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Fixed Wireless Equipment

Grounding Your Business

Although wireless antennas are situated in tall towers, the laws of physics demand a connection to the ground.

[August 3, 2004]

On the ISP-Wireless list in July, GM asked at the end of a long discussion (see The Guys You Depend On):

What's your opinion on linking tower ground to electrical ground for towers attached to buildings?

SC responded in the same tone:

"The key difference here is that I would not consider that in good condition since it did not follow the manufacturer specs. There is a huge process that goes into designing and building a tower that is overlooked more often than not. We really haven't gotten into soil samples, bittle tests, concrete slump tests, wind zones and wind loading, or FAA and FCC registration requirements. I am simply pointing out a couple general things that would be cause for concern—at least to look further and get more information. Honestly, I have never seen the tower so for all I know it could be rusted through and all this is a non-issue.

As far as grounding goes, my opinion is this: Put in a separate ground system. Just as an example (there is some irony here) my house was hit by lighting some time ago. Everything in the house had surge protectors to include the fuse box. I ended up with over $10,000 in damage. No one has any idea what really causes lighting or how to predict it or its power. Granted, the damage could have been much worse. The only reason the cost was that high was really due to the high cost of some electronics. That being said, I would never purposely allow lightning to pass through a building in any way, shape, or form. Put in a separate grounding system away from the foundation of the buildings and the tower. Do soil tests, CAD welds, etc., and make sure you are protected."

BG had a terse response to the suggestion that every tower should have a separate ground system:

"Not a good idea. Not only does this violate code; it can actuallymake the damage worse if you get a hit. Basic physics."

[SC replied] "What?? You are saying that if I build a tower with a concrete foundation and a radio head shed next to it that I should run the grounding from the tower to the head shed? I am curious as to what code and physics you are speaking of? The whole point of grounding is that you have the easiest path to earth so you pull the charge away from whatever you are trying to protect.

A real tower will have a ring of grounding rods all interconnected with potentially multiple connections from the tower to the ground system. You would never run a wire into the building to ground it."

[BG replied] "No; that would be a bad idea as well. You must bond the grounds together properly. I am referring to the National Electrical Code and the basic physics of electricity: E=IR.

If you ground the tower separately, then you will have a differential between the tower ground and the ground in any building that's near it in the event of a lightning strike. That differential is sufficient to blow out equipment, start fires, and worse. Your grounds must be properly bonded."

[JO intervened, noting] "We bond all our grounds just as BG suggests, using 2 gauge copper wire as a minimum size and have yet to have equipment die due to a lighting strike. We did have one antenna weld itself to the tower, but the drains on the coax and grounding system did its job and that radio was fine.

What protects your equipment is not the best path to ground, but rather having an equal potential across all your gear so that your equipment does not provide a path to ground through which any current travels. We usually install four 10 foot grounding rods at the base of the tower, welding them to the tower and three or four 8 foot rods near our huts, all of them are bonded together with large bare copper wire.

The best reference that I have found on the web, regarding proper grounding procedures (which are used in broadcasting, which is where I learned them) can be found here. "

[BM added] "A seperate ground line from a lightning rod at the top of a tower to the grounding down below is not necessary. The tower steel will have the same, if not better current carrying capacity because of the overall surface area. Many manufacturers do not recommend down conductors on towers, just rooftops where possible."

[SC replied] "BG, we were trying to stay at a very high level for the purpose of this conversation. There are about 100 different things that anyone would need to look into in order to build a tower properly—or install a wireless system for that matter. The entire point I was trying to make was that grounding must be done properly. We are making far too many assumptions to be of value at this point. The net result is that you should contact someone who knows what they are doing before simply slapping a 25G to the side of your building. I would hope we could agree on that.

JO, your point about simply putting in a fixed number of ground rods may not provide the necessary grounding that you need if you haven't done a proper soil test. This goes back to my point from the beginning: there are so many things that make each tower job unique it is hard to sit here on a discussion list and make a fixed list of absolutes."

[BG replied] "Yes, we do agree on that. I am now helping an acquaintance to improve upon a 25G which he erected without knowing much about grounding or how to route cable to resist lightning. He's a ham but will also be a wireless customer."

—End