Fuse for 60VDC rail can be an AC fuse?

[Faringdon]

Hi,
Just doing a hobby prototype...multiple SMPS's.
We want to have a 1A fuse on a 60VDC rail.......why can't we use this.?..

   MP001593
https://www.farnell.com/datasheets/3665667.pdf

....we know its an AC fuse, and  could possibly  release shrapnel if it blows.....since its not DC rated...but we will have protective eyeware on.

DC rated fuses are more expensive as you know.

[Doctorandus_P]

Have you ever considered there is a reason for DC fuses being more expensive?

An important factor for fuses is the maximum short circuit current that has to be disconnected by the fuse.
At the moment the fuse blows a plasma arch starts, and that has to be extinguished, and with a higher short circuit current (and with DC) the arch is more difficult to extinguish.

A simple and effective design has a fuse wire and a spring under tension.  When the fuse wire melts, the spring pulls the pars from each other and thereby greatly increases the distance to be bridge by the plasma.

Alternatively, maybe you can use PPTC's.

[Faringdon]

Thankyou very much for this.
And i believe this means we can expect fire burst and a loud bang...so thankyou, we will be ready for this.
We cannot afford the DC fuses unfortunately.

Also, for our 4kW LLC converter, which runs off 400Vdc input, we will fuse the input with two of these cheap 7A fuses in parallel.

0251 007.
https://www.farnell.com/datasheets/2189447.pdf

...And will be ready for quite some fireworks if it blows...as discussed, we have the fire extinguisher in the room, and safety eyeware on.
Unfortunately, the only alternative for our well hammered wallets , is no fuse at all.

[Doctorandus_P]

Also, protective eyewear usually does not work too good.

You use that for some time, but the day that the fuse finally blows you just happen to have forgotten your eyewear.

It's just more reliable to make some kind of (possibly transparent) shielding over the fuse.
That way it's just always there.
Or use an enclosed fuse holder.

It's a bit like those CAT ratings for DMM's.
I don't really care much for those expensive fuses, and if it has to be replaced I put in some affordable fuse. (I also do not measure on CAT IV equipment anyway).
Even if the fuse explodes, the outside casing of the DMM is strong enough to contain the pieces.
There is a chance that this damages the DMM further, but that's a risc I'm willing to take.

[MarkF]

Also, for our 4kW LLC converter, which runs off 400Vdc input, we will fuse the input with two of these cheap 7A fuses in parallel.

Why in the world would you think that is a good idea?   

Gut feeling, fuses in parallel would be like LEDs in parallel.  The fuses are not going to be identical and will NOT pull equal amounts of current. 
Furthermore, if one fuse blows the other one may still be connected.  Not a clean disconnect at any rate.

[richard.cs]

Both those fuses have pathetically low, and AC-only breaking ratings. The most likely outcome is not shrapnel, it's failure to break the current flow, continuous arcing, leading to total destruction of the PCB, and possibly starting a fire.

[Gyro]

Hi,
Just doing a hobby prototype...multiple SMPS's.
...

I wish you'd cut out the false "hobby prototype" narrative!

[golden_labels]

Having a choice to use a wrong fuse, no fuse or cancel the project, the best option is to stop the project. But since most people will take the risk, if you really must then use even a wrong fuse. Still at least some chance, not even a small one, of providing protection. Certainly do not go for no fuses.

The reason you need a DC fuse is not because an AC fuse will explode in your face. It’s about the ability to do their job. A 50Hz alternating current waveform hits 0A each 10ms. If arcing happens, a hundred times per second there is nothing to sustain it. With DC there is no such option: the fuse must ensure that the arc is never formed or quickly extinguished. This makes them more expensive.

The first fuse you mentioned has only 50A breaking capability. A short across 60V will instantly reach much more than that, if only the power supply can deliver.

