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Because the rod would have to extend significantly higher than the blade tip at its highest point and be mounted from the ground. A length like that would require either a substantial lattice tower or guy wires. A tower would be impractical and guy wires would impede the blade locus. It is a matter of economics, vastly increasing the installation cost vs. the slight chances of a lightning strike.
Incidentally, some of the images of burning turbines are actually caused by seizing bearings or electrical faults igniting the oil in the lubricant tank. The forces on the shaft are huge so forced lubrication is essential to prevent the bearings overheating.
Yes you are no doubt right, thing is, the resistance of the blades "earth point" to earth has to be regularly tested, to ensure its resistance to earth is still low enough.....this requires quite an effort with roped climbers going up on platforms and carrying electrical meters with long earth wires hanging down to earth...so they can tell what is the resistance to earth of the "Lightning point". Given that, i wonder if some latticed tower might be worthwhile.
Earth bonding probably isn't worth testing, a motor and gearbox will have sufficiently low resistance anyway. If a few million volts hits it, a clearance of a few oil molecules won't make much difference!
Thanks yes, I would tend to agree, however, I know they do test it, so I am wondering why?
It’s a test of the earth point at the blade end, right along the lightning conductor that goes all along the blade…then down through the station pole to earth ..presumably after a few strikes the lighting conductor path may get broken somewhere , so it needs regular testing? (by staff on ropes and platforms)
The danger isn't to the blades or tower, it is to the controls, cables and feeds to the transformer. A standard conductor will protect them against most lightning flashes and static discharges. Lightning damage to the visible parts is unlikely, when you see dramatic fires in the nacelle it is far more likely to be the result of mechanical breakdown. Several turbines near here have broken down with cracked gears, obviously they don't work at all in that state but the mechanisms are still prone to many tonnes of torque and the heat it creates through stress and braking.
I can see 27 turbines from home - it's funny how people complain about them spoiling the landscape, the first things visitors (in the pre-Covid days) say when they get here is "Wow, what a fantastic view you have". I can see four counties from my office window with views to as far as 50Km away.