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Author Topic: Yet more on grounding  (Read 3433 times)

Posts: 6252

« Reply #15 on: November 17, 2004, 08:03:11 AM »

Granted, direct strikes are less likely to happen than indirect "induced currents".  A person who is just setting up a station is unlikely to have the lightning protection that a fully set up station should have (a mistake, but one that frequently happens) and is looking for the best course to follow.

As it happens, I overlooked the fact that co-ax switches frequently ground the unused center connections, but most of us starting out to build a station are worried about three things:  1.  The rig,  2.  The feedline  3.  The antenna,  overlooking the fact that item 3 should be an ANTENNA SYSTEM.

In the discussion I overlooked the fact that Jim's antennas are indoors and he was asking about the ground system he intends to put in place.  

I now realize that others are probably correct about the grounding of the center conductor--the small space of the dialectric between the center conductor and the shield won't make any difference if the antenna system is struck directly, but if the strike is nearby and indirect currents are induced it may make a big difference.

You learn something different every day.

In the case of the beginner and his starter setup, I'll still say the best course is to disconnect the antenna co-ax and toss it out the window!


Posts: 1819

« Reply #16 on: November 17, 2004, 08:36:33 AM »

I don't think that throwing the feedline out the window will offer the protection you need unless:

1. There are NO OTHER wires coming into your shack from outside, including telephone, TV cable, relay control lines, etc.

2. You have already tied your electrical system ground to your entry panel and station ground.

When there is a nearby strike, currents WILL be induced in metal conductors in the near vacinity and strong currents will flow through the soil as well.  The result can be a high potential difference between two points even if both are grounded through a rod, but not tied together.  For example, if you have a station and/or entry panel ground and it's not tied to the electrical system ground, the substantial difference in potential that can exist may be resolved inside your rig, or between the rig and a nearby box that is not grounded.  The Polyphaser articles that appeared in QST a while back (series of three) had a good approach to planning for protection.  It involves drawing a diagram of everything in your station and all the lines coming in and out of the room.  I uncovered some situations I didn't think about before when I did that.  

I suspect that many are in the same boat as I am regarding the number of conductors going into and out of the shack.  There are so many that there's simply no way one could disconnect everything (and achieve adequate separation)when a sudden storm occurs.  The last thing I want to be doing when that happens is disconnecting cables behind the operating table!

Everyone has to make their own choice as to how much protection they want to consider.  My only advice is - no matter what you decide, make sure you have ham radio and PC insurance.  A year's worth cost about what a Polyphaser arrestor costs!

Posts: 21764

« Reply #17 on: November 17, 2004, 11:39:50 AM »

I'm evidently no expert on this, either; however relying on NEC and momma NFPA for any worthwhile advice seems futile also.

About eight weeks ago I had eleven -- count 'em, 11 -- conversations with various parties at NFPA regarding grounding issues, and couldn't find any two who agreed with each other unless they were in the same conference call.  And I'm still looking for anyone there who understands electricity.  

I have to remember to tape record my next call to them, it should be worth a lot of laughs.

Happy to be where lightning seems to never happen,


Posts: 14491

« Reply #18 on: November 17, 2004, 03:23:27 PM »

I think its primarily a matter of following the current flow in light of Ohm's law. Most of the current is going to flow thru the lowest resistance (impedance actually) path. If you can provide a low impedance path that doesn't go thru your equipment then you are better off. It's like many parallel resistors, most of the current flows thru the lower resistance and the lower it is, the less voltage appears across it and the rest of the resistors. If you leave a wire ungrounded then the resistance is very high so if there is a direct or induced strike then the voltage can rise very high. If it gets high enough, the air will ionize and there will be a flash to the closes ground connection. There are plenty of ground connections around. The electrical wiring in the walls, moisture in wood and concrete, plumbing, etc. The other issue is one of heating. The higher the resistance, the more heat is created for a given current thru it. If lightning strikes a wooden pole and the current is permitted to flow thru the moisture then it gets hot and the vaporized moisture can splinter the pole. If you run a heavy wire down the pole to a ground connection then the current flows in the low resistance wire instead and there is much less heat developed. Thats why telephone and power poles have a ground wire run top to bottom down the outside and wrapped around the base under ground.

Things get more complicated when you have multiple paths to the equipment such as power, phone lines, rotor cables, etc. The ideal solution is to ground them all to a single point at the entrance to the building. The problem is that most homes are already constructed incorrectly and not easily modified.

Bob  AA4PB
Garrisonville, VA

Posts: 1819

« Reply #19 on: November 17, 2004, 05:47:39 PM »

My last comment - I promise!  The single point ground entry panel is the way to go from all I've read.  However, if you have long runs of cable between that entry point and your station, it may not be terribly effective.  After studying my situation where there was damage to the TV cable system in my home (even though it enters through my ground panel and is protected there), It occurred to me that there is something like 100 feet of coax running around through the walls before it gets to the point where I had damage.  Apparently that was enough for a reasonable current to be induced and allow a high potential to exist at the wrong place. If your station is close to the entry panel, I guess you would not experience such a problem.  If it is far from the panel, maybe an additional common ground bus at the operating desk is a good idea after all.
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