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Storage Tank Lightning Protection

Storage Tank Lightning Protection

Storage Tank Lightning Protection

(OP)
I have the problem in designing lightning protection of above ground water storage tank as follows;

I designed lightning protection based on NFPA780 standard. The tank has more than 5 mm. thickness. So I do not have air terminal on the top of tank but uses the tank itself as striking termination device and as well down conductor. The tank bottoms are bonded to the underground main ground grid of the power plant grounding system.

My engineering manager who is not electrical engineer but mechanical engineer commented on my designed of water storage tank to have air terminals around the tank handrails at the top of tank with the reason that there is possibility that the working guy might need to do some work on the tank even when the storm coming and lightning strike is likely to happen.

My point is that I understood that even though separated air terminals are installed on the top of tank, that guy may be protected against direct stroke but he may die from the high current flowing on the tank surface he is standing on and some portion of current shunted through his body.

He argued that two legs of person is at the same potential, so there is no voltage across his legs and as such no current can flow through his body.

In my opinion, I understand that it is practically not possible to have two legs situated at the same location, there must be some distance apart and step voltage has never been zero and with such uncontrollable high current he will die. This is what I understood that why NFPA780 allows tank itself to act as air terminal and down conductor, so it is assumed that nobody should still stay on the tank when storm is coming and lightning strike is likely to happen.

Can anyone explain the followings;

1. Is my understanding correct ? that with some distance apart of two legs, there will be step voltage even though the tank is well grounded at the bottom since the current is high and uncontrollable.

2. Based on the reason in the above item 1, so installing air terminals on the tank violates the NFPA780 or not since if it could be interpreted as allowing person to work on the tank when lightning strike comes.

3. Is there any practice to install air terminal on the top of tank or not ?

Thank you for your helps.
 

RE: Storage Tank Lightning Protection

In order to protect a person of 1.9 m from lightning strokes a mast of more than 18 m has to be provided in the middle of a tank of 20 m diameter of the tank or at least 11 m height mast for 10 m tank diameter. One cannot protect person with air terminals of 0.6 m all around. See NFPA 780 art 7.3.3.2:
"Since the lightning stroke can strike any grounded object within the striking distance of the point from which final breakdown to ground occurs, the zone of protection shall be defined by a circular arc concave upward, shown in part (a) of Figure 7.3.3.2"
 

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