Amps are amps...right?

The real problem is electrons. When them little buggers get loose you can't put them back into the wire even if you could catch them.

Sam Spade wrote:

mike-s wrote:
But that's the wattage within the fuse itself, not the number of watts in the circuit being fused.

Which is why the voltage makes a difference.

The voltage drop across the resistance of the fuse causes heat, which melts the fuse element.

30 amps flowing at 12 volts makes about 10 percent of the heat across the fuse as 30 amps flowing at 120 volts.

No sir, I'm sorry but that's not correct. Voltage doesn't make heat - current does. Your body can conduct hundreds of thousands of volts (with tiny current levels) and not be burned. Do the oposite with current and you fry!
The fuse doesn't care about voltage at all unless it was so high it could jump across or out of it. It's very easy to contain such low voltage - so easy that most wire is rated well beyond 300 volts, 600 being more the norm.

Someone else said it had to do with ohms law - it doesn't. It is soley the heat caused by current that melts the fuse.
Those that are suggesting voltage has something to do with it need to read up on the current/coulombs relationship with special attention to what is not involved.

ScottG wrote:
Someone else said it had to do with ohms law - it doesn't. It is soley the heat caused by current that melts the fuse.
Those that are suggesting voltage has something to do with it need to read up on the current/coulombs relationship with special attention to what is not included.

As I said above, an AGS-30 fuse will have a voltage drop of about 0.12 V @ 30 A current. That works out using Joule's Law (which is commonly combined into Ohm's law), to be 3.6 W which the fuse has to dissipate. Go to 100 A, and you're closer to 100 W, which melts the link in under a second.

But yes, the circuit voltage doesn't make any difference to when the fuse blows, only the current.

mike-s wrote:

ScottG wrote:
Someone else said it had to do with ohms law - it doesn't. It is soley the heat caused by current that melts the fuse.
Those that are suggesting voltage has something to do with it need to read up on the current/coulombs relationship with special attention to what is not included.

As I said above, an AGS-30 fuse will have a voltage drop of about 0.12 V @ 30 A current. That works out using Joule's Law (which is commonly combined into Ohm's law), to be 3.6 W which the fuse has to dissipate. Go to 100 A, and you're closer to 100 W, which melts the link in under a second.

But yes, the circuit voltage doesn't make any difference to when the fuse blows, only the current.

To that end, I've seen where a person put way too small of a fuse in a circuit and while the current was insufficient to blow said fuse, the fuse acted as a current limiting resistor and limited the divices output.

But it depends...

A fuse isn't always the same rating as the wire. Sometimes a fuse rating can be higher than the wire in the circuit, as in the case of motor circuits like stab. jacks and slides. And sometimes a fuse can be smaller than the wire if voltage drop is an issue.

A fuse "blows" because the element melts from heat. Heat, in watts, is dependent on the amps flowing through the fuse and the voltage drop across it as per Ohms law. The resistance of an element in a fuse is normally dependent on the voltage rating of the circuit.

However, sometimes the voltage rating of a fuse is not linked to the circuit voltage. A fuse voltage rating can sometimes relate to it's ability to extinguish the arc that is generated as the fuse element melts under fault conditions and the type of circuit it is in. A fuse in series with an inductive circuit can have voltages several times greater than the supply voltage during the time it is clearing the circuit and needs to be rated according.

Choosing the right fuse for a specific application can require a range of factors to be assessed such as let-through current rating and speed rating. Time-current characteristics are sometimes important like the graph below that shows an example of a 10 amp "littlefuse". And sometimes a fuse has to coordinate correctly in conjunction with a circuit breaker.

In summary, if a blade fuse in your converter panel blows, simply replace it with the same size. :)

mike-s wrote:

ScottG wrote:
Someone else said it had to do with ohms law - it doesn't. It is soley the heat caused by current that melts the fuse.
Those that are suggesting voltage has something to do with it need to read up on the current/coulombs relationship with special attention to what is not included.

As I said above, an AGS-30 fuse will have a voltage drop of about 0.12 V @ 30 A current. That works out using Joule's Law (which is commonly combined into Ohm's law), to be 3.6 W which the fuse has to dissipate. Go to 100 A, and you're closer to 100 W, which melts the link in under a second.

But yes, the circuit voltage doesn't make any difference to when the fuse blows, only the current.

This diagram kinda makes me wanna kill myself. And I'm an engineer. Thank God my technical side is offset by the fact that I'm a natural blonde. Can't we just roast some marshmallows or go for a swim?

tarnold wrote:
Be nice ok? Size of wire determines current capacity (amps) as I believe. Recently had to replace a 30 amp fuse(U shaped). Had small size about same as a dime but needed next larger, about size of a nickel. Went to Autozone and got what I needed. They had same style that was about the size of a 50 cent piece, if you can remember what they look like. All 30 amp, so question, why the much different physical sizes, if all are still 30 amp rated?

Different sizes can handle different voltages but I think the real reason is the Almighty Dollar.. Yup, Profit.

You see with 3 different sizes of blade fuses. You have to buy 3 times the fuses to insure you have a spare, this means more PROFIT for they manufacturers.

Duplicate.

road-runner wrote:
The voltage across the fuse, and the heat it generates, is a function of the current and fuse resistance, nothing else.

This is correct of course.
Embarrassing mistake on my end.

As I understand it, the voltage rating is the interrupt rating- that's why those easy to blow if you're not paying attention fuses in the Fluke meters is so expensive- the 650 volt rating.

Another neat fact- fuses are precision enough they can be used as current shunts- there are tables out there for mv/amp conversions. Easy to measure across the fuse.