DC Fuses

[4827]

Can you use AC fuses for 24 VDC control circuits? We commonly fuse all 24 VDC using say a MDL-1 style fuse but they are AC. I thought that in 508A somewhere it was stated that you needed to use DC fuses on DC circuits but I cannot find it. This would be for low voltage wiring on control circuits.

 

[GBShade684]

It depends on how you are looking at the problem. From a UL perspective, it is not a UL listed / recognized component with a DC rating, so I imagine the answer would be no. But from a manufacturer perspective, the fuse is more than capable of interrupting a 24Vdc circuit. In fact any fuse in that size and the 5mm x 20mm size should be capable of interrupting up to 32Vdc without additional testing.

 

[Jraef]

A-C rated fuses should not be applied in D-C voltage
circuits unless D-C application ratings are provided by the
fuse manufacturer. Except for some special purpose fuses,
D-C ratings are not usually shown on fuse labels.

http://www.cooperindustries.com/con...es/Data Sheets/Bus_Elx_DS_2004_MDL_Series.pdf
No DC listing

If you look at the GBB line, it is basically the same thing, but rated for DC.

http://www.cooperindustries.com/con...es/Data Sheets/Bus_Ele_DS_2013_GBB_Series.pdf

 

[petersonra]

I seem to recall some of these small glass fuses are only voltage rated, but don't specify either AC or DC.

 

[dicklaxt]

Simply put I don't think the fuse knows the difference between AC & DC it has to do with the point of has it been tested under those conditions.The voltage rating value is the level that can still be safely(and been tested for) applied when the fuse is blown and not arc across the gap thus still conducting a current.

If AC or DC is different in this regard would someone explain that or post a link.

dick

 

[fmtjfw]

Simply put I don't think the fuse knows the difference between AC & DC it has to do with the point of has it been tested under those conditions.The voltage rating value is the level that can still be safely(and been tested for) applied when the fuse is blown and not arc across the gap thus still conducting a current.

If AC or DC is different in this regard would someone explain that or post a link.

dick

AC VERSUS DC OPERATION
It is true that some AC fuses are also suitable for DC circuit
operation; however, testing is the only sure way to
determine the DC voltage capability of a particular AC fuse.
There is no ?rule of thumb? that safely converts an AC
voltage rating on a fuse to a DC voltage rating. To ensure a
fuse will safely interrupt a DC circuit, the fuse must have
been tested using circuit parameters representing the specific
application. The key question is how much DC capability
does an AC rated fuse have?
On a 60 Hertz, AC system, the current crosses zero 120
times per second. This natural zero crossing helps the fuse
extinguish any residual arcing that occurs after the element
melts. There is no natural zero crossing on DC circuits;
therefore, the fuse must be capable of absorbing and
extinguishing all of the energy in the DC arc

see this for complete paper:
http://www.arcadvisor.com/pdf/Fuse-Protection-of-DC-Systems.pdf

 

[iceworm]

... If AC or DC is different in this regard would someone explain that or post a link. ...

Dick -
I can't see fmt's paper - so this could have been covered.

DC circuits have inductance - sometimes a lot. If the fuse opens, the magnetic field collapses, and energy is dumped into the fireball inside of the fuse. As noted, with AC, the current g0es to zero 120 times per second - the fireball self quenches.

ice

 

[mikedionne]

24 VDC Fuses for Control

24 VDC Fuses for Control

UL Does require that a fuse used on a DC Circuit must be Listed. To replace your MDL fuse I would use a Bussmann ABC. The ABC has both AC & DC Ratings.

 

[dbaird]

I don't know your application and/or if it matters but both the Bussman GBB and ABC fuses are fast acting fuses. The MDL is a time delay fuse.

 

[petersonra]

I get caught on MDL fuses now and then because they are not rated for 120V above about 6 or 8 amps.

There are other slow blow small fuses of this type that are rated up to 20A or so for both AC and DC.

For some reason years ago I started using MDL style fuses of this size and it just stuck with me. They are not the cheapest or have the widest application, but for whatever reason they are my first choice just out of habit.

 

[renosteinke]

Let's look at electricity for a moment.

AC is typically at 60hz. That means it has zero volts for an instant 120 times per second. DC maintains a steady voltage at all times.

How far an arc can 'jump' is a functon of voltage. Ac the AC voltage drops to zero, the arc is interrupted for an instant. Thus, an AC arc is easier to interrupt than a DC arc. 60Hz is easier tointerrupt than, say, 400hz.

So fuses have to be rated for the type of current they interrupt. Many fuses have both AC and DC ratings; the DC rating is always less.

Put a 100-a AC fuse on a DC circuit, and there's a real good chance the fuse will not be able to interrupt a 100-a DC fault. The fuse will blow, but the current will continue to flow.

 

[templdl]

Simply put I don't think the fuse knows the difference between AC & DC it has to do with the point of has it been tested under those conditions.The voltage rating value is the level that can still be safely(and been tested for) applied when the fuse is blown and not arc across the gap thus still conducting a current.

If AC or DC is different in this regard would someone explain that or post a link.

dick

It is to be noted that AC current goes through "0" volts 120 time a second with 69hz where dC doesn't. As such when a fuse clears an even AC has the advantage of seeing "0" volts where DC current doesn't. As such fuses, or even breakers for that matter, will clear and event much more quicky and efficiently than with DC current.