Re: NASA and braid was Re: [TowerTalk] There's 'ground', andthen there's 'ground'

[Jim W7RY <w7ry@centurytel.net>]

At Motorola the "R56 Site Standards and Guidelines for Communications 
Sites" strictly prohibits braid of any kind on a communications site. 
Mostly because of the noise it generates when it gets old and deteriorates.


The underground conductor interconnecting all of the ground rods is #2 AWG 
tined solid copper. This conductor is also attatched to the MGB (Master 
Ground Bar) inside of the building just below the cable entrance window. 
There is also a EGB (External Ground Bar) on the exterior of the building. 
The exterior bar is where the coax shields are grounding before they enter 
the building.

The MGB is where the lightning surge suppressors are mounted or connected 
directly to. (Within 18 inches using #6 AWG minumum).

Each equipment rack has a RGB (Rack Ground Bar). Each piece of equipment in 
each rack is bonded to this bar. Each RGB is bonded back to the MGB with 
stranded jacketed #2 AWG wire in a single point ground fashion.

The AC power grounding conductor is also connected with a #2 AWG conductor 
to the MGB.


The MGB is the single point ground location.

It's not rocket science... Heh Heh.....

BTW the Motorola R56 is based on the following INDUSTRY STANDARD DOCUMENTS:

NEC, The National Electric Code.

TIA J-STD-607-A   (This is the best document to refer to)

ANSI  T1.333 and  T1.403

As for NASA, I don't have their document or documents.

When you have fully read the above documents and understand their content 
and proper application techniques........  You will know.....


73
Jim W7RY



At 08:30 PM 1/16/2005, Jim Lux wrote:


> >
> > Many construction standards prohibit braided straps in RF or
> > lightning paths unless the connection absolutely must have
> > braiding in order to withstand flexing, and then the braid
> > is often substantially oversized to make up for its reduced
> > current capacity. It's my understanding NASA restricts use
> > of braiding, and I can cite many cases where braiding will
> > either cause excessive loss or actually fail in high current
> > RF systems.
>
> I'm going to guess that NASA doesn't like braid because of the possibility
> of small metallic fibers coming out and winding up in the wrong place in
> flight hardware, not because of any RF properties.
>
> I'll also have to say that what NASA requires or advocates may or may not be
> relevant to amateur radio (or any other industry) practice. Without knowing
> the details of the ostensible NASA proscription on braid, I can't really say
> why they do it, but it might be because of some reason totally unrelated to
> the RF properties.
>
> Tom, you've written several times about NASA not liking braid, but I can't
> find any definitive reference in any NASA documentation that says this.
>
> Back in 2001, you wrote (in part):
> NASA and others prohibit the use of braiding in lightning grounds,
> and you'll never see it in BC stations where the ground is involved
> in lightning or RF applications unless it is way overkill size and very
> short.
>
> and in 1997: NASA prohibits use of braiding for lightning protection and RF
> grounds unless
> the connection absolutely has to be flexed.
>
> I found these when hunting for  NASA requirements for something regarding
> the use of braided conductors.
>
>
> I did find the following NASA specification (June 2004 date)
> http://www.ccb.org/docs/NASAASC/NS16065.pdf
>
> which specifically allows braid for bonding conductors.
>
> And then there is the following NASA document:
> http://klabs.org/DEI/References/design_guidelines/design_series/1231.pdf
> "Design considerations for lightning strike survivability"
>
> which makes no mention of braid, pro or con, except in the context of
> requiring braid shields to be "grounded"
>
> Then, there's JPL D-8208 (rev I) "Electronic Packaging and Cabling", with
> which I have entirely too much familiarity...
> http://standards.jpl.nasa.gov/contractor/docs/d8208i.pdf
>
> One might want to look on Page 292 of the spec, which says, in part:
> " The S/C chassis provides very low impedance between all subsystems. When
> the RFS (Radio Frequency Subsystem) is powered from a source other than the
> S/C, a low impedance path between the RFS chassis and its power source
> chassis must be provided.  Heavy *braided* wires will provide an acceptable
> impedance. The power source shoudl also have a heavy *braided" wire
> connecting it to solid earth ground."
>
> This isn't lightning protection related, of course.
>
> There's a whole raft of stuff about using braid as a thermal path, as well
> (since it has many parallel wires, and is flexible under vibration and
> thermal distortion)
>
> I also note that we use braid as a bonding conductor at JPL a lot
> particularly in ESD protection schemes (not that it might not be prohibited
> by some spec, or even a bad idea, but I would think that some auditor would
> have raised the flag by now, and we would have had a lab-wide purge of braid
> (like we have had for "pink poly" or for "disposable" wrist straps, and
> such)).
>
> I grant that using small braid in a lab for a ESD ground isn't lightning
> protection.
>
>
> But the summary is that I can't find any reference in NASA procedures or
> documentation about prohibiting its use. If you could give me some pointers
> to where it might be, I'll hunt them down.
>
>
> Jim, W6RMK
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 
> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
> any questions and ask for Sherman, W2FLA.
>
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See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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