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Author Topic: Station grounding.....RF grounding.....???  (Read 9970 times)
LA9XSA
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Posts: 376




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« Reply #30 on: July 20, 2012, 04:31:06 PM »

Concerning induced current in the house wiring remember that without the single point entry configuration the house wiring lightning current is 16 kA or 8 kA depending on if a shunt wire is used.
I would think that depends on the gauge of the shunt wire, where it's placed, the soil resistance, and the physical distance from the antenna ground rods to the AC service ground rod.

The induced current due to the coax running along the house will be less than this. I can calculate this if you want. And when a lightning shunt wire is used it too will induce a current into the house wiring. I'll think about all of this some more.
To be clear, I was describing hanging the coax on the exterior wall, a few inches from the house AC wiring, as a very bad thing to do that could induce current or cause arcing to the AC wiring, and then damaging the wiring on the path to the AC service entry or even to appliances near water or gas pipes. I compared this to scenario 2 with a shunt wire between the AC and antenna ground points, but this shunt wire should be buried outside the drip line of the building. If you run coax to a common entry point (scenario 3), the right thing to do would be to bury this coax a certain distance away from the house and only let it approach the house when it's perpendicular to the common entry point.

A shunt wire or coax hung inches away from the house AC wiring is bad - burying them a sufficient distance from the house would is good. Agreed?

The point I tried to make by that example is that a correct modification of scenario 2 would be far better than a poor execution of scenario 3.

I was analyzing the pseudo single point entry where the RF cables are brought in at one end of the house and the AC is brought in at the other. The analysis is to see what is the advantage of mounting the power outlets on the RF panel vs. not doing so.
OK. I'm still not sold on the figures, but yes I'd mount the power outlets on the panel in scenario 2, and as you say in scenario 1 it could at least save the radio equipment while not protecting the rest of the house.

LA9XSA, note that the coax that is routed along the building - from the AC service ground to the station - carries no lighting current and will not induce current into the house wiring. It is however at a potential relative to the ground under it and that difference can be calculated for purposes of insulation coordination.
In my example of poor execution of scenario 3, the coax is simply hung on the exterior wall of the house. It then acts as the only connection between the four ground rods at the antenna and the AC service ground rod, and the lightning current flowing in the plasma left behind by the evaporated shield would be substantial. Right?

So we have two distinct scenarios: Route the coax to the station and deal with the lightning current through the house AC wiring or route the coax to the AC service ground then to the station, have no lightning current through the house, and deal with the potential vs. ground on the coax.  
We have three scenarios here, but the difference between them is - I think - a matter of degree. If you execute scenario 2 with a small shunt wire hung on the exterior wall, it's not much different from scenario 1. And if you execute scenario 3 by likewise hanging the coax on the exterior wall, it's almost as bad as scenario 1. But if you execute scenario 2 with a good enough external ground ring/grid between the entries, it looks electrically almost like a well executed scenario 3.

Either scenario can be designed such that when lightning hits the antenna the house and station survive.
My weakness here is that I can't quantify the wire sizes and configurations myself, since I'm not an electrical or RF engineer. I can only refer to the work done by others. If you use sufficiently long buried coax in scenario 1, yes I imagine you'd limit the strike energy that makes it to the house so much that further protection is not needed, but I imagine that the losses in that long coax would make the installation unusable. However, it would only protect against a tower strike of the design size, not any other type of transient, so only scenario 2 or 3 could be executed safely, in my opinion.

Both scenarios point to more antenna ground rods and fewer AC ground rods.
If you are sure lighting will only strike your tower, yes. But if you have a risk of lighting or other transients coming into your house on the AC service, seeking your antenna ground, then it would be better to have more AC ground rods and fewer antenna ground rods. Since transients can come both ways, both sets of ground rods need to be sufficiently bonded with wire buried outside the drip line - that way they help each other dissipate the transients into ground with minimal conduction through the house.
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WX7G
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« Reply #31 on: July 20, 2012, 05:42:22 PM »

LA9XSA, did you read the Lightning Rod Man? Might that be state-of-the-art for 1860?

I've been waiting for you to mention a strike from the other direction; the power pole. In the case of coax entry at the station one wants many ground rods at the house AC ground to reduce lightning current through the house, but that means more lighting current coming from the tower through the house if hit from that direction. In the case of coax entry at the house AC ground it doesn't matter. The lightning current through the house is zero whether the pole is hit or the tower is hit.

About the coax along the house or through the house, that is the coax from the house AC ground to the station. The coax from the tower to the house AC ground is routed along the ground as hams usually do it. So, the coax along or through the house is connected to the house AC ground at one point and to the station at the end of its run. It is at the same potential as the house AC wiring and as with the AC wiring it carries zero lightning current.
« Last Edit: July 20, 2012, 06:10:02 PM by WX7G » Logged
LA9XSA
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Posts: 376




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« Reply #32 on: July 29, 2012, 08:45:29 AM »

LA9XSA, did you read the Lightning Rod Man?
Yes.
Might that be state-of-the-art for 1860?
I doubt that but if you knock on my door to show me your rod, I'd be likely to show you the door.

In the case of coax entry at the house AC ground it doesn't matter. The lightning current through the house is zero whether the pole is hit or the tower is hit.
A wood wall is not a magical artifact. How the coax is brought to the entry point will matter - too close proximity to the house will cause induction and arcing to wires and other conductors in the house. The walls themselves can conduct too. And proper bonding of the tower ground to the house ground system will also matter, as will the ground rods.

Lighting has a fast rise time - not slow. Its AC/RF component is substantial. As far as I know only an EMP would have a higher dV/dt than lightning.

About the coax along the house or through the house, that is the coax from the house AC ground to the station. The coax from the tower to the house AC ground is routed along the ground as hams usually do it. So, the coax along or through the house is connected to the house AC ground at one point and to the station at the end of its run. It is at the same potential as the house AC wiring and as with the AC wiring it carries zero lightning current.
Wait. Hold the phone. Are you seriously suggesting that one could just lay the coax on the ground, then bring the coax inside the house, and run it THROUGH the house to the AC service entry, and then back to the station? Or hang the coax along the exterior wall? Without any ground kits or suppressors between the tower and the AC service entry?

I'd put it to you that in a real sense, the AC service entry is no longer your antenna entry. Your actual antenna entry is where the coax comes near the house, and it is an ungrounded antenna entry point.

Please correct if you meant something else.
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