DC fuses and arcs

"George Herold"

** Of course.

Once the wire inside develops a break, an arc will develop between the nds - what happens next depends on how many DC volts there are and how much current is available.

Suffice to say that standard fuses (ie 20 x 5 mm & 1-1/4 inch ) cannot be relied upon to quickly and efficiently break more than about 32 volts DC if large fault currents exist.

In extreme cases, an arc can jump from one fuse terminal to the other, causing the glass to shatter and damaging the fuse holder.

BTW

AC and DC fuses have maximum breaking current ratings - one has to pay significantly more for " HRC " or High Rupture Current types.

.... Phil

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Ahh, Thanks Phil. Silly me I was trying to imagine an arc *before* the fuse blew.

George

Ayup!

I assume he means at the moment the fuse blows. When the metal is melting and there is a gap forming the AC would prevent an arc from forming because when the AC was momentarily 0(or close) there would be no hard but the metal would have melted and went further apart requiring more voltage to arc. If it did arc then the next step would increase the distance even futher requiring even a higher voltage. Eventually after only a few cycles the distance should be large enough so that there is no arcing.

For DC, there is a "continuous" arc. It would last longer since once the arc gets going it is hard to stop since the gas is already ionized. Unlike the AC version there is nothing stopping the arc except distance. I'd imagine that as the distance increase the more ionization you have too which doesn't counteract act against the arc much(up to a point).

Yes. As the wire breaks an arc forms. With DC the metal vapour sustains the arc because of the continuous current through it.

--
Best Regards:
                     Baron.

On a sunny day (Thu, 3 Dec 2009 13:49:59 -0800 (PST)) it happened George Herold wrote in :

That is easy, it is called 'lightning'. ;-)

I've seen fuses explode, when the shop gorillas used 32V fuses in a 240V circuit. It literally blew the cap half-way across the shop. It was a good thing nobody was standing in the "line of fire" when it went.

Cheers! Rich

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Thanks All, (hope I=92m not breaking too much netiquette by only responding to the last post.)

Perhaps my misunderstanding can lead to a more interesting question. So, if 34 V DC is enough to sustain an arc, how much current do I need? What about at a higher voltage say 40 - 50 Volts?

George H.

Hmm, An experiment for tomorrow?

"George Herold"

** Here is what I posted:

" Suffice to say that standard fuses (ie 20 x 5 mm & 1-1/4 inch ) cannot be relied upon to quickly and efficiently break more than about 32 volts DC if large fault currents exist."

So, with small and moderate FAULT currents ( say up to 10 amps) standard fuses work OK with 32 volts DC and possibly up to 100 volts.

It all depends on how fast you NEED to break the circuit and if winding up with a very blackened fuse or shattered glass is acceptable.

With large fault currents, say 100 to 1000 amps DC, expect major damage from end to end arcing of the fuse - which generally means the fuse never breaks the circuit and something else has to give.

Like connecting wires or PCB tracks !!

Such damage is NOT limited to DC circuits - much the same will happen with 120 /240 V AC circuits if similar magnitude fault currents exist and the fuse is NOT rated for them.

I also posted this:

" AC and DC fuses have maximum breaking current ratings - one has to pay significantly more for " HRC " or High Rupture Current types. "

HRC fuses come in ratings from 500 amps up to 20,000 amps breaking capacity. Ones rated at 1600 amps are available in 1-1/4 inch size.

Of course, fuses and breakers fitted in the wiring of the premises should act as backups in the latter case.

..... Phil

Hi Phil, Sorry, I wasn't asking about fuses anymore. (I do appreciate your responses, but do you think you could lighten up a little? It's clear to all of us that you know a hell of a lot more electronics than I do.)

So my question is, can I pull an arc from my 35V 3A power supply? How about if I add another in series? (What about if I lower the pressure to a few Torr?)

George H.

One interesting data point is the Thomson-Houston street lighting system used in the 1880's, had a 1500 V DC dynamo feeding 25 to 30 arc-lamp in series with a loop current of 6 A, thus the voltage drop across each lamp was 50 V so each lamp consumed 300 W. The actual arc voltage was slightly less due to the voltage drop in the series solenoid used to maintain constant distance between the carbon electrodes.

