Wow.. that is all I can say. The amount of voltage running through your standard 'ped' is fairly low. The tap on many systems usually stops the voltage from hitting the home.
I guess a spark of some sort ignited a fire, I really don't know. There aren't a whole lot of flammable parts in the system..
If I were TW, I would definitely get to the bottom of this, and figure out what the cause was.
Glad the damage was limited to several planks on the fence. -- Life is not a problem to be solved, but a reality to be experienced.