[Heathkit] Differntial earth

[Robert & Linda McGraw K4TAX]

One thing that is often misunderstood within the ham community with regard 
to grounding and lightning protection.

All driven grounds must be bonded together.  Station ground must be bonded 
to the AC mains ground.  Failure to do this will place the radio equipment 
between grounds of two different potentials during a nearby lightning 
strike.  Most think that ground is ground.  Well it is except that between 
any two ground points there is some value of resistance. During a nearby 
lightning strike the energy is dissipated across the surface of the earth. 
Studies have shown that 80% of a strike is dissipated within the first 18" 
of surface depth.  Thus with two or more ground points, and a value of R 
between the points, there will be a voltage differential between the them. 
If your radio is connected between any two grounds then the path of least 
resistance is through your radio.

Solution:  Connect all driven grounds together outside of the structure with 
a #6 or larger conductor.

73
Bob, K4TAX


----- Original Message ----- 
From: "Steve Beveridge" <steve.b at internode.on.net>
To: "Duane Fischer W8DBF" <dfischer at usol.com>; "Robert Dorchuck W6VY" 
<w6vy at verizon.net>; <HEATH at LISTSERV.TEMPE.GOV>; "Heathkit" 
<heathkit at mailman.qth.net>
Sent: Saturday, February 04, 2006 5:15 PM
Subject: [Heathkit] Differntial earth


>
> Greetings folks,
>
> This little snippet from Duane is an interesting/unfortunate observation
> which has a very powerful message.
>
>>due to lightning getting into the Shack via my ground rod system. I did
>>not disconnect the ground during an  electrical storm. Unhooked all
>>electrical connections from wall outlets,  disconnected all antennas >and
> set switches to ground enable the Poly Phasers etc. You ground >them to
> protect them and them must disconnect the ground to
>> protect them! Sigh.
>
> Everything seems to be right, but the problem was probably inside the
> "boxes". In effect, poor earth connections between boards within the 
> radio,
> etc. can mean that there are small (probably very small) resistances 
> between
> the original earth and the "earth" which the board sees. At the sort of
> potentials associated with lightning strikes, this means that there is a 
> PD
> between true earth and the earth as seen by the board - thus a 
> 'differential
> earth'. We know that PCB screws come loose - as may be seen by the myriad
> 'fixes' which simply involve re-tightening PCB screws.
>
> I have been doing a total rebuild on a TS-850 which suffered a serious
> lightning surge through the house wiring - not mine fortunately. This was 
> a
> 'not-commercially-repairable' moment-of-weakness purchase (from the
> insurance company) on my behalf.
>
> There were quite substantial earth tracks 8 mm wide (or ~1/3") which had
> simply been blown away, leaving gaps of 3 mm (1/8"). In one instance, 
> there
> were three blown sections in no more than 25 mm (1") of track. I am not
> exactly sure how the rig was earthed, but suspect it had been earthed
> through the PS, which was blown beyond repair. Most of the SMD transistors
> on the IF board looked like little carbon blocks (and tested the same!).
> Electros 150 mm (6") away had been blown up. A protection diode (on a
> different board), nearly as big as my (average-sized) little finger, was 
> in
> full conducting mode.
>
> It seems as though if everything goes "up" then "down" (with respect to
> genuine earth) at the same time, there is no (little?) damage. However, 
> even
> a slight 'lag' in the return to true earth potential leads to damage. That
> energy has to go somewhere.
>
> A colleague (ex RF design and manufacturing engineer) always uses the 
> outer
> braid of RG58 to earth boards within his radios. Doesn't rely on the 
> typical
> push-on connectors. Sounds like a lot of hard work, but it seems to do the
> trick.
>
> The 850 is nearly there, but this has been a valuable learning foray into
> something I had always taken as a fairly simple aspect of out hobby - the
> earth!
>
> Cheers
> Steve
> VK2XWL
>
>
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF
> ** For Assistance: dfischer at usol.com **
> $$ See the vintage area on the HCI web site - http://www.w9wze.org $$
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