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Dielectric breakdown with insulating electrodes

  1. Dec 28, 2015 #1
    Consider this stack of materials:
    metal - dielectric - air - dielectric - metal

    Assume that air has a lower breakdown field than the dielectric.

    As the voltage (DC) on the metal electrodes increases the field inside the air gap increases and eventually reaches breakdown causing it to become conductive. However since the dielectrics are still insulating no current can flow.

    My understanding has been that damage to circuit components is caused by the effective short circuit during breakdown and the resulting large current flow. Since this current is suppressed, does any damage still occur?

    My questions are:
    1) can an arc be supported with these insulating dielectric layers?
    2) Do you expect any permanent damage to the dielectrics?
     
  2. jcsd
  3. Dec 29, 2015 #2

    jambaugh

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    You can still have a transient current across the dielectric, the Maxwell current, and you indeed might get arching in this scenario.

    Consider for example the slight arching on the surface of a plasma ball as you brush your fingers across it. The glass dielectric is not breaking down but due to the high frequency you are getting significant transient effective current across the glass surface.

    as far as question 2.) is concerned, damage would be minimal since the arcing, if it occurs, only occurs within the air gap. Any dielectric damage would be on the surface. Of course this depends on scale, a very thin dielectric would be "all surface" and easily damaged.
     
  4. Dec 31, 2015 #3
    What you are describing is similar to the classic problem problem of a small air bubble in the electric insulation of a high voltage power cable which eventually causes the cable to fail, although the cable is operated on AC. See: https://en.wikipedia.org/wiki/Partial_discharge and the diagram of partial discharge within solid insulation. When 60-Hz power is applied there are 120 mini-arcs per second within the air bubble, which slightly burn the surface of the insulator. With time the bubble of air and slightly burned insulation grows and the cable shorts out. You asked about DC:

    1) can an arc be supported with these insulating dielectric layers?: Yes, as long as the breakdown voltage of the air gap is exceeded.

    2) Do you expect any permanent damage to the dielectrics?: Yes, but it can take some time as the damage is often cumulative. Sometimes an arc can even punch a hole in an insulator with one shot.

    Interesting story: 35 years ago I was working at a high voltage facility where 2 people were zapped in the same week. (1) A workman mistakenly handled a 2300 VAC cable he thought was not energized but it was and he died. Not so surprising. (2) The same week another workman touched a large capacitor that was charged to 140-kV DC. It knocked him on his butt but he was back to work the same afternoon to the surprise of everyone. The investigation showed his life was saved by chance because his leather boots had a rubber sole. One of soles had a pin hole punched thru by the high voltage arc. His body apparently experienced little voltage drop during the arc and almost all the energy of the discharging capacitor went into blowing a hole in his shoe.
     
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