Lightning Protection for Water Steel Tanks and Zone of Protection

The only way to answer the question is to review and apply the applicable standards for lightning protection in whatever location these structures are on the planet. The standard cannot be seen from here, and the location has been withheld from the forum for some reason that cannot be determined. The contents of the tank influence the outcome: the approach might differ if the tank were holding aviation spirit as opposed to water, for example.

The only way of protecting structures from lightning, in principle, is to give the lightning a preferable path that makes the lightning go nowhere near the structures. Typically, it is an earth electrode pointing up to the sky; in the UK, radio masts, mobile telephone masts and church spires are typically used in this way, giving a measure of protection to the other structures in the immediate vicinity. However the forum has nothing to go on about the location of anything suitable in the immediate area of the chosen structures.

Therefore any form of useful reply to the original question is going to be somewhat unlikely.

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Why not go full tilt, and protect above the tank with lightning rod, and copper rope.

Also protect the buildings, depending on what is worth anything to you.

My understanding is that air terminals provide roughly a 30 degree cone of protection around them. It seems that a 23 meter high air terminal will not provide much protection to structures about twice the distance away.

You will find a number of standards at the bottom of the Wikipedia page

https://en.wikipedia.org/wiki/Lightning_rod

Good luck with your quest.

As long as the tank is well grounded, it is plenty of protection for itself. No action should be needed. As to the rooms some distance away, same answer - If you are worried, sink ground rods along the connecting pipes and ground the pipes.

If you intend to follow NFPA 780/2012 Chapter 5 Protection for Miscellaneous Structures and Special Occupancies 5.4 Metal Towers and Tanks “shall require only bonding to grounding electrodes” if that have a metal thickness of 4.8 mm (3⁄16 in.) or greater shall require only connection to the lightning protection system by welding, brazing or plates having a surface contact area of not less than 5200 mm2 (8 in.2).

However, the other two structures are too far located –in my opinion. Following rolling sphere method, the horizontal protected distance has to be [in excel language]:

d=sqrt(h1*(2*R-h1))-sqrt(h2*(2*R-h2)) where:

d = horizontal protected distance (m or ft)

h1 = height of the higher roof (m or ft)[23 m]

R = rolling sphere striking distance radius (m or ft)

h2 = height of the lower roof (top of the object)(m or ft) [6 m]

R<=h1-h2[17 m]

d=1.63 m only.

Sorry.The last sentence has to be:

R>=h1-h2

For a 2.9 kA lightning peak value R=10*2.9^0.65 =20 m

Then d=4.28 m.

Thanks to all for enriching the discussion