Bonding a Tower

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luckylerado

luckylerado

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Lake of the Ozarks
We have a 50' tower that is installed approx 25' away from a single story structure. The structure has a lightning protections system. The downleads terminate at individual ground rods. The tower was installed with a counterpoise with rods at the corners, tied to the steel in the footing and leads up to each leg but independent of the adjacent structure.

My interpretation of NFPA 780 is that this is a compliant installation.

Our EE has made a punch item to connect the tower to the adjacent structure GES. His concern is that a lightning strikes the tower and will make its way through the earth into the structure and fry sensitive electronics. My argument against this is that providing a conductor from the tower GES to the building GES would give some portion of the energy in a lightning strike a freeway path back into the building.

As a compromise I might suggest tying the tower counterpoise to one of the structures downlead rods but even this I think is beyond the requirements of NFPA and UL. Looking for input from the group on

"The concern is , without this tower bonded to the adjacent facility building grounding system a strike to the tower may cause catastrophic damage to any electronic devices contained within the facility. "

Any advise on how to make this argument? At then end of the day we will do as directed to cover our butts but I think it is a mistake and some code language to support this would be very helpful to change hearts and minds on the subject.
 
Lightening protection systems are required to be listed. The drawings that come with them are required to be followed. What do those drawings say?
 
petersonra said:
Lightening protection systems are required to be listed. The drawings that come with them are required to be followed. What do those drawings say?
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The existing structure is UL listed but the addition of the tower on the property is being done under another contract by a different AE firm and per contract the tower is not required to be UL listed. The drawings say to tie to existing counterpoise but there is no counterpoise. the GES for the existing structure appears to be ufer/triad combo. We put in an RFI and the answer came back to run a conductor from the tower counterpoise back into the building and bond to the MGB.
 
I am not a lightning expert, but my understanding is that all earth electrodes must be tied together.

Think of the earth electrodes as antennas in the soil sampling the local voltage of the soil they are touching. If a potential gradient exists in the soil then that will show up as a voltage difference between two separate grounding electrodes.

You current have two sets of grounding electrodes (building and tower) connected via your equipment (antennas, coax, radios, etc). If lightning strikes anywhere nearby causing current in the soil, your equipment will be the path for equalizing the voltage between these two electrodes.

Jon
 
winnie said:
I am not a lightning expert, but my understanding is that all earth electrodes must be tied together.

Think of the earth electrodes as antennas in the soil sampling the local voltage of the soil they are touching. If a potential gradient exists in the soil then that will show up as a voltage difference between two separate grounding electrodes.

You current have two sets of grounding electrodes (building and tower) connected via your equipment (antennas, coax, radios, etc). If lightning strikes anywhere nearby causing current in the soil, your equipment will be the path for equalizing the voltage between these two electrodes.

Jon
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So, I am hearing you but I think equipotential grounding plane like I would in a swimming pool is outside of the scope of NFPA 780. The cabling between the structure and equipment on the tower will have TVSS or some kind of surge suppression. Once the energy is in the earth by way of the tower counterpoise, I cannot imagine it will find a better path by going back up into the building. Maybe I am just being stubborn.
 
Like I said, I am not a lightning expert.

I'd suggest you walk away from the idea of 'once the energy in in the earth'. Lightning is a discharge between two charged surfaces; a large patch of cloud and a large patch of earth. Current must flow through the soil over a considerable area during a lightning event.

Consider both what might happen during a direct lightning strike, where current will want to spread from the tower through all available paths into the patch of earth.

Also consider what will happen during an indirect strike, where lightning hits something nearby and then spreads through the soil and anything sitting in the soil.

-Jon
 
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