What is this?

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FionaZuppa said:
this particular DC does have a zero "crossing" @ 120Hz :thumbsup:

that same video using full bridge rectifier at say 30Hz or 200Hz (AC) will show way different results.

a 240 battery, even more different results.
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I may be wrong but since it is not AC I don't see that it has zero crossing. Simple rectifier output will pulse from zero up to a peak but will never go below zero, or the opposite and will never go above zero.

I can see where it still may be more difficult to extinguish the arc from a constant DC source like a battery, but can also see this situation that reaches but doesn't cross zero being more difficult to extinguish an arc then for AC current that does cross zero.
 
kwired said:
I may be wrong but since it is not AC I don't see that it has zero crossing. Simple rectifier output will pulse from zero up to a peak but will never go below zero, or the opposite and will never go above zero.

I can see where it still may be more difficult to extinguish the arc from a constant DC source like a battery, but can also see this situation that reaches but doesn't cross zero being more difficult to extinguish an arc then for AC current that does cross zero.
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It does make a difference whether or not the current changes sign, but it also makes a difference how long the current stays in the general vicinity of zero. I also have a hard time seeing how that arc can be maintained with only a half-wave rectifier unless there is a hidden inductance in the circuit somewhere.
 
FionaZuppa said:
this particular DC does have a zero "crossing" @ 120Hz :thumbsup:

that same video using full bridge rectifier at say 30Hz or 200Hz (AC) will show way different results.

a 240 battery, even more different results.
Click to expand...
I previously was not looking at the DC power supply. But from what I can tell, there is no 0V time span long enough because of the capacitor in the circuit stores enough energy during the charge half cycle to sustain the arc during the discharge half cycle.

Roobert33
+High Voltage First of all, this video is not accurate, there is a mistake on my part to write 220 volts DC. It really is 310 volts DC. I have corrected the mistake with the annotation to the video, the minute 1:12. Electrolytic capacitors are 4 x 740 mF. 475 V in parallel.
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GoldDigger said:
It does make a difference whether or not the current changes sign, but it also makes a difference how long the current stays in the general vicinity of zero. I also have a hard time seeing how that arc can be maintained with only a half-wave rectifier unless there is a hidden inductance in the circuit somewhere.
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So single diode gives us half of the AC cycle at zero, where full wave rectifier gives us shorter time near zero - now start using capacitors to fill in the voids and we get closer to pure DC current. Kind of what I was thinking all along there.
 
kwired said:
So single diode gives us half of the AC cycle at zero, where full wave rectifier gives us shorter time near zero - now start using capacitors to fill in the voids and we get closer to pure DC current. Kind of what I was thinking all along there.
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PIC.93.JPG
 
i had to go back to the video, the 1st time seeing that vid i missed what the wires were in upper right...... thats a cap bank, which makes it look more like a batt.

the resistors in the video seem to be wire wound type so they have some inductance, coupled to the caps you have a tank circuit which may be exaggerating the sustained arc, but none-the-less the video shows diff between AC and DC.
 
infinity said:
That makes sense but why a #1/0 jumper for 200 amps, wouldn't the terminals be rated for only 75° C?

Also for AC would you remove the jumper?
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Years ago it used to be that the ampacity tables for UL508A were different from the NEC tables (allowing more amps), which made for a lot of tense phone calls in the field. Now UL has aligned themselves with the NEC for what panel shops can do, but that does not apply to a UL listed DEVICE itself. The mfr can use whatever they think will pass the UL specifications, which include temperature rise inside of the enclosure at whatever current it is rated for. So if 1/0 does not cause the internal air temperature of the switch to go above 75C when 200A is flowing in a 40C ambient, then it passes.

If you remove that wire, then decide later that you want to put one back in, YOU cannot use 1/0 again.

When switching DC, the arc will last longer because unlike AC where the voltage crosses zero 120 times per second, DC stays steady and even as the contacts start to open, the ionized air helps to keep current flowing until the arc is too long for it to sustain itself. One way to do it is a very long disconnect switch distance so that the arc length is increased quickly as the switch opens. But that's impractical in most cases because the switch itself becomes long. But since it is the total arc length that counts, another way is to split the arc between multiple sets of contacts, allowing the size of the switch body itself to be smaller. In this case, each set of contacts is 'double break", so from the total arc length standpoint, two sets in series gets you 4 times the arc length and the switch itself can be more compact.

It's in here, page 19.

PS:
Oh damn, I forgot to look at the number of pages again. I'm late to the party on this....
 