I've pulled a sustained arc from a high current 24V lab power supply. OK I did have a 1 Henry 10 A choke in series. Had a pencil lead as the electrode and could cut thin sheet steel. It was definitely an arc and not a contact process as if you touched the steel with the tip, the pencil almost instantly got red hot. I estimate I had between 1/32 and

1/16" separation. If there is enough inductance in circuit, you can get a continuous arc at even lower voltages.

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rge

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You need to get hold of J.J. Thompson's " Conduction Of Electricity Through Gases", first published in 1906, or a more recent textbook on the subject.

I think that Thompson was the first spell out that while it was difficult to start a a discharge through a gas, sustaining a discharge was just a matter of generating enough free electroncs at the anode to carry the current, and - for the small gaps that you see in arcs around fuses - most of the voltage drop across such a discharge is at the anode surface.

He distinguished glow discahges, where the electrons are generated by positve ion bombardment of the anode surface, which seems to require about a hundred volts or so, and arc discharges where the anode surface gets hot enough to melt, so that it can distort under the influence of the high electric field at the anode surface to form a pattern of very sharp spikes, where electrons are emitted at the tips of the spikes by field-stimulated thermal emission.

Typically, the voltage drop across a short arc is around twenty volts. The exact amount of current required to sustain an arc depends on the shape and nature of the anode. My impression is that it usually takes a few amps.

The incremental resistance of an arc is low, and can often be slightly negative.

-- Bill Sloman, Nijmegen

With DC (particularly if the load is inductive) the arc will increase in length as the fuse wire melts and the gap lengthens. The heat generated at the points where the arc contacts the metal of the wire is sufficient to generate a plasma cloud of conductive ions which allow the current to keep flowing.

With AC, the embryonic arc momentarily goes out as the current swings through zero and then cannot re-start because the plasma has cooled. A really big AC arc will keep going - similar to a DC arc because it contains so much hot material. The trick is to make sure it doesn't get beyond the stage where it is self-quenching.

Fuses for DC are much longer than those for AC. Typically a ceramic fusewire carrier would have about 6 inches spacing between the terminals for 660 volts and 60 amps rating. If the arc wasn't broken promptly, it would continue to eat back into the terminals and eventually would burn its way back into the switchboard wiring, which accounted for several switchboard fires on early DC systems.

Since about 1890, DC systems (such as traction supplies to tramways and railways) have used circuit breakers with magnetic blowout. The wire carrying the load current is wrapped a few turns around the iron frame of the breaker, which is shaped in such a way as to concentrate the flux at the contact gap. When the arc strikes, the electromagnetic forces cause it to expand circularly and lengthen to a point where it is no longer self-sustaining. (Some modern breakers use permanent magnets)

Sometimes an 'arc chute' of fireproof material is fixed to the breaker to guide the arc; in other systems, a fan of steel plates (like the magazine of a Kodak Carousel projector) intercepts the arc, breaks it up into short sections and quenches it.

When the DC breaker blows at a sub-station or on a tramcar, you know about it! The arc breaks with a deafening 'crack!' or sometimes an unearthly shriek

--
~ Adrian Tuddenham ~
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Interesting, I hadn't realized arc lamps worked at 'low' voltages.

George

TIG and stick welding is somewhere around 20Volts once the arc is started

-Lasse

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Hmm, that's a big inductor. I've got some 'air' coils that can take several amps, but nothing near a Henry. Do you remember how much current was being drawn? I just sharpened two ends of a pencil and measured a resistance of about 12 ohms. Did the pencil get hot?

Thanks, George H.

OK I've used the secondary from a transformer (20mH, 6 ohms)... Now I've let some of the smoke out of a pencil! Totally different smell from resistors. I'll need some welding glasses to continue... That sucker is bright!

G .

Never made an arc with sharpened carbon rods from Zinc-carbon batteries?

Nope, but just made one from a pencil. Eyes are still recovering.

George H.