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When a tree branch touches a high voltage power line, the first result is typically a small arc at the touching point at first, and then, for some time, vapor emerging from the point of contact, and the point where the plant connects to the ground. While the branch gets hotter, there is more vapor along the branch and stem.

After a while, parts of the connecting starts to burn. The flames may later change to arcing.

I think that the resistance of the wood and bark will increase a lot during the time the vapor emerges, and then even more when it burns, which should dry the wood more.

Looking at a recording of the process, like this slow and detailed example, it seems like the resistance does not significantly drop during the process. Why is that?

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  • $\begingroup$ "it seems like the resistance does not significantly drop during the process": why do you think so? $\endgroup$ – valerio Feb 27 '18 at 12:23
  • $\begingroup$ @valerio Because the visible effects seem to increase over time. Or at least do not decrease in an obvious way. $\endgroup$ – Volker Siegel Feb 27 '18 at 12:27
  • $\begingroup$ If by "visible effects" you mean fire and smoke, that's because the tree starts to burn. There is no obvious correlation between resistance and fire/smoke... $\endgroup$ – valerio Feb 27 '18 at 12:28
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What you see at the start is indeed water vapor leaving the tree. Then, after some time (at around 1 minute in the video) the tree catches fires and starts releasing smoke.

There is no obvious correlation between what you visually see (smoke, flames) and the conductivity of the tree, so you cannot really tell what is happening to the conductivity from the video alone.

On the other hand, Wikipedia gives an electrical conductivity of $10^{-4}-10^{-3}$ S/m for damp wood and $10^{-16}-10^{-14}$ S/m for oven dry wood, so I think that we can conclude that burnt wood/coal has an electrical conductivity which is pretty higher than that of wet/"live" wood.

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    $\begingroup$ Soot is a pretty good conductor of electricity, so while the branch is burning the carbon on the surface and the soot particles in high concentration right near the branch will conduct quite well. $\endgroup$ – Jon Custer Feb 27 '18 at 14:45
  • $\begingroup$ @JonCuster Didn't know that. Anyway, my main point is that it is hard to tell something about the resistivity just by watching the video. $\endgroup$ – valerio Feb 27 '18 at 15:16

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