# Fuse at 250 volts versus fuse at 12 volts

I'm getting mixed answers. Will a 250 volts, 140 amperes fuse blow as fast as with 12 volts?

140 amps through a fuse is 140 amps regardless of the supply voltage in a system and fuse will blow because of current through it.

Now the difference between a 12V fuse and 250V fuse is that the 250V fuse can safely handle 250V when it has blown open, but a 12V fuse cannot hande safely 250V because it is not rated for 250V and it may continue to arc over.

• There can be other functional differences between a 12V fuse and 250V fuse, like the 12V fuse would usually have a lower resistance thus lower power loss. I also think it can be faster. But speed is a parameter of fuses, and it's possible to make a fuse intentionally slow, thus the data at hand is not enough to say which fuse will be faster. Aug 19, 2023 at 16:01
• @fgrieu For a given construction and environment the active part of an xxx amp fuse will be the same regardless of system voltage. It's the energy dissipated in the fuse that melts the link and as this is a product of current squared and resistance the resistance will be the same in both cases. If you change thermal paths or cooling then the resistance would need to change to compensate. Aug 20, 2023 at 13:22
• @RussellMcMahon: the need for increased robustness against arcing of a 250V fuse compared to a 12V fuse (as explained in the question) typically means a higher blow temp, hence need for higher power dissipation, hence higher loss and higher resistance since P=RI². Aug 20, 2023 at 14:01
• @fgrieu (1) I essentially agree with you :-). || (2) " ... need for increased robustness ... " --> If we ignore my "For a given construction and environment ...", then, sometimes. I have probably 100's of fuses in my fuse cabinet drawers. For the fast-blow glass ones of a standard size the construction of those sold for 12V or 230 VAC use looks identical. I'll try to get to measure some with a milli-ohm-meter sometime. None are 140A rated :-) Aug 21, 2023 at 3:00

At some point, a high enough voltage rating will imply a high voltage drop at rated current, and therefore 12V won't even be able to sink 140A through the thing.

I don't know numbers offhand, but I suspect even distribution line fuses (10kV+) don't drop quite that much, so this will be quite a low order effect, and certainly no 250V fuses I know of, would suffer such a problem.

Notice that opening time depends on fault current, typically well above rated current.

The amount of fault current possible, similarly depends on voltage, and so it could be that a fuse opens faster at 250V than 12V -- because the 250V circuit has a fault capacity of say 2kA and more than enough voltage to force as much through a fault condition; whereas a 12V circuit might only do 1kA into a properly "short" circuit (say a car battery into the proverbial screwdriver), whereas with the resistance of real wiring, an only-mostly-shorted load, and the fuse, maybe it's only 500A instead?

In any case, the correct answer is not some set of assumptions that will inevitably be wrong in enough cases to be meaningless; the point is it can be calculated, given sufficient data. Take the short-circuit fault current, whatever that is -- it depends on source impedance, wiring resistance, fuse resistance, and whatever the expected fault resistance is -- and apply it to the fuse's time-current curve. Whatever the result is, between the two particular applications (say 240V mains vs. whatever the 12V thing is), there lies your answer.

One concern is whether the 250 volts and 12 volts are both AC, or whether the 12 volts could be DC? An AC fuse will (generally) not interrupt DC power. A DC arc through a blown fuse will not extinguish and the power will not be interrupted (until the fuse terminals and holder has been eroded by the arc to a much larger separation than is inside the fuse).

As others have answered, if both circuits are AC or both circuits are DC, the fuse opens when it gets hot, in proportion to:

current * current * fuse_resistance

And doesn't have any dependence upon the system voltage.

• THIS. AC and DC specifications on a fuse are two entirely separate things! Aug 19, 2023 at 0:10
• Doesn't it depend on the type of fuse? E.g., can the (DC) arc be maintained if the fuse has silica sand in it? Aug 19, 2023 at 1:37

As noted, a fuse blows on the current passed and circuit voltage does not affect the time taken.

But, as Justme (NOT JustMe) notes, voltage rating matters for safety purposes.
In higher voltage circuits where high fault currents are possible, a 10A fuse my blow in well under millisecond if subject to say 100A fault current. However, the fuse needs to be rated to break 100A and to not sustain an arc with the available circuit voltage across the fuse. A 12V fuse may not properly interrupt an arc at 100A in 230 V circuit.

In circuits where very high fault currents are expected and are required to be safely interrupted HRC (High Rupture Capacity) fuses may be specified.