You do not want to parallel fuses to protect a single line.⁽¹⁾ Surprisingly the primary conern is not their failure; contrary: they may blow below the expected current. Two 7A fuses in parallel are not guaranteed to carry 14A.⁽²⁾ That’s because their actual limits vary considerably and so is current each of them may see. Two 7A fuses may in fact blow at 7.5A and 9A, and passing 14A through them in parallel they may see 8A and 6A, leading to premature trigerring. The other end of the problem, the risk associated, is not observing that you can’t parallel breaking capability and creating more points of failure. You can’t get 600A breaking from two 300A fuses: one of them dies first, exposing the other to the full 600A. And if you have two fuses, there is much larger probability at least one of them forms an arc.

⁽¹⁾ Not impossible, but you would need to go deep into statistics, understanding theory of operation and modelling to parallel them safely.
⁽²⁾ No fuse is, but the probability of a 14A fuse blowing below 14A is acceptably low, since manufacturers have error margins. That’s why in this Littlefuse datasheet you can see that one rated for 7A will on average pass 10A without blowing.

[tggzzz]

Hi,
Just doing a hobby prototype...multiple SMPS's.
We want to have a 1A fuse on a 60VDC rail.......why can't we use this.?..

   MP001593
https://www.farnell.com/datasheets/3665667.pdf

....we know its an AC fuse, and  could possibly  release shrapnel if it blows.....since its not DC rated...but we will have protective eyeware on.

DC rated fuses are more expensive as you know.

Thankyou very much for this.
And i believe this means we can expect fire burst and a loud bang...so thankyou, we will be ready for this.
We cannot afford the DC fuses unfortunately.

Also, for our 4kW LLC converter, which runs off 400Vdc input, we will fuse the input with two of these cheap 7A fuses in parallel.

0251 007.
https://www.farnell.com/datasheets/2189447.pdf

...And will be ready for quite some fireworks if it blows...as discussed, we have the fire extinguisher in the room, and safety eyeware on.
Unfortunately, the only alternative for our well hammered wallets , is no fuse at all.

Hobby project? Yeah, riiiiiight.

This kind of silly stupid corner-cutting leads to the kind of PSU problems that treez experienced. I wonder what became of him?

Best approach: do it right, do it once or don't do it at all. Much cheaper than bodging something multiple times, particularly when considering the court case resulting from injuries due to known dangerous activities.

[tooki]

We cannot afford the DC fuses unfortunately.

Don’t give us that crap. You can get cylindrical glass fuses for pennies. Standard 5x20mm glass fuses are rated to 250V DC and 1A is a standard value.

[tooki]

This kind of silly corner-cutting leads to the kind of PSU problems that treez experienced. I wonder what became of him.

Hmm, what indeed… https://www.eevblog.com/forum/supporters-lounge/treez-how-many-more-accounts-are-acceptable/

[tggzzz]

Don’t give us that crap.

It is worth being aware of the OP's previous moniker.

[xavier60]

I use SCHURTER fuses for high current DC, https://www.schurter.com/en/datasheet/typ_SHF_6.3x32_Pigtail.pdf "20 kA @ 63 VDC" and are very reasonably priced.

[Zero999]

https://uk.rs-online.com/web/p/cartridge-fuses/7897797

[jonpaul]

DC arc continues till external ckt current is zero.

AC arc is extingusihed and reignited on each mains zero crossing eg 100 Hz for 50 Hz mains\\

Thus a DC fuse is a very different construction than AC fuse.

Jon

[Zero999]

It's much more difficult to break the arc at higher voltages. This means a high voltage AC rated fuse can normally be used at a much lower DC voltage. As a general rule of thumb a 250VAC fuse can normally safely used for DC voltage up to 30V, but always check the data sheet.

[Faringdon]

You do not want to parallel fuses to protect a single line.(1) Surprisingly the primary concern is not their failure; contrary: they may blow below the expected current.

Thankyou for this....i must admit i recently opened up a 750W offline 100-265VAC PSU which i was told had a superb longlife record.
It had two 8A, 5x20mm axial solder_in fuses at the mains input....

...So youuve got me thinking now......750W from 100VAC is about 8A.....was the reason they over-rated the fuses so much (ie using 16A's worth)  because they were in parallel?

