Do Fuses restrict currentvoltage flow

[Anseo]

I'm trying to understand how currentvolts flow is not being restricted by some fuse types.

E.G. In 12v solar panel set-up, we are ancouraged to install relatively large cross section wires, so if we take a 10 AWG gauge wire (2.59mm approx) and then install a 25amp rated blade fuse somewher along the wire, can the 25amp fuse be a 'bottleneckobstruction to currentvoltage ?

A proprietary 25amp pre-wired 12v automotive type fuse comes with a blade fuse that has an internal conductor having a significantly smaller cross section than the the 10 AWG entering and exiting the fuse link.

 

[Guzzilazz]

Short answer is that it doesn't materially. the Automotive fuse holder holding a 20 A fuse will conduct 20A. If you were using a conductor that size along the total length of the conductor, it may do... But a 6" piece of smaller cable and the associated blade type fuse, it doesn't. The Prof's comments in the Solar thread is relevant...

 

[steve2019]

I would suggest that the installation guide cabling is err'ing on the side of caution, since these cables may be hidden it is always advised to install heavier cable than really necessary for the job,the heavier cable will have heavier protection too,
I know what you are saying about fuse holders having flimsy cables attached, I am sure if you bought one for a 20 amp fuse it would have been tested to 20 amps at least, but personally i would find a good quality fuse holder that has some decent cable at either end.
i very much doubt there would be any resistance anyway, resistance can of course be measured if you calculate the voltage and current (look up Ohms law)

 

[ProfJohnL]

I'm trying to understand how currentvolts flow is not being restricted by some fuse types.

E.G. In 12v solar panel set-up, we are ancouraged to install relatively large cross section wires, so if we take a 10 AWG gauge wire (2.59mm approx) and then install a 25amp rated blade fuse somewher along the wire, can the 25amp fuse be a 'bottleneckobstruction to currentvoltage ?

A proprietary 25amp pre-wired 12v automotive type fuse comes with a blade fuse that has an internal conductor having a significantly smaller cross section than the the 10 AWG entering and exiting the fuse link.

Technically a fuse does provide some restriction to the flow of current, but the actual amount of added resistance is not much in the overall scheme of things.

The purpose of a fuse is to stop the flow of current if it rises above a chosen threshold, and it does this by using the current passing through the fuse to self heat the fuses material. The greater the current, the hotter the fuse will get, and if the current is great enough and for long enough the fuses material will melt, and break the circuit. It becomes the weakest link.

Good circuit designers will choose a fuse that will rupture at a current that will not be enough to over heat the conductors in the circuit, so the fuse will protect the circuit wiring from damage including fire. If a fuse with a to big rating is used, there is a danger the wiring will be the weakest link, and under fault conditions the wiring may overheat and start a fire or cause a short circuit which can have catestophic effects on appliances and the installation.

Fuses are usually very short in length, so the total additional resistance it adds to a circuit is usually quite small. Generally their contribution in caravan circuits can be ignored, but in ultra low power circuits, a fuses contribution does need to be considered.

 

[Woodentop]

I'm trying to understand how currentvolts flow is not being restricted by some fuse types.

E.G. In 12v solar panel set-up, we are ancouraged to install relatively large cross section wires, so if we take a 10 AWG gauge wire (2.59mm approx) and then install a 25amp rated blade fuse somewher along the wire, can the 25amp fuse be a 'bottleneckobstruction to currentvoltage ?

A proprietary 25amp pre-wired 12v automotive type fuse comes with a blade fuse that has an internal conductor having a significantly smaller cross section than the the 10 AWG entering and exiting the fuse link.

Technically a fuse does provide some restriction to the flow of current, but the actual amount of added resistance is not much in the overall scheme of things.

The purpose of a fuse is to stop the flow of current if it rises above a chosen threshold, and it does this by using the current passing through the fuse to self heat the fuses material. The greater the current, the hotter the fuse will get, and if the current is great enough and for long enough the fuses material will melt, and break the circuit. It becomes the weakest link.

Good circuit designers will choose a fuse that will rupture at a current that will not be enough to over heat the conductors in the circuit, so the fuse will protect the circuit wiring from damage including fire. If a fuse with a to big rating is used, there is a danger the wiring will be the weakest link, and under fault conditions the wiring may overheat and start a fire or cause a short circuit which can have catestophic effects on appliances and the installation.

Fuses are usually very short in length, so the total additional resistance it adds to a circuit is usually quite small. Generally their contribution in caravan circuits can be ignored, but in ultra low power circuits, a fuses contribution does need to be considered.

A good explanation - as ever - by the Prof.
One thing however is how long it takes a fuse to blow. If you switch on and instantly draw say, twice the fuse's rating it will blow in milliseconds. If however you start with a low current and the current increases slowly over a period of time it is quite possible for the fuse to carry two or three times its rating without failing - but it will be glowing by then and could be a source for a fire!
In most applications - like the fuse in a mains plug - its purpose is to protect the cable, not whatever is connected to the other end of the cable. If the appliance or whatever needs protection then it will have its own fuse.
Miniature circuit breakers (MCB's) will trip according to their spec - B3, B6, or B10. A bit of Googling will show you what those specs are.
Finally, note that provided it is with the cable/fuseholders rating, the voltage in use is irrelevant. The power consumed will change - 3A @ 12V is 36W, 3A @ 240V is 720W - but it is the current that matters.

 

[ProfJohnL]

Hello WT

I did allude to the time and scale factors when I wrote, “if the current is great enough and for long enough.”

I didn’t expand on the time and current interaction. I also didn’t mention MCB’s as the OP was referring to solar panels and 12V dc systems.

Voltage drop can be a point to bear in mind, especially on low voltage circuits. If a fuse at its rated power were to have vol of the avtage drop of 0.5V in a 230V system that would be less than 0.25% of the available voltage.

If the same fuse were used in a 12V circuit at its rated current 0.5V drop would be about 4%, and in some voltage sensing circuits that needs to be included in any circuit design.

Hopefully the designers will have done their homework properly to take into account their effects.

One point about MCB’s they too have a delta operation time, Whist they are generally faster and more accurate in their current measurement, they are not instant, as the magnetic field that causes them to trip takes time to establish. Where MCB’s are used in AC circuits, the supply frequency will also affect the tripping time.