Everything You Need To Know About HRC Fuse

Complete Guide to High Rupturing Capacity (HRC) Fuses

High Rupturing Capacity (HRC) Fuses

Web search HRC fuse ratings

Typicla fuse time to blow curves for a fast blow fuse family.
I've highlighted the 1 amp fuse curve and added a few intercept points to show the blow times at various currents.
Note that current is the only parameter considered. (In practice ambient temperature and air cooling can have a minor effect.).

Image from here

• TECHNICALLY, this is not correct - what blows a fuse is power, not current, but I can't see any way that that would make a critical difference. Aug 18, 2023 at 9:57
• @MikeB Technically it's temperature :-) . ie the fusible link fuses. Next in line is energy. But time and thermal conductivity and a little radiation and convection and ... . But current is a reasonable proxy. As you note, power/energy dissipation matters, but the key external parameter is the circuit current. I know you know that :-) Aug 18, 2023 at 15:32
• @MikeB: low power can also blow fuses. Power is not what blows fuses. Fuses melt at their rated temperature, and since temperature is an effect of current, for any given resistance, one could say current is what blows fuses. Aug 20, 2023 at 6:14
• @Zimba Your first part is correct, but your conclusion absolutely not. Imagine for a moment that your fuse will blow at 500K (or any temp you like, on any scale) and your ambient temp is 1 degree below - how much energy will be required to make it blow? Now compare with the same fuse, at an ambient temp of 1K and tell me the same current is required. Aug 21, 2023 at 15:42

Fuses are designed to melt at some temperature. This temperature is based on the fuse resistance and amount of current flowing through it. This resistance is selected to the required max. current that the fuse is designed to allow. Voltage rating of a fuse is a safety limit to prevent arc and undesired short circuits).

For your question, a 140A fuse will blow around 140A (A hot summer day will blow fuse just after 140A, a cold winter could delay until a few amps higher). Since current is the result of potential difference (voltage) across some resistance, 250V will blow the same fuse much faster than 12V since it'll reach melting temperature faster.

As an example, see this image showing 25A fuse blown to disconnect a 240V load when drawing power at about 9.2kW, which should have blown at 6kW (if you do the math). Voltage spikes were transient and not persistent long enough to heat up the fuse to melting point.

Notice the gap during which no power was being supplied until new fuse replaced and power reset at under 6.9kW

If the fuse was rated for 12V, the higher 250V voltage may create a short circuit to some other ground, in which case the fuse may stay intact and the short circuit will make fire long before the temperature around the fuse wire reaches the melting temperature for it to blow (from the fire that is, not the current flowing through it).

Some plastics create poisonous gases when melting so in case of fire, so you'll have additional poison and explosion hazard to deal with, in case of fuse material and other plastics or volatile compounds nearby.

• You appear to have some misconceptions about fuses: melting temperature is determined by element material; resistance by material and geometry; and power by heat dissipation from the element versus resistive heating. I suggest reading a few datasheets: the rated current is the maximum current at which no failure will occur; typically fusing is guaranteed at some additional margin e.g. double rated, within some time frame (seconds to minutes), and faster thereafter. High voltage cannot keep the fuse from blowing, but it can keep it arcing long after melting, which is the real hazard. Aug 20, 2023 at 6:20
• The 140A is overwhelmingly a result of external circuit factors. eg in the 12V situation (1680 watt) it may be the load from a motorcycle starter motor. In the 250V situation (35 kW) it may be a small commercial building. The fuse resistance will be a very small part of the overall circuit resistance in either case, so time to reach 140A will be essentially fuse resistance independent. Aug 20, 2023 at 13:37
• @Zimba Do not insult other users.
– Null
Sep 6, 2023 at 17:16
• @Zimba The edit record shows that your "paragraph 3" reference to temperature was added 2 weeks + after the comment of mine that you are "correcting". I'd usually ignore such things, but note it now as it's something that you need not to repeat with other people in future. Sep 7, 2023 at 1:58
• @Zimba Thanyyou for the interesting links relating to fuse construction and operation. || Other: The original question did not specify AC or DC in either case. Usually you'd assume AC and DC at the two voltages given in isolation. You are correct in noting that fuse construction is affected by which is being considered. || You wrote: " ... Arcing is a characteristic of DC, not AC ..." --> Arcing is a common occurence on AC circuits, as I hope you are aware. Characteristics vary but both are important. AC arcing in even residential installations must be designed for. Sep 7, 2023 at 2:00