Lightning Protection for 'Underground' Building

[deb4523]

I have a project I'm working on to install a lightning grid for a building. There is a large basement that extends well beyond (~50 ft) the exterior walls of the above ground section. It is a few feet below the surface of the ground. Is there a section of NFPA 780 that covers this situation? I haven't been able to find anything.

Thanks!

 

[charlie b]

The lightning risk assessment process only deals with the height of the building (i.e., above ground). The basement does not come into any equation.

 

[deb4523]

I wasn't sure if I should do some kind of counterpoise system (like for airfield lighting circuits). If the basement was only underneath the building, I wouldn't worry about it. However since it extends so far beyond the above ground structure, I am wondering. It would be unwise to only protect part of the building, right?

If it isn't address through codes/regulations. What would you guys suggest as best practice? Ignore it? Install counterpoise?

Thanks

 

[Adamjamma]

I could be wrong but I think you basically ignore it except for your ground rod perimeter if that is what you are using... that needs to be run outside of the footprint of the basement... so you do not introduce more fault voltage above the basement...

However, if I recall the experiments done by the ARMY in the 60s they found the ground protected the basements from atmospheric conditions already, so it was a case of not introducing the atmospheric conditions to the underground systems through electrical means... just as a grid ground system over a tent protected the tent from radio waves.

But, as I said, I could be wrong... when I was reading these old reports it was from an antenna and groundpoise, not an electrical standpoint.