Why does a tree branch under high voltage not stop conducting?

Question

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?

Answer 1

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.

Answer 2

Burning (oxidizing) organic material incompletely leaves carbon. That's what "blackened, zorched wood" or (when all the way through) "charcoal", is - carbon with lots of little voids and the like in it from gas bubbling as well as any spaces that were in the material prior.

Carbon is a decently good conductor of electricity, at least in its graphitic form, which this can be considered to be a type of, with a conductivity of about 128 kS/m. For comparison, sea water has about 0.005 kS/m, and iron has around 10 400 kS/m (3 s.f.). So as the wood is charred and converted to charcoal, a form of carbon, the conductivity rises to meet that value.

Note, however, that if the current is left on, the heat will permit the carbon to continue to react with the air, and that means eventually you burn away enough completely (i.e. convert it to carbon dioxide or, at least, monoxide [don't try indoors]), that the newly-formed "wire" simply breaks due to shrinkage and gasification.

FWIW, carbon also can work as a semiconductor, just like its heavier and far more famous-for-the-role congener, silicon, when in its diamond form and in particular when a suitable dopant is added.