I am sure I can address it. Maybe if you asked in a "real life scenario"question I can tell what you are trying to get to.

If I cannot I will be happy to opt out and let another tech take over.

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Customer reply replied 1 month ago
My original post was specific, three questions about how fuses work.

I agree with the suggestion that another tech should jump in and assist.

Please hold your rating for the next technican.

I am opting out now and wish you the best.

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David Bowden
David Bowden, General Contractor
Category: Home Improvement
Satisfied Customers: 337
Experience: Owner at Floridaco
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This question came up and I thought I would try to answer it, however if I am unsuccessful just let me know and I will also opt out so another expert might be able to explain it better. Please only rate me if I am helpful otherwise another expert will not be able to access the question or answer it.

Here is a link to better understand how these particular ATO fuses are designed to fail. Basically they fail for 3 reasons. Excessive heat, excessive amps, and excessive time.

http://www.littelfuse.com/products/fuses/automotive-aftermarket-products/blade-fuses/ato.aspx

Excessive heat; This is measured 2 ways, ambient temperature and applied friction. Fuses can fail if the suns heat exceeds designed temperatures (increasing the ambient temperature softens/weakens the metal) or if the applied friction (resistance) exceeds the melting point of the metal.

Excessive amps; The amps are electricity's resistance (heat) of electron movement. When electrons move, they produce friction (heat) when they are forced to slow down (resistance) they also produce heat. Both of these occur at a rapid rate.

Excessive time: Fuses can be made in a way to either reduce or increase the time necessary to fail. If you look at these particular fuses you can see that the left and right blades are made of a thicker and wider metal, whereas the fusible link metal in the middle is shaped smaller and thinner. This is the time feature before the fuse will fail. If you filed some of the metal away you can alter the time required for the fuse to fail. Some fuses, like in an air conditioner for example, have a time delay or slo-blo feature built into the fuse. These are typical where motors or compressors are involved, since it takes more amperage (resistance) to start a compressor or motor, than it does to keep it running. A mass in motion requires less force to keep it moving.

To answer the last part of your question regarding voltage increase vs. decrease, here is a link to better understanding why that is.

https://physics.stackexchange.com/questions/141786/given-ohms-law-how-can-current-increase-if-voltage-increases-given-fixed-resi

I hope this answered and explained all of your questions,

Best,

Dave

David Bowden
David Bowden, General Contractor
Category: Home Improvement
Satisfied Customers: 337
Experience: Owner at Floridaco
Verified
David Bowden and 87 other Home Improvement Specialists are ready to help you
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Thank you very much for the positive rating, I sincerely ***** ***** If you need further assistance or help with another question, just let me know,

Best,

Dave

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