DC Interruption vs. AC Interruption: How the Vacuum Arc Physics Differs - Blog

Pull an arc between contacts in vacuum-AC breaker on one side, DC breaker on the other-and the way each arc dies takes a different route. The whole thing comes down to one question: is there a current zero lying around to use?

The Measurement Standard of Contact Erosion - How Many Millimeters of Wear Require a Circuit Breaker to Be Taken Out of Service?

AC: The Arc Gets to Walk Out on Its Own

AC has it easy. The zero crossing shows up on its own, so nobody has to strangle the arc-the trick is keeping it alive and well until that zero arrives. Add an axial magnetic field and the arc spreads for two or three milliseconds, then flattens into a diffuse column across the contact gap, burning at a relaxed pace. It's not racing any deadline; it's waiting for one. Once the current crosses zero, ions and neutrals in the gap scatter and recombine in microseconds, and dielectric strength comes back up. If the anode spot hasn't run too hot, whatever residual vapor it leaves isn't enough to stir up trouble, and the interruption holds. The arc basically ends itself.

DC: The Arc Gets Jumped Midway

DC doesn't have that zero, so you have to fake one-push reverse commutation current into the gap and drag the net current down to nothing by force. No peaceful ending here; the arc gets jumped midway. A column that was diffuse a moment ago goes dim in tens of microseconds. The column, though, isn't the headache-the anode is. Under heavy DC current the anode spot sits on heat it can't shed, and after the forced zero the gap is still packed with leftover metal vapor. Let recovery voltage climb before that vapor thins out, and the arc is back in business. DC interruption ends up feeling less like putting out a fire and more like waiting out a bad hangover-with a window that's painfully tight.

One lets the arc finish; the other cuts it off. Same vacuum, same metal vapor, two very different endings.