Towertalk
[Top] [All Lists]

Re: [TowerTalk] Does prevailing grounding scheme promote large ground lo

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Does prevailing grounding scheme promote large ground loop?
From: jimlux <jimlux@earthlink.net>
Date: Mon, 25 Jul 2016 07:55:00 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 7/25/16 7:17 AM, David Robbins wrote:
"If you have one piece of gear with a 10 foot cable to the lightning
impulse. And another piece of gear on the bench connected with a 100
foot cable to the lightning impulse, and then you interconnect the two
with a short jumper, you can see that there might be a problem."

And THAT is the crux of the problem when bonding equipment for lightning
protection.  NOTE:  I said BONDING not grounding! Consider this...  the 10'
cable is connected from a rod/shack/power 'ground' to the radio and the 100'
cable is a coax that is not connected to the rod/shack/power 'ground' but
goes right from the antenna/tower to the radio.

And this starts to get at the whole thing about what works and what 
doesn't.
There are various devices that clamp the voltage differential (e.g. 
polyphasers and other transient suppression devices), so that sort of 
takes care of the "inner conductor to shield" voltage problem. And then, 
if all the shields are bonded together at the point where all those TVSS 
devices are, you've got a common voltage reference point.
But the farther downstream you get from that point, the more risk there 
is of the "thing being protected" seeing something radically different 
than expected.  For instance, you might have a entrance panel for your 
coax. Run the coax from that panel 50 ft to your shack, and then you 
plug your rig's power supply into the wall socket in the shack.  Oops.. 
there's a long wire between the greenwire/chassis and your nice coax 
entrance panel.
This kind of thing is really hard to avoid: maybe you've been 
conscientious about the rig: running its chassis ground back to the coax 
entry panel (perhaps via the coax shield..that's a decent way), but the 
PC next to it is plugged into the wall, and the chassis ground of the PC 
is common to the "ground" pin of the serial port going to your remote 
control interface.
Wired ethernet is nice: it has galvanic isolation (although typically 
only 5kV breakdown in the coupling transformer). Wireless ethernet is 
even nicer: no galvanic path at all.
Wall warts typically have no third wire pin, but that doesn't mean 
they're well isolated. They often have substantial capacitance between 
the output and the AC power pins. They *do* usually pass some sort of 
HiPot test at the factory, so at least there's no easy path.
I've always thought that ham rigs should make more use of the TOSlink 
type optical interconnect. These are getting more rare on 
stereo/audio/video equipment since HDMI came out, but you can't beat a 
meter of plastic for getting good isolation.

Moral of the story, bring your AC power distribution next to where you bring your coax in, and bond all those shields and "grounds" together at that point. That's also where you put the TVSS clamps on the AC line/neutral conductors.


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>