Also, maybe the customers were only using it at 220VAC, i dont know.

[strawberry]

this fuse should work

[tooki]

Don’t give us that crap.

It is worth being aware of the OP's previous moniker.

I’m obviously well aware, given my reply immediately above yours.

[tooki]

You do not want to parallel fuses to protect a single line.(1) Surprisingly the primary concern is not their failure; contrary: they may blow below the expected current.

Thankyou for this....i must admit i recently opened up a 750W offline 100-265VAC PSU which i was told had a superb longlife record.
It had two 8A, 5x20mm axial solder_in fuses at the mains input....

...So youuve got me thinking now......750W from 100VAC is about 8A.....was the reason they over-rated the fuses so much (ie using 16A's worth)  because they were in parallel?

Also, maybe the customers were only using it at 220VAC, i dont know.

Are they actually in parallel? I doubt it. More likely, they fused both AC lines.

[golden_labels]

We would need to see the photo, associated with PCB reverse-engineering to say anything. I suspect what tooki said: fuses in series, each for one side of the power supply. Also note the footnote. You do not want ≠ you absolutely can not. As an example: fuses made for carrying huge currents are produced by paralling multiple smaller ones. You can see one in the video from strawberry. Each of the metal strips is an individual fuse. Designed to have very similar characteristics, all connected to a massive block of metal that equalizes currents. But there is a difference between a well-engineered device, carefully designed for that purpose, and ad-hoc slapping two random $0.25 fuses, which individually have so large variance the manufacturer must keep an error margin of 30%.

On a separate note: fuses may have higher rating to avoid premature triggering from inrush current. But there may as well no be a particular reason other than “being good enough”. Their purpose is to prevent overheating and potential fire, if the circuit shorts, not to exactly match the current circuit takes during normal operation. Consumer products are also expected to already have two breakers in series⁽¹⁾ in the electric installation. If they suck a few amps without shorts, the internal fuse acts as a third line of defence, not the sole protection. It may be chosen to let the other two breakers, which are resettable, act earlier.

⁽¹⁾ In Poland that would typically be 10A for a circuit, and in apartments 16A shared across all circuits.

[TimFox]

Also, remember the difference between the two common rating systems for fuses:  UL and IEC.  From a Schurter document:
"Fuses can be in accordance with IEC 60127 or UL 248-14.
Because of the different definitions between the two standards, fuses are not directly interchangeable as follows:
fuses in accordance with IEC 60127 may be operated continually at 100% of the rated current value,
whereas fuses in accordance with UL 248-14 only at 75%.
UL 248-14 specifies a minimum of 4h operating time at rated current."

[bdunham7]

DC rated fuses are more expensive as you know.

Can you afford 35 cents, 15 cents in quantity? 

https://www.mouser.com/ProductDetail/Bel-Fuse/0697H1000-02?qs=GtFly9OVs880JpSy6RRCrg%3D%3D

[Faringdon]

Thanks, with the component shortage, we just cant garantee that proper cheap DC fuses will remain in stock throughout our testing phase.....as such, for an LLC  converter which is 2kW, 400vin, can we fuse it's 400v input  with a dirt cheap 250VAC , 7A fuse, but put a cheap  10 milliOhm  flame proof resistor in series with this (admittedly dodgy) fuse?

eg

12 milliOhm resistor......flame proof ness..
https://www.farnell.com/datasheets/2710520.pdf

[nightfire]

Question: If this is for experimental equipment, where obviously some fuses are used up- would this be an option to replace them with a RCCB that is certified by manufacturer for DC?

I did this some time ago, when I wanted to build some cheap solution for partying a bit with friends whilst traveling, and came up with a 12V lead battery, some standard RCCB  (from ABB) that was  documented to be able to break the prospective currents i calculated for that battery.
So it does not always have to be a breaker that is certified for railway applications...

Question #2: What are the prospective currents that fuse or other overcurrent protection has to be able to protect against?