lightning protection of a tower crane

[sberbece]

Hello!
I am looking for advice regarding lightning protection of a tower crane.
I have a tower crane, 55 m high. It is the highest structure on a few km around. The crane’s structure is connected at the base to the plant earthing grid (< 1 Ohms).
The Client wants to have an lightning rod installed in the top of the crane, and a down conductor from there.
To install that lightning rod is not a big problem, but I have a problem with the crane’s top rotating part and the down conductor.
Since the crane is a metallic structure, someone could say that is in fact a big lightning spike. However, could be safe to assume that the baring system of the rotating top part has enough contact surface so that the lightning discharge current won’t weld the crane in place?
How it is done? Does anyone have experience with tower cranes and lightning protection?
Thank you in advance for your support.
Regards,
Stefan

 

[ausphil]

No experience designing a system, but have seen a crane hit by lightning and it is spectacular. The relevant clause from our local standards for lightning protection states:

"Cranes and other tall lifting appliances used for building construction purposes, shipyards and port installations should also be bonded to earth. For cranes or revolving structures mounted on rails, efficient earthing of the rails, preferably at more than one point, will usually provide adequate lightning protection.
In special cases, where concern is felt regarding possible damage by lightning to bearings, additional measures may be justified.
Mobile towers, portable cranes and similar structures mounted on vehicles with pneumatic tyres can be given a limited degree of protection against lightning damage by drag chains or tyres of conducting rubber such as are provided for dissipating static electricity."

It is vague enough to be interpreted as you see fit.

ausphil

 

[kithiljr]

The tower crane IS ALREADY a "de facto" lightning rod, just as is a metal radio tower. The question to answer is "what are the assets to be protected? Operator inside enclosed cab
is safe (Faraday Cage) under most circumstances. Electrical/electronic devices require surge protection devices. Grounding is a tracked crane is adequate. If rubber tired mobile crane some mods may be needed.
More info see:

http://www.lightningsafety.com

 

[7anoter4]

I have not experience with an isolated high crane lightning protection but I found some links that may help.
Honeybee is developing a lightning protection system for rotating structures using an array of rolling contact connectors, or conductive wheels, to transmit lightning strike current loads across a rotating interface. See:

http://www.honeybeerobotics.com/lightning.html

This system is applied also- but not clearly for Lightning Protection- by Cavotec see:

http://www.oskom.cz/files/uploaded/UserFiles/File/Cavotec/SK_Specimas_aj-2.pdf

pp 16 Medium voltage slipring