[R-390] Grounds

[rbethman]

Don,

The document that Tisha is offering to send out to those that want it, 
even gets into the inductance factoring in the lightning strike instance.

The document is an excellent treatise on the topic!

It says it much better than I have been attempting to.  The diameter of 
the ground grid conductor is not as critical as is the skin effect issue 
of large outer area.

You'd have to go for over 2" diameter copper pipe to come "close" to 500 
MCM cable.

If it is okay with Tisha, I'd gladly put the document on my web site, 
and tell everyone on the list exactly how to get it.

That way folks don't have to deal with a large E-Mail.  I realize not 
everyone has that service.

The file is approaching 6MB in PDF format

Bob - N0DGN

2002tii wrote:
> Because lightning can draw tens of thousands of amps of current 
> (albeit for a very brief time), it can develop substantial voltages 
> across any resistance in its current path.  So, for lightning 
> protection, everything that you count on to be at "ground" potential 
> should be connected with very low resistance paths.  Then, the 
> difference between the potential of a "ground" that is in the 
> lightning's current path will be close to the potential of all other 
> "grounds," and the risk of getting a shock between the two "ground" 
> points should be low even during a lightning strike.  Note that 
> lightning current is often conducted laterally through the soil, so 
> "ground" stakes separated by some distance can be at different 
> potentials during a lightning strike even if the lightning current is 
> flowing in neither of them, due to the resistance of the soil itself.
>
> Otherwise -- that is, at all other times besides during a lightning 
> strike -- there should be very little current flowing into any 
> "ground" (or through the ground past one "ground" point and 
> another).  Therefore, you should be able to count on the various 
> "grounds" being at close enough potentials not to cause a risk of 
> shock if you touch more than one at a time.  If you get a shock when 
> touching two different "grounds," something is wrong with at least one of them.
>
> RF grounding is another, separate issue.  In the RF case, it is not 
> only the resistance of the path to earth ground that matters, but the 
> inductance as well.  If the path is any longer than 1/10 wavelength 
> or so at the frequency of interest, it will have significant 
> inductive reactance and, therefore, you can develop significant 
> voltage down the "ground" wire even if it has low resistance.  You 
> can minimize this by making the ground conductor out of wide strap or 
> braid rather than a round wire, but only to a certain extent.
>
> So:  Best practice is to establish a robust "station ground."  Bond 
> all equipment securely together with 2" or 3" wide copper strap, then 
> connect this to a good, solid earth ground (my suggestion is a 12 
> foot length of 1" copper pipe driven into the ground) with the 
> shortest possible length of copper strap.  If it is unavoidable that 
> this length will be more than 20 feet, use wider strap (6") or 
> multiple straps.  If your station ground is near the service entrance 
> ground, great.  If not, connect the two ground stakes (at ground 
> level, or buried) with the shortest possible length of copper 
> strap.  Use the widest strap you can find.
>
> Best regards,
>
> Don
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>   

-- 
+------------------------------------------------------------------+
|             AM Amateur Radio Operator    NØDGN                   |   
+------------------------------------------------------------------+
| Bob Bethman                \\\|///     " The absence of a danger |
|                           \\ ~ ~ //      signal does *NOT* mean  |
| rbethman at comcast.net      (/ @ @ /)      that everything is OK " |
+-------------------------oOOo-(_)-oOOo----------------------------+