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I am trying to figure out a good (but inexpensive) way to protect a house's electrical circuits from a highly unlikely lightning strike to an outdoor antenna. Here is the setup:

There is an antenna on a short metal mast, which is attached to the side of the house, and extends about 1.5 feet above the roof. It is attached via screws to the wood frame of the house. There are tall trees nearby, so keep in mind that the likelihood of a direct lightning strike to this antenna is pretty low. The mast itself is not currently grounded, but it soon will be. From the antenna, a coaxial cable travels down, and into the house. It then must attach to an electrical unit, which itself attached to the house's 120v AC service on a circuit which is shared with a few other devices.

How best can the house be protected?

I'm thinking of the following:

  1. Add a dedicated grounding rod, and run wire (either 10 gauge, or 6 AWG - I've heard conflicting recommendations) up to the antenna mast, and bond it there.
  2. Add a grounding block to the coaxial cable (somewhere), and ground that out to the dedicated grounding rod. Much like an arc arrester, this only protects the coaxial shielding. The data rail (copper core) is still unprotected. (This ground connection could itself became a threat, as explained below in issue "C")
  3. Add a Tripp Lite brand "Coax F-type Lightning / Surge Protector" to the coaxial line, inboard of the grounding block, so as to protect the data rail from lightning-related surges. This probably must also ground to the new grounding rod, and will be the only protection for the copper core.
  4. As a last resort, perhaps add a 120v outlet-based surge protector between the AC plug of the indoor powered unit, and the power socket.

My questions and concerns are as follows:

A. The grounding block (point #2) and coaxial surge protector (point #3) must connect to the same dedicated ground. (We don't want to use the house's grounding system, as that would likely backfeed and fry everything in the house) However, these don't seem like the sorts of things which should be exposed directly to the elements. We also don't want to drill holes in the house, just to run the dedicated ground inside. And tips?

B. Can an AC outlet surge protector work as a "last resort" protection option, in this case? We will be asking it to work in reverse. I did see this discussion where someone said that they can work in reverse, but only up to a point. If the other protection measures fail, this could receive a significant portion of a lightning strike. Could it work to protect the house? Of course, it would use the interior grounding system, which is exactly what you don't want. The house circuit it is on does NOT have a 3rd wire for ground, but rather uses the electrical conduit as the ground, where it connects down to the grounded circuit breaker panel...not where you want lightning going, if it can be avoided.

C. Is it safe to have each subsequent protection measure on the coaxial cable, connect to the same (dedicated) ground directly? Is there something like a diode stack which could be used to prevent backfeed? In other words, if the antenna mast is struck, the charge will (presumably) go mainly right down to ground. However, some of that current will likely backfeed to the proposed grounding block, and the ground of the coaxial surge protector. This will definitely electrify the coaxial shielding, and if the coaxial surge protector fails, it could allow it to leak onto the data rail which is otherwise unprotected.

D. Am I seriously over-thinking this?

Thank you!

Edit: This is NOT a TV antenna. The only electrical connection between the antenna and the house is the single 120v AC power connection.

DavidB
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  • What are you using 75 ohms wire for that is not TV? – Jasen Oct 01 '21 at 10:28
  • Just so you know. The "dedicated lightning ground" must be **bonded to the building electrical ground.** They are not all that separate. https://lightning.org/wp-content/uploads/2014/12/Bonding-2013-ULPA-LPI-rev1.pdf 4.14.3 This interconnection shall include all building grounding electrode systems including lightning protection, electric service, communication, and antenna system grounding electrodes. – Ecnerwal Oct 01 '21 at 12:52
  • How many joules is the surge protector rated for? How many joules is the lightning? – Harper - Reinstate Monica Oct 01 '21 at 16:37
  • @Ecnerwal The house's electrical system already has a grounding rod, but it is on the wrong side of the house for this purpose. Wouldn't bonding this new grounding rod to the existing electrical system cause lighting to backfeed into the house? Why not keep it isolated? – DavidB Oct 01 '21 at 21:22
  • Lightning protection is rather serious business. Code requires bonding the systems, which should tell you "it needs to be done" - in short, having the entire system ground potential rise some number of kilovolts is much, much safer (like two birds on a wire) than having one thing go to kilovolts and another thing go to hundreds of volts, resulting in kilovolts between things. – Ecnerwal Oct 01 '21 at 21:56
  • not bonding it is worse – Jasen Oct 02 '21 at 11:46

1 Answers1

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1: add a dedicated lightning grounding system and run a 1" wide bar to it. anything less is a fire hazard.

2: yes, grounding block

3: connect the inner lightning trap to internal safety ground

between 2 and 3 use a coaxial isolating transformer if possible.

Jasen
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