Fuses, 2 in series to comply with needed voltage rating

This one talks about a 30% inballance added:

So perhaps 3 in series would be ok....

** You must find out the breaking capacity of the fuse.

Eg:

10 x 38 mm Bussman cartridge fuses have a breaking capacity of 120kA at 500VAC.

They sell for about $1 each.

** It's not the voltage rating alone, you goose.

All fuses have a max rated breaking current at rated voltage. You too stupid and lazy to look it up ??

.... Phil

I don't think it works that way, as it depends on breaking amps and how much voltage it will clear on.

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

Why do you need a fuse? No matter what your answer, get a fuse rated for your application. There may be building code requirements if that's relevant to your use. Or you can just ask the fireman about it when he arrives.

+1

d

This is for a industry product, so internal fuses for protection of interna l devices to avoid a catastrophic failure of a power module, not for the bu ilding supply

I have a question pending answer from UL about this, but I want to dig deep er before they respond. If the fuse does not have the correct rating, UL (a nd IEC) will approve any current limiting device by specific tests. But I w ould like to avoid the testing if possible, because it will probably be pro blematic

Cheers

Klaus

Me very stupid, yes :-)

No, I know about the current rating. What I want to know is if one of the fuses open, can I count on the other one to open by the arching action, provided the I2t rating is sufficiency, so it will not "explode"..

Cheers

Klaus

Enough Said... If I were the project manager, I'd summon our product liability legal counsel. We'd take you out back and waterboard you until such thoughts were washed from your head.

Intentionally building a product with components unsuitable for the task is bad design. Circumventing safety is criminal.

so internal fuses for protection of internal devices to avoid a catastrophic failure of a power module, not for the building supply

Klaus, Can you share with us your "strange reason" for wanting to do this? (Are 500 VAC fuses particularly expensive?)

George H.

Yes. I have a fuse that complies, leaded version. But that fuse fails after several hundreds of thermal cycles, so I think I need to find an alternative.

We are pretty sure the root cause is thermal expansion coefficient differencies between the PCB and the ceramic

So I was looking for SMD fuses, but could not find any resonable prices at more than 250Vac

Actually it's just a wild idea, I like pushing the limits

Cheers

Klaus

nd

t

e.

I am not circumventing safety.

There are no requirement for a fuse in the product and there are no rules t hat govern the selection of the fuse, except when we do internal short circ uiting of the power module, the fuse must blow fast enough that the rectifi er dies inside the module does not crack the substrate transfer mold packag e.

One of the tests we will conduct with UL is the test of the fuse

Cheers

Klaus

If the over-curewnt is slight and the load is inductive probably one of the fuses will become an arc lamp, the current will drop slightly and if you're lucky the fuse holder will turn to glass,

--
umop apisdn

** You certainly appear not to.

The breaking capacity tells you what happens when the fuse opens.

If the current flow is LESS than the rated figure, any arc will instantly self quench. As my example shows, breaking capacity can be a HUGE number compared to the carrying capacity of the same fuse.

** Not if the breaking capacity of the fuses involved is being exceeded. You app involves 500VAC and about 15kW right?

You need a *real* fuse - not some PCB mount excuse for one.

.... Phil

As page 3 of the URL above states, each fuse in series must be capable of clearing the fault independently. Assume a moderate overload and a minor variance in fuse characteristics. Fuse 1 might melt without fuse 2 melting and then you're in a single fuse situation. Just buy a properly rated fuse.

Interesting, I hope you'll post results. Any chance you can cut a little slot in the pcb under the fuse?

George H.

The 2nd fuse will be subjected to the same voltage when it opens, exceeding the 500v rating. I have a good hunch UL will say its a bad idea.

Cheers

Not good. To break X volts, you need some certain minimum dimension between electrodes. You can't add a series connected succession of smaller gaps together to reach that minimum.

Ed

Yes, we can cut a slot to increase creepage, no problem

Two fuses in series is absolutely NOT supported anywhere.

A fuse is a fail open device and if you are exceeding its rated voltage, you risk continued conduction via arc after the fuse link itself 'opens' in a failure event.

This isn't about 'pushing limits', this is blatant lack of grasp of the concept, and shows a lack of grasp of electronics as well.