[Heathkit] Differntial earth

[Duane Fischer, W8DBF]

This was not the problem OM, see my previous post about the lightning 
hitting a tree, then the soil and through the soil we go, up the ground rods 
etc.
----- 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 **
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