[TowerTalk] Grounding thru a bulkhead???

[jimlux]

chasm at texas.net wrote:
> to the Towertalk reflector...
> 
> I have a Georgia Copper bar for a single point ground behind my gear 
> using flexweave between each unit and the bar.
> 
> the bulkhead is the MFJ wooden board product with a short piece of 
> metal plate on both indoor/outdoor sides connected with barrel 
> connectors, etc. those ground both sides to each other.
> 
> I have drilled a couple of holes away from the plate to pull my 240V 
> thru the bulkhead from an outside service panel to provide juice for 
> the amps.  The 120VAC also comes thru the bulkhead from that same 
> service panel to supply the radios, etc.  grounds for the service 
> outlets go to the ground bar also.  mixing DC and RF grounds. hmmmm


The idea of tying, or at least referencing, things to one point (or one 
low impedance reference) within the shack isn't a bad idea.  If a 
transient happens, everything goes up and down together.

Tying the electrical safety ground (greenwire ground) to that point 
isn't necessarily such a hot idea.  The greenwire "ground" really isn't 
for grounding (in the RF or signal common sense).. it really should be 
called a bonding conductor, and the real purpose is to carry enough 
fault current if some equipment has an internal short to the case that 
the breaker will trip.  As such, it needs LOW line frequency resistance 
back to the panel.  Connecting other systems to it doesn't buy you much, 
and can actually aggravate RFI problems (i.e. your electrical safety 
ground becomes another antenna)

OK.. as a reality, the RF common (shields of coax, etc.) wind up being 
connected to the electrical safety ground through the power cord's third 
pin, etc. (since the chassis is connected to both).. at least for a lot 
of AC powered gear. (Hmm, is the 12V negative connected to chassis 
ground on that power supply?)

And, if you're following the NEC on antenna cables (which I suspect less 
than 5% of hams actually do), then the coax shield is permanently bonded 
to the building's "ground" system via the appropriate wiring.

So, the question is.. given that you have one wire going from equipment 
back to the service panel ground(the greenwire ground) and another wire 
connected to the same point by a more circuitous path (chassis, coax 
shield, bulkhead, shield bonding point, wire to service entrance), 
you're doomed to have one or more loops.

Does it hurt to have an extra connection in the middle (i.e greenwire 
ground to tie point)??

I'd have to think about it.  In general, randomly adding ground 
connections is a bad idea. (sometimes its better to have a single 
connection that fails open than to have one good and one poor 
connection, because if the good opens, then everything goes through the 
poor connection, and "bad things happen"... MGM Grand Hotel Fire is an 
example)

> 
> question.  how do I connect that inside ground bar thru the bulkhead 
> to the ring of 8' ground rods outside my window???
> 
> I plan to use 1.5" wide copper strap from the bulkhead-outside to the 
> ground rods and to tie the ground rods together.  but I cannot figure 
> out how to get from the copper single point ground bar THRU the 
> bulkhead to that outside copper grounding strap.
> 
> do I drill a hole thru the wood and coil up some strap and push that 
> thru the hole?   do not know how to physically connect the two sides 
> both mechanically and electrically.

Why not just a bolt through the whole thing?  Presumably you're using 
strap outside to provide a low inductance path for lightning transients 
to your ground ring.  Hopefully, you're not trying to conduct a 
transient from inside the house to outside, but just providing a 
reasonably low impedance connection.
Or a wire? Connected to your ground bar inside and run through a hole in 
the bulkhead to a lug on your outside tie point?
> 
> The bulkhead DOES have a screw with insulators which goes thru the 
> bulkhead much like the two screws for the ladder line.  but it is for 
> a single long line.  could I use that?

Sure.. why not?


> 
> help???
> 
> thanks
> chas k5dam
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