Fuse for 60VDC rail can be an AC fuse?

[Gyro]

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

If it's a hobby project, just stock up on them (fuses) while they're available.

[tggzzz]

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

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...

Those two statements clash. Can you confirm you are being open and honest with us?

IMNSHO anybody working with such high energies (2kW, 400V) should not need to ask opinions of random people on a hobby forum.

[tggzzz]

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

If it's a hobby project, just stock up on them (fuses) while they're available.

That thought occurred to me.

I won't mention some other thoughts that have occurred to me

[Faringdon]

OK thanks, any thoughts on the combo of cheap AC fuse in series with cheap 0.01R flameproof resistor?......for use with 400Vdc and 6A...(input to LLC converter)
(and eyeware and fire extinguisher in the room  of course!.....and the building has upstream breaker etc etc).

I mean one of my biggest explosions was when i accidentally plugged a three phase supply into the output of a 15kW electric drive......it sounded like a cannon going off!!!...the PCB  had a huge burned melt hole!.....i mean...surely the 400Vdc and 6A (dodgy AC) fuse blowing will be no worse than that?

[Faringdon]

Thanks for replies...another point is that surely, when fusing for an AC  mains supplied PSU....even if its got a DC output....it essentially calls for just an AC fuse in that DC output...because once the short has discharged the capacitors.......the whole thing is back to being AC....because the mains is AC.....so surely we have some leeway with offline PSU fusing, and we can revert to AC fuses?........unless they have enormous amounts of  storage capacitance of course....and i am not talkign about eg bigoffline  battery chargers here.. (EDIT....i  refer to fusing of the DC output here, or fusing of the post PFC DC Bus)

[tooki]

What the hell, man… that makes no sense. Input and output fuses serve fundamentally different purposes.

You shouldn’t be going anywhere near mains (never mind 15kW anything) if you don’t understand this. And all this because you’re too cheap to spend 18¢ on an ordinary glass fuse?!?

[TimFox]

What the hell, man… that makes no sense. Input and output fuses serve fundamentally different purposes.

You shouldn’t be going anywhere near mains (never mind 15kW anything) if you don’t understand this. And all this because you’re too cheap to spend 18¢ on an ordinary glass fuse?!?

In my "electricity" shop class in 8th grade junior high school, we had a good teacher, who lectured us on fuses.
His parable was the guy whose fuses kept blowing, which he assumed was caused by "old cheap fuses" so he kept replacing them with higher ampere ratings.
When his house burned down, he blamed that fire on the "old cheap fuses".

[Faringdon]

Thanks, yes, but its strange theres no differentiation between a 1A fuse for say a 10MW.hr battery with 400VDC output, and a fuse for a 400Vdc ouput of an AC mains supplied  Boost PFC......The Boost PFC requires a lesser fuse.

[Zero999]

Thanks, yes, but its strange theres no differentiation between a 1A fuse for say a 10MW.hr battery with 400VDC output, and a fuse for a 400Vdc ouput of an AC mains supplied  Boost PFC......The Boost PFC requires a lesser fuse.

That's not true at all look up breaking capacity, which is the maximum current the fuse can safely interrupt, without posting a safety hazard.

[nightfire]

Thanks, yes, but its strange theres no differentiation between a 1A fuse for say a 10MW.hr battery with 400VDC output, and a fuse for a 400Vdc ouput of an AC mains supplied  Boost PFC......The Boost PFC requires a lesser fuse.

That's not true at all look up breaking capacity, which is the maximum current the fuse can safely interrupt, without posting a safety hazard.

A look in the datasheet of the model of the fuse is often necessary. The question as always is: which voltages (AC/DC and amount of) and prospective currents has the fuse to deal with?
Especially the curves and Integral values (necessary to determine timing of when the fuse begins to act) are needed to determine if a certain fuse is appropriate for handling a short at the position where it is deployed.

The form of the curve is mostly different, and for selecting to correct model of fuse you have to look at the situations and conditions when a short can occur- and determine, which parts of the system you want to protect.

[mc172]

You don't need a fuse, just make the PCB traces really thin.

[Gregg]

Hey Treez.... Why haven't you asked about putting cheap AC fuses in SERIES for your DC hobby project?? 
Certainly that would increase the potential spark gap if they all should blow.

[Zero999]

Hey Treez.... Why haven't you asked about putting cheap AC fuses in SERIES for your DC hobby project?? 
Certainly that would increase the potential spark gap if they all should blow.

I know you said that in jest, but it would probably increase the breaking capacity and voltage. It would be a very good idea to both fuses must be replaced, even if only one of them blows, to avoid nuisance tripping.