Jraef said:
Years ago it used to be that the ampacity tables for UL508A were different from the NEC tables (allowing more amps), which made for a lot of tense phone calls in the field. Now UL has aligned themselves with the NEC for what panel shops can do, but that does not apply to a UL listed DEVICE itself. The mfr can use whatever they think will pass the UL specifications, which include temperature rise inside of the enclosure at whatever current it is rated for. So if 1/0 does not cause the internal air temperature of the switch to go above 75C when 200A is flowing in a 40C ambient, then it passes.

If you remove that wire, then decide later that you want to put one back in, YOU cannot use 1/0 again.

When switching DC, the arc will last longer because unlike AC where the voltage crosses zero 120 times per second, DC stays steady and even as the contacts start to open, the ionized air helps to keep current flowing until the arc is too long for it to sustain itself. One way to do it is a very long disconnect switch distance so that the arc length is increased quickly as the switch opens. But that's impractical in most cases because the switch itself becomes long. But since it is the total arc length that counts, another way is to split the arc between multiple sets of contacts, allowing the size of the switch body itself to be smaller. In this case, each set of contacts is 'double break", so from the total arc length standpoint, two sets in series gets you 4 times the arc length and the switch itself can be more compact.

It's in here, page 19.

PS:
Oh damn, I forgot to look at the number of pages again. I'm late to the party on this....
Click to expand...
Can you use the same one you removed again?
 
Jraef said:
When switching DC, the arc will last longer because unlike AC where the voltage crosses zero 120 times per second, DC stays steady and even as the contacts start to open, the ionized air helps to keep current flowing until the arc is too long for it to sustain itself. One way to do it is a very long disconnect switch distance so that the arc length is increased quickly as the switch opens. But that's impractical in most cases because the switch itself becomes long. But since it is the total arc length that counts, another way is to split the arc between multiple sets of contacts, allowing the size of the switch body itself to be smaller. In this case, each set of contacts is 'double break", so from the total arc length standpoint, two sets in series gets you 4 times the arc length and the switch itself can be more compact.
Click to expand...

Another thing you can do is seal the contacts in an evacuated enclosure. Without air present there is nothing to ionize to support the arc.
 
ggunn said:
Another thing you can do is seal the contacts in an evacuated enclosure. Without air present there is nothing to ionize to support the arc.
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is a vacuum better ??
 
ggunn said:
I dunno; maybe it's because it's AC. I know that there are vacuum contacts made for high amperage DC connections.
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Just because the pump is called a vacuum pump it does not necessarily produce a perfect (or even a very good) vacuum.

What you have in the bell jar is a low pressure. The voltage required to produce an arc for a given electrode separation initially goes down as the pressure drops from atmospheric pressure. At some point as you get a better and better vacuum the voltage starts to increase again.

The fact that you see a blue arc trace between the electrodes is a direct sign that the arc is ionizing a gas, the residual gas in the imperfect vacuum.

Vacuum breakers and switches generally pull a much higher vacuum than you see in the video and the chamber contains a chemical (called a getter) which will capture stray gas molecules that work their way into the chamber. The getter increases the service life of the switch before the vacuum is degraded enough to support an arc at working voltage.
 
Paschen's Law describes the arcing distance in a partial vacuum as compared to the arcing distance with normal air pressure.
 
don_resqcapt19 said:
Paschen's Law describes the arcing distance in a partial vacuum as compared to the arcing distance with normal air pressure.
Click to expand...

For those that are interested, the mechanism relates to whether or not the electric field is great enough that an ion moving in that field will hit a neutral gas molecule with enough energy to ionize it also. If that happens the cascade will insure enough ions to carry arc current.

At higher pressure an ion does not move very far before hitting another gas molecule, so the field must be higher to build up enough kinetic energy over that distance.
As the pressure goes lower, this average distance (called the mean free path) gets longer and so the electric field required gets smaller.
But as the pressure gets low enough the moving ion may get all the way to the electrode without hitting another gas molecule and so the arc extinguishes. Under those conditions a current can only flow if the field is strong enough to strip electrons away from one of the metal electrodes. (Think vacuum tube, although the heated cathode helps make it easier in that case.)
 
ggunn said:
I dunno; maybe it's because it's AC. I know that there are vacuum contacts made for high amperage DC connections.
Click to expand...

Paschen's Law explains why the arc doesn't strike at ambient atmospheric pressure, strikes as the pressure is reduced, and goes out as the vacuum increases.

https://en.wikipedia.org/wiki/Paschen's_law



EDIT: Dang. Beaten to the punch.

SceneryDriver
 
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