Cecil,
You are spot on. A direct ground strike will develop up to 400 kA-pk. At 5
ohms that is a serious voltage surge. The Voltage Surge will have a very
fast rise time (less than 100nS), the current surge will have a lower rise
time 40uS or so. Within 1mS the impulse will have decayed back to zero.
If your tower takes a direct strike, the grounding at the base of the tower
had better be able to "take it" up.
W8JI claims his towers have taken direct strikes and has a section of his
web site dedicated to lightning protection. Lots of straight talk and no BS.
I feel blessed to live in Western Washington. Very rarely do we have serious
lightning activity.
73, Don KO7i
-----Original Message-----
From: chacuff [mailto:chacuff@cableone.net]
Sent: Tuesday, May 22, 2012 6:14 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] RFI Issues
Just my 2 cents worth....
Your grounding work shouldn't start just outside the wall from the radio
gear...it really should start at the tower and be an antenna/tower grounding
"System" designed to stop/minimize the lightning impulse at its source as
you work back towards the station.
A single rod lightning protection system is of little protection because
much like Bob has pointed out the resistances involved between the multiple
ground points there is also resistance between a single point of ground and
earth. If the ground resistance was measured at the single rod it would
probably be measured as several hundred ohms. If your station lightning
protection ground was measured at that level and subjected to several
thousand amps of instantanous current simple ohms law will tell you what
kind of voltage potential you can expect on the chassis of equipment...and
that gets dumped onto the electrical system of your shack/house. Lots of
damage.
The lightning protection grounding has to be approached as a system. The
ground rods can be looked at like resistors...put more in parallel and you
bring the total system resistance down. That's done by driving multiple
rods at 2 times there length apart and bonding them together with large
gauge wire, preferrably bare. The Cellular industry uses #2 bare solid (not
stranded) wire welded to the ground rods. It's not unusual to have a dozen
or more rods driven to get the system resistance down to 5 ohms or
less...the lower the better.
The tower should be tied to this ground "System" and all feedlines should
have ground kits installed on them which ties the shield to the ground
system. Each feedline and rotor cable should have an inline commercial
protection block...ie Polyphaser or ICE connected and tied to a common point
with a low impedance path to the ground "System". Finally all this should
be tied to your house electrical panel at the point where your meter can is
located which should be where the small ground rod the electrican drove and
tied to the house panel.
As you can see this can get expensive (and complicated) but so can loosing
all your nice gear and a bunch of stuff in the house that's not related to
the ham shack. It can come to 10s of thousands of dollars easily....all in
a millisecond. Been there and done that. Hope to not have to deal with it
again.
Again just my 2 cents worth...
Cecil
K5DL
----- Original Message -----
From: "Bob McGraw - K4TAX" <RMcGraw@Blomand.net>
To: "Jim WA9YSD" <wa9ysd@yahoo.com>; "Discussion of Ten-Tec Equipment"
<tentec@contesting.com>
Sent: Tuesday, May 22, 2012 7:30 AM
Subject: Re: [TenTec] RFI Issues
>
>
> Here's where I view that people get in trouble. They drive a ground or
> two
> and connect it to their radio or station equipment. They plug the radio
> in
> the wall plug which has neutral and ground. Neutral and ground are
> connected at the breaker panel for the house per NEC. The ground at the
> breaker panel is attached to a driven ground outside, again per NEC. A
> lightning storm approaches and they disconnect their antennas. A nearby
> strike, meaning up to 5 miles away, causes the two or more ground points
> to
> have different voltage potentials as they are separated by some distance
> of
> a few feet to several hundred feet. There is resistance in the earth
> between the ground points thus with current flowing through the earth
> there
> is a difference in voltage between the two or more grounds. Now, what's
> connected between the two ground points? The radio and station equipment.
> Therefore, even when antennas are disconnected and the radio is turned off
> there is a path through the ground and neutral back through the radio or
> station equipment. It spells failure and we often hear........"but my
> radio
> was grounded, my antennas were disconnected and the radio was turned off".
>
> The point is the fact that ALL grounds must be bonded together and
> preferably outside of the structure. This includes a hard electrical
> connection back to the AC mains ground point.
>
> 73
> Bob, K4TAX
>
>
>
>
> ----- Original Message -----
> From: "Jim WA9YSD" <wa9ysd@yahoo.com>
> To: "TenTec .com" <tentec@contesting.com>
> Sent: Tuesday, May 22, 2012 12:52 AM
> Subject: Re: [TenTec] RFI Issues
>
>
> Sorry Jim
>
> When the FCC had checked out my installation cause of an RFI issue, they
> found that I had a bad solder joint on my shielded ground and fixed it for
> me. All was well. If the the shielded ground was BS they would have told
> me
> and not fix it?
>
> Tying station ground to electrical ground low impedance please describe.
>
> Existing wiring. The 2 grounds were separated at time of inspection was in
> code then but is not in code now, residential wiring does not have to be
> upgraded unless there was remodeling, house was sold, and the sort, so
> then
> it needs to be brought up to code at that time.
>
> Stay on course, fight a good fight, and keep the faith. Jim K9TF/WA9YSD
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|