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.
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