Thanks, yes, but its strange theres no differentiation between a 1A fuse for say a 10MW.hr battery with 400VDC output, and a fuse for a 400Vdc ouput of an AC mains supplied  Boost PFC......The Boost PFC requires a lesser fuse.

That's not true at all look up breaking capacity, which is the maximum current the fuse can safely interrupt, without posting a safety hazard.

A look in the datasheet of the model of the fuse is often necessary. The question as always is: which voltages (AC/DC and amount of) and prospective currents has the fuse to deal with?
Especially the curves and Integral values (necessary to determine timing of when the fuse begins to act) are needed to determine if a certain fuse is appropriate for handling a short at the position where it is deployed.

The form of the curve is mostly different, and for selecting to correct model of fuse you have to look at the situations and conditions when a short can occur- and determine, which parts of the system you want to protect.

I was hoping the original poster would have figured that out for themself.

The maximum prospective short circuit can be calculated by measuring the impedance of the wiring and applying Ohm's law. If it's a battery powered circuit, it's a little more tricky. The easiest way is to apply large load to the battery and measure the voltage drop. R = V_DROP/I.

Come to think of it, what about a power supply with a large capacitance? It might be able to supply surges of an order of magnitude higher, than the continuous current. I suppose it then becomes a matter of energy, as well as current. I'd imagine a time delay fuse would better cope in an application with a large capacitance, since it will only break, once the current has already decayed somewhat.

[tooki]

I worked on a project last year involving two 2kV 800μF capacitors in parallel. Let’s just say that they could supply far, FAR, FAAAR more current than they were charged with. (Peak measured output current was 6kA. Not a typo.)

[nightfire]

I worked on a project last year involving two 2kV 800μF capacitors in parallel. Let’s just say that they could supply far, FAR, FAAAR more current than they were charged with. (Peak measured output current was 6kA. Not a typo.)

For how long could they deliver this amount of current?

[tooki]

A ms or so.

[Gregg]

Hey Treez.... Why haven't you asked about putting cheap AC fuses in SERIES for your DC hobby project?? 
Certainly that would increase the potential spark gap if they all should blow.

I know you said that in jest, but it would probably increase the breaking capacity and voltage. It would be a very good idea to both fuses must be replaced, even if only one of them blows, to avoid nuisance tripping.

It was somewhat in jest but more of a feeble attempt to jumpstart Faringdon’s (aka Treez) brain; a daunting task at best. Or as you stated, hoping he could figure it out for himself. 

The OP reminds me of the student that copies others’ homework and doesn’t grasp the disservice he is doing to himself.  He has come to rely on the people here for answers he really should be able to derive himself; perhaps this blog has kept him employed. 

The old saying prior to the internet was, “engage your brain before opening your mouth.” 
Now it should be, “engage your brain before posting on the internet.”  (Never going to happen) 

[Faringdon]

For how long could they deliver this amount of current?

A ms or so

Thanks, this is the crux of it......in an ACDC offline PSU...if you fuse any DC rail in it...then once the caps on it have discharged...then that fuse is essentially in an AC circuit......as  the short cct current will be going to zero every 10ms (8.33ms in US)......so you would think an AC fuse could be gotten away with.

[golden_labels]

He has come to rely on the people here for answers he really should be able to derive himself; perhaps this blog has kept him employed. 

Writing from a softdev perspective: I wish you knew how many jobs are kept solely thanks to Stack Overflow and keyboards having keys Ctrl, C and V.

Meanwhile, he always adds “thank you”, even if you do not deliver any useful answer or openly insult him. What a nice person!

[tooki]

For how long could they deliver this amount of current?

A ms or so

Thanks, this is the crux of it......in an ACDC offline PSU...if you fuse any DC rail in it...then once the caps on it have discharged...then that fuse is essentially in an AC circuit......as  the short cct current will be going to zero every 10ms (8.33ms in US)......so you would think an AC fuse could be gotten away with.

It’s been pointed out to you more than once that a 250V DC-rated 1A 20x5mm glass fuse can be had for pennies. So why is there even a discussion? Just buy the damned fuse and put it in.

[tggzzz]

For how long could they deliver this amount of current?

A ms or so

Thanks, this is the crux of it......in an ACDC offline PSU...if you fuse any DC rail in it...then once the caps on it have discharged...then that fuse is essentially in an AC circuit......as  the short cct current will be going to zero every 10ms (8.33ms in US)......so you would think an AC fuse could be gotten away with.

It’s been pointed out to you more than once that a 250V DC-rated 1A 20x5mm glass fuse can be had for pennies. So why is there even a discussion? Just buy the damned fuse and put it in.

