Another 'What happened here?' (Electrical Question)

A couple months ago, I was home when the power went out. A few seconds later it came back on, accompanied by an extremely loud “buzzing” sound outside.

I stuck my head out the door to see what was happening. The power lines in front of my house seemed to be acting as loudspeakers. Up the street I saw what I thought was a transformer burning. I went back in and got my camera and got a picture of the arc:

The repair crew arrived and worked on something about 5 poles down from where i saw the arc light. I told them what I had seen, and they said that a short to ground had occurred and the arc must have travelled to the point where I saw it.

This picture was taken later in the winter, after all the leaves on the trees had fallen. The arrow points to the pole where the actual short circuit took place:

I have a number of questions about the occurrence, if anyone can help with them:

  1. Why did the powerlines produce such a tremendous volume of sound?

  2. How did such an arc travel along between the wires without the wires being vaporized?

  3. Why did the arc produce a blue light?

Thanks to all for your explanations.

electric arcs are noisy, huge electric arcs are very noisy.

the metal vaporizes at the point of a sustained arc. if the arc travels and bounces the metal can cool.

nitrogen and oxygen get excited by the arc, they give off blue/violet light as it calms down.

The blue color of the arc is more likely to be vaporized copper.

1. Why did the powerlines produce such a tremendous volume of sound?

Yeah, arcing can be pretty loud. Did it sound something like this?
http://www.youtube.com/watch?v=xLlOjWmFaS4

2. How did such an arc travel along between the wires without the wires being vaporized?

I suspect that in your case, wind or a falling branch or something moved the wires so that one of the high voltage wires got too close to the ground wire and started arcing. As the wires moved back into position, the arc followed the narrowest point between the wires until it was some distance from the original fault.

It takes a lot to get an arc started, but once it starts you can “draw out” the arc to a much longer distance. This is actually a problem for high voltage DC transmission lines since you need special equipment just to switch the line on and off. AC will draw an arc too, but it’s much easier to extinguish the arc because the AC voltage drops to zero twice every AC cycle (1/60th of a second).

This video shows a switch drawing an arc. The arc starts out small but grows larger as it follows the switch contacts back.

http://www.youtube.com/watch?v=PXiOQCRiSp0

The wires didn’t vaporize because they are big hunks of copper. Some of the copper around the arc probably did vaporize.

3. Why did the arc produce a blue light?

The color depends on the gas that the electricity is arcing through, which in the case of regular old atmosphere is of course a mix of oxygen, nitrogen, carbon dioxide, etc.

I’ve heard that the temperature of the air makes a difference in the color arc you get, but I don’t know how true that is.

I’d expect vaporized copper to be more green, but I dunno. I could be wrong.

in my experience with overtaxed DC motors (where the commutator starts burning away) it’s green to bluish-green.

I’m pretty sure transmission lines aren’t copper. Don’t know what’s in a transformer.

Those are old overhead distribution lines, so they probably are copper, though they might be aluminum. Transmission lines these days are usually aluminum reinforced with steel.

Transformers are copper or aluminum.