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I was watching this video, where they were demonstrating the difference between breaking AC and DC current.

https://www.youtube.com/watch?v=Zez2r1RPpWY

When they try to break the DC current, an arc forms. It continues for a while, but then breaks.

Does it break because the material gets so hot, that the resistance increases and consequently reduces the voltage experienced over the gap, thus requiring the gap to be reduced to maintain the arc?

I've experienced a similar effect when I was arc welding. The material would get so hot that maintaining an arc was impossible. I didn't understand why until I saw this video.

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  • \$\begingroup\$ Excellent video! There are several things going on. The arc is made of plasma which is very hot and any hot gas or plasma will rise up in air. As it does, new plasma needs to form below it or the arc will travel upwards until the arc too long for the voltage across it to break down the plasma and it will extinguish. The material heating will probably play a role too but I would put the plasma temperature and air around it as the major mechanism for extingushing the DC arc. \$\endgroup\$ – winny Sep 9 '16 at 7:45
  • \$\begingroup\$ Have a look at jacobs ladder videos \$\endgroup\$ – PlasmaHH Sep 9 '16 at 7:50
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No the effect breaking the arc is definitely not related to the resistance of the metallic material.

If you watch closely you may notice that the shape of the arc does not maintain a straight line between the contacts. An arc basically consists of ionised gas. That's why it is conductive. And it's ionised, because it is hot. But hot gasses are of lower density than the surrounding cold gasses and get buoyant, i.e. they tend to rise. So the shape of the arc gets deformed until it is long enough to die off.

So when does it break? Any power source connected via conductors to an arc can deliver a limited amount of current to it. And as the arc has relatively low resistance the current is clearly defined by the voltage of the power source and the resistances of the source and connected conductors.

After the arc is built up it tends to elongate, as the plasma is migrating upwards. This increases the run length and because even a good plasma is not as superconductor, the resistance increases. This leads to an increased voltage drop which in turn decreases the current supplied by the source over the conductors.

At some point in time, the current has dropped so much, that the plasma is not heated sufficiently any longer. The resistance will rise suddenly. It will quench and fall apart. This is when the arc breaks.

Arc welding cannot be compared with this, because the power sources for arc welding are designed to deliver a current just enough to hold a very short arc for a longer period. Those short arcs are so small they cannot extend and change their run length significantly. Perhaps the heating of the material can in fact play a role there. While in the video you linked, it doesn't.

There are plenty other videos on YT showing that effect in greater detail.

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