Tuesday, May 22, 2018

The sound makes this so much better



Explanation:  There were problems with the circuit breaker (CB) at this end of the line. The things coming open in the video are disconnects. This is absolutely not a normal operation.
A CB has a mechanism to extinguish an arc. The contacts may be contained in vacuum tanks (no air = nothing to conduct, no arc) or they may be surrounded by SF6 (sulfur hexafluoride, a non-conducive gas). Disconnects are normally operated once the CB has been opened and interrupted the current flow. The disconnects are needed to establish safe working conditions (visible air gap, to see that the circuit is open).
IIRC, the CB was stuck closed. They opened the CB at the remote end, so that the disconnects are not interrupting full current. The arc is from interrupting the capacitance that is inherent to an unloaded line. If they had interrupted this line with disconnects, when it was carrying normal loading, it would be an explosion, not an arc.

And why do the arcs migrate upward?  There is a lot of electricity going through those lines when they separate. And electricity always follows the path of least resistance, like water flows downhill.
As they seperate, a small amount of air gets in the way of that nice easy path of conductive conduit. Air is a pretty good insulator usually, but this is a pretty small amount, imagine a deep, steep, river, with a nice uniform bottom, interrupted by one large rock. You may see some disturbance on the surface, but overall it won't significantly alter the flow. So the electricity makes a short hop through the air to continue on its way.
Now, anything an electric current runs through will have some resistance (aside from superconductors) this is kind of like electrical friction. The higher the resistance, the more electricity is "wasted" and becomes heat. Good conductors are low resistance, good insulators are high, so when the charge moves through the air, it makes it extraordinarily hot. In this case, it doesn't just got hot, it turns the air into plasma. Unlike normal air, plasma is a great conductor, so the electricity will naturally try to follow the path of plasma. But the plasma is also much less dense than the air around it, so it starts to rise. And that is how we wind up with this rising arc, the plasma is acting as a bridge while floating up.
If the power had remained for a very long time, eventually the long path through the plasma would have cost more energy than punching a new hole through the air, so you would wind up with a cycle of a continuously rising plasma stream that would hit X height, then jump back down to a straight line, and begin rising again, over and over.

5 comments:

  1. I've seen one of these at night, at a substation that was just off the highway. The arc was at a safe distance from the road, but the light generated was much, much brighter than the sun, and this was the real hazard. People driving with their night-adjusted vision were suddenly driving at 70 mph across a stretch that was lit to an extent that was too bright for welding goggles, much less conventional sunglasses. It was bad.

    ReplyDelete
  2. That was an excellent explanation, thanks!

    ReplyDelete
  3. Great explanation of the arc, thank you!
    and the temperature of the plasma inside the arc was above 35,000 degrees and that is somewhat hotter than the surface of the sun. the arc was generating hard Ultra Violet and just about all of the RF spectrum including X-ray. looking at the arc with the naked eye is not a good thing. being up close and personal with it is really bad.

    ReplyDelete
  4. I took arc flash training in my role as a manitenance electrician.
    Normally, I did not deal with anything above 480v three phase, but
    I did rack out (and in) some 12.5kv breakers in a cold state. Even
    then, I was wearing a 40 calorie overall, a heavy 40 calorie jacket,
    leather over rubber gloves, a 40 calorie hood, and a hard hat mounted
    with a full-face UV visor and rubber boots.

    I called the PPE I had to wear the Satan's Beekeeper Suit. When an
    arc flash explosion sends out red hot copper oxide at the speed of
    sound, you need all the protection you can get. Current OSHA standards outlaw "hot swaps" of breaker buckets.


    ReplyDelete
    Replies
    1. I agree with Leonard in this. used to teach the course.
      almost anyone does not understand "calorie". put it this way. get your bic lighter out light it and hold the flame tip against the skin of your hand for one second. that is a rough approximation of one calorie per square centimeter. now, hold it there for forty seconds. that is forty calories per square centimeter. now picture that occurring over your entire body turning you into a carbonized cinder.the arc flash 40 cal suit can provide some protection from the heat flash of an arc. it can't protect you from the blast which is akin to having a 82mm mortar round explode right next to you, if the energy state is high enough. 480v 3ph has more than enough energy when the only thing between you and the powerplant is one fuse which may or may not open in time.
      the best protection against the effects of an arc flash is to not be there when it happens

      Delete