[time-nuts] GPS-Attached Equipment Lightning Protection

Robert Vassar rvassar at rob-vassar.com
Sat Mar 1 14:24:53 UTC 2008



Lightning simply doesn't behave like you might expect.  It's a DC  
current, but the rise times and current magnitude gives rise to AC  
behaviors.  The problem with "attractive protection" is they have to  
carry the full burden of the strike without fail.  If it fails, the  
current simply moves to the next easiest path.  This could be your  
equipment or even you.  Seemingly low impedance conductors like RG-8  
coax braid simply become copper patina at these energies.  Conductors  
need to have substantial mass, and straight conduction paths to  
ground.  Keep in mind that "ground" is a vague electrical proposition  
that the soil beneath your feet may or may not agree to.



Rob
KC6OOM/5


On Feb 29, 2008, at 7:33 PM, Alan Melia wrote:

> Hi All, all the comment has been about in-line protectors so far.  
> We in the
> UK do dot have the ferocity of lightning that is seen in some parts  
> of the
> USA but surely like all problems "prevention" is better. It is  
> probably not
> 100% but would it not be better to have a higher metal rod say 6  
> feet above
> the GPS antenna spearately grounded as with a building protection  
> system.
> The GPS antenna should then be placed in the "cone of protection"  
> of this
> rod. The likelyhood of getting any significant voltage on the centre
> conductor I would think would be very small. The most likely  
> scenario is for
> induced voltages/current on the braid due to nearby strikes. This  
> should be
> grounded in a way that does not allow the whole ground system that  
> it is
> connected to to to be "pulled up".
>
> As has been stated if you do get a direct hit on the GPS antenna  
> nothing
> will save your kit, but again (I believe I am right here) the vast  
> majority
> of the current from the strike will flow down the braid (if it doesnt
> vaporize it), So grounding the braid, preferably outside the  
> premises if a
> good idea. The size of the antenna is such that the build up of static
> should not be significant....it is not a 200 foot long wire as I  
> use on MSF
> !
>
> If this is rubbish please say so but it sounds much more sensible  
> way of
> proceding to me.
>
> Cheers
> Alan G3NYK
>
>
>
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