dc fuse on ac circut

[domnic]

what is the difference in 32 volt dc fuse and a 120 ac volt fuse same size same amps?

 

[mdshunk]

Basicly, the "heavy-dutyness" of the fuse.

 

[LarryFine]

Another difference or two:

Typically, the low-voltage fuse has a thick link with a narrowed middle, which helps reduce voltage drop, while the high-voltage fuse usually has a constant-thickness link, and can open a larger gap because it has to interrupt a higher voltage.

 

[coulter]

Fuses in DC circuits with an inductive load have a heck of a fire to put out. Here is my understanding (do not confuse this to mean I know all about this):

Fuse starts to open.
Current starts to drop.
Magnetic field collapses, raising the voltage across the inductor.
This keeps the arc established until the mag field goes to zero.

All the energy stored in the magnetic field is released in the arc inside of the fuse.

This is not as much as a problem with ac circuits. With AC, the current goes to zero twice each cycle. The arc is self-extinguishing.

This "DC inductive load, current interupting" issue is why you see free-wheeling diodes across DC solenoid and relay coils, but not on AC coils. The diode dumps the energy in the mag field and keeps the control contacts from rapidly pitting and burring up.

It is also why CBs (and fuses) that have both a DC and an AC fault current rating, the DC rating will be a lot lower than the AC rating.

I didn't exactly answer your question. You left out one important part for your comparison. Is there a difference in the fault current rating?

carl

 

[davidr43229]

Genereal rule of thumb is that you can use the fuses' 1/2 the AC rating in a DC application. The exception being is the slew rate is 25ms or greater. In other words if, from 0-FLA it takes more that, you need to adjust for that.
32volts DC is an old UL standard for the auto industry and electronic fuses.
In the last 5-10 years the telephone industry (Bell-core-standard) has called out for DC protection, due to massive batteries being used to hold up power, in case of a power outtage. (due to a fire in Chicago (hindsdale fire)

Carl, in his post, stated that it is harder to inturrupt a fault in DC than AC due to DC not crossing through 0, which is true.

Another reason that a fuse may have a 120volt AC rating and a 32volt DC rating is that the fuse has not been tested any higher in the DC area, and approved by UL.
Some fuses will have a equal AC & DC rating, it all depends on if the manufacturer has tested that fuse for a specific End User to fulfill a requirement.

Generally the 32volt DC is a standard default rating.
Just my $.02

 

[LarryFine]

I believe the OP was asking about the physical differences in fuses such as the 3AG glass fuse, and that was reflected in my response.

Of course, a fuse must be rated within the manufacturer's testing parameters, but there are obvious, visible diffrerences in the fusible link.

 

[ELA]

Another reason that a fuse may have a 120volt AC rating and a 32volt DC rating is that the fuse has not been tested any higher in the DC area, and approved by UL.

Generally the 32volt DC is a standard default rating.
Just my $.02

The OP's subject line said "dc fuse on ac circuit". I am wondering if the question was, "can you use a 32Vdc rated fuse on a 120Vac circuit?"

I have always replied no to this. Although it is true that a dc arc is more difficult to extinquish, I have never felt comfortable that the 32Vdc fuse was designed well enough to interrupt 120 or 250vac. I had thought that they were probably designed more cheaply since they were for lower voltage circuits. Am I wrong in that assumption?

32Vdc rated fuses are common and easily accessible. I have many times found replacement 32Vdc rated fuses in 120Vac rated equipment and that concerned me. I always change them out with 250Vac rated fuses.

 

[davidr43229]

I have always replied no to this. Although it is true that a dc arc is more difficult to extinquish, I have never felt comfortable that the 32Vdc fuse was designed well enough to interrupt 120 or 250vac. I had thought that they were probably designed more cheaply since they were for lower voltage circuits. Am I wrong in that assumption

With the "glass" 3AG fuse there is no special DC link that is installed them, like the AGC, MDL 1/4" X 1 1/4" glass. Now the ATC/ATM bladed fuses are DC rated only and should not be used in any AC application. Why? becasue it's never been tested.
The narrow neck in the fuse is the "bridge" which continous current has to travel through. It is intented to be the weakest area of the fuse, while carrying 110% of it rated current at 25 degrees C. It's the bridge that gets burned back on.
You are correct to just say "No".
Just my $.03, adjusted for inflation

 

[LarryFine]

Now the ATC/ATM bladed fuses are DC rated only and should not be used in any AC application. Why? becasue it's never been tested.

Another reason, besides the fact that you won't find a holder rated for line voltage (or probably UL-anything'ed), is that contact with the blades is accessible from the tops of the fuses for testing.