Some people seem to like repeatedly asking a question and ignoring answers. Perhaps the answer isn't The Right Answer, perhaps they don't understand the answer, perhaps they think the answer will be different next time.

The last of those is the reason my daughter's dog repeatedly brings you a ball to throw even when you have told it "no". If one ball doesn't work, he tries another, or a snake. If that doesn't work, he has a primitively frightening tactic: lunge at your face with his mouth open wide. Yes, that is a dogs' standard way of saying "I want to play", but even when you know that, your hindbrain kicks into action

[floobydust]

A fuse is a safety component, engineers here will understand to not get cute about it.
Yet there is a strong drive to use anything but the proper part. We all need to feel special.

Regulatory and safety certification agencies will always first check to see if a fuse is suitable for the application. They look at the voltage, interrupt ratings, approvals.

I had one control panel design, for the 24VDC power I specified automotive blade fuses and DIN rail holders because that's what we had on hand. The certifier failed those fuses, the argument he had was they needed to be able to clear incoming mains (voltage) in the event there was a primary-secondary insulation failure in the power supply. So he wanted 250VAC-rated fuses even though it wasn't obvious for a 24VDC rail. We just swapped out the fuses+fuseholders to 5x20mm and all was good.

[CJay]

Some people seem to like repeatedly asking a question and ignoring answers. Perhaps the answer isn't The Right Answer, perhaps they don't understand the answer, perhaps they think the answer will be different next time.

I usually add a charge for wasted time and then eventually dump clients who rephrase questions repeatedly in an attempt to change reality.

I wonder if anyone has ever recorded the number of hours of free consultancy Treezingdon has had and calculated the fees he'd have had to pay.

I reckon at least enough Ocset clubcard points for a family holiday to Disney

[wizard69]

This is totally asinine!   I don't mean to be unkind but if you have ever spent some time reviewing a failure of an overload devices failure to protect then you would realize just how stupid i is not to size components properly.   I do mean stupid and if that offends you at least you have been warned.   A failure of a fuse to work can lead to things blowing up - literally!!!

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.?..

You have to be trolling.   It is pretty simple the fuse isn't rated for it.

   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.

This is so stupid that I can't even understand how somebody could be so foolish.   What good is eye protection if the shrapnel cuts your neck or penetrates your heart?

DC rated fuses are more expensive as you know.

Not that much more.   Beyond that have you even considered how much short circuit current is available?

As an aside I've had to investigate a couple of arc flash events in my life.   it is fantastical amazing what a electrical system can do with all that power available.   In one case, a small 30 horsepower compressor apparently seized up and the starter, the local fuses, and the nearest sub panel fuse did not break the circuit.    This resulted in rather significant destruction inside the compressors control panel literal[y blowing the starter apart, melting most of the 3 phase wiring, the wiring back to the sub panel, frying the breaker there and eventually tripping a 500 amp breaker.   Shockingly the aluminum in the rotors even melted to the point of running out.

Now I'm not sure whom was most likely at fault here.   Thankfully my responsibility was with physical rebuild / repair of the instillation.   I any event it is pretty obvious that somebody forgot about the potential short circuit current and the hardware required to handle that.   My point is that if the fuse or breaker for that matter, fails to do its job a lot of stuff will go up in smoke in the best case, in the worse case you have an arc / flash explosion and significant damage to equipment.   As for people near by you will be lucky if no one dies or get seriously hurt.

Now I understand you think that you only have 1 amp on the rail, thus not comparable.   That is normal operation, what you need to ask is what is the short circuit potential and does that exceed the fuses interrupting capability.

[Faringdon]

Thanks, that was interesting about the molten aluminium etc...

I must admit here that we are fusing a prototype 2kW offline SMPS.  In particular we will fuse the 400VDC output bus of the Boost PFC stage.

AYK, this is DC....but AYK, fusing of this rail is not mandated by regulations......the only fuse required by regs in an offline PSU is the AC fuse in the mains AC input.
No other fuses are required.

...As such, why should we, in this prototype, use a more expensive DC fuse in the 400VDC rail, when even regs dont mandate a fuse in there at all?
My apologies , but i should have explained, this is a free-of-charge prototype for a group of engineers.....if they like it, they will pay me to do the thing properly and fully, for now, all they want is to see 180V at 2kW (into a dummy load) and they're happy.
The mosy likely event, is that they will look at the prototype, and even if it works,  they will  just get their Chinese partners to do it   anyway....or take the PSU on themselves and do it themselves.......so i deffo dont want to spend a penny or cent more on this thing than i  have to......there is no payment whatsoever to the   demo'd working prototype stage.