On 5/10/2013 1:17 PM, K8RI wrote:
When we look at today's rigs it is very common to see shields
connected to PC boards when they should go to a real ground/earth not
a circuit ground. Icom even uses the shield for the mik as a control
return that if grounded will lose functionality.it will actually pick
up enough AC in my den to key the vox.
Not quite -- you're absolutely right that the shield should NEVER go
inside the box, but the cable shield must to to the SHIELDING ENCLOSURE.
A connection to EARTH is NOT necessary, or even beneficial.
The late Neil Muncy, ex-W3WJE, who first taught us about "The Pin One
Problem," often lectured EMC fundamentals to audio professionals at
conventions. One of his favorite lines went like this:
"Grab a case of your favorite beverage, find a comfortable spot near the
end of the runway of any major airport, and PHONE ME COLLECT when you
see an aircraft take off trailing a ground wire."
You can kind of get away with pouring a bit of household
60hz into the ground, but don't count on it to work for a
primary on a power line either!
Actually, we CANNOT use the EARTH as a conductor for AC power either --
the resistance is far too high to carry enough current.
For the same reason, the use of SEPARATE grounds for RF or for audio
with no "green wire" between the equipment is VERY unsafe -- the reason
for the safety ground is to blow a fuse if something "breaks" and causes
the chassis to be "hot," exposing people to electrical shock. The REASON
for the green wire is to blow a fuse, and if there's no green wire, only
separate ground rods, there's not enough current through the earth to
blow the fuse and remove the shock hazard.
73, Jim K9YC
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