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Tom Horne[_4_] · January 7th 08, 04:43 AM posted to rec.radio.amateur.moderated

wrote:
On Nov 27, 8:46 pm, Dave Heil wrote:
James Barrett wrote:
Does it really matter if my equipment is ground to the house ground or
should it really be connected to its own external ground? And if/when
I ground to guard against lightning strikes, would I use that same
external ground or go with yet another external ground?
I'm still doing my research, but figured it couldn't hurt to ask.
Jim,

You'll see numerous references on the web to "single point ground." It
is very, very important stuff. I got nailed on this stuff back in
August, 2003. Here's how it played out:

We had a violent thunderstorm early one morning about 7 A.M. Suddenly
there were three near strikes (simultaneous lightning and thunder)
within a ten second time frame. In addition to telephones, computers
and home entertainment equipment, my four month old Ten-Tec Orion and
numberous shack accessories were zapped. My house ground is a single 8
foot copper plated ground rod. The lightning ground for the shack was
an identical ground rod driven about thirty feet away from the house ground.

When the strikes conducted a surge into my home via the power lines, all
three wires had very high voltage on them. The shack ground was still
at its usual potential (0). The house ground was elevated to high
voltage. There was a potential difference between the shack ground the
the house ground. That potential quickly equalized inside my equipment.

Ten-Tec reported that six boards inside the Orion had their ground
traces evaporated.

For a shack ground, you want the shortest possible distance between your
rig and earth. The house ground will be as close as possible to your
electrical service entrance. If, like me, you have to use two ground
rods, you need to bond the two of them together with a big, fat wire so
that the two can never be at different potentials.
If you have a tower, I'll assume that you have at least one 8 foot
ground rod driven at its base. Bond your coaxial cable sheaths to the
rod at the tower. The sheaths should also be bonded to your shack ground.

Dave Heil K8MN

This whole topic area seems to be eternally confused and confusing and
I'm in the parade of the confused. On a common sense basis I
absolutely agree with your connecting the station grounds to the
'lectric service entrance ground for the reasons you've stated.

But somewhere along the line somebody in the ham groups stated that
the National Electrical Code states that there shall be one and *only
one* grounding point per power drop and the neighborhood code cops and
the insurance companies reportedly get stiff about it.

So is it legal to connect a phalanx of ham station ground rods to the
service entrance ground?? Or not.

w3rv

What the code enforcement types get vexed about is were you have two
different Grounding Electrode Systems (GES) that are not bonded together
into a single system. So if you have one concrete encased electrode in
your tower base, another concrete encased electrode in your homes
footer, and a driven rod array electrode just outside the radio room
wall and you do not bond them together into a single GES that is a code
violation. It is also a code violation to connect any conductor that
carries power and light current to ground at any point in the wiring
system that is not at the Service Disconnecting Means (SDM). The SDM is
the first switch, breaker, or fused pull out that can be used to
disconnect the ungrounded current carrying conductors of the wiring
system from the service entry conductors. In most homes this is the
main breaker or the main fuse pull out. The code name for the white
wire in North American wiring practice is the "Grounded Current Carrying
Conductor." In many cases it is also the neutral conductor but that is
not always the case. It is that white wire that most of us call the
neutral; albeit sometimes mistakenly; that must be connected to the GES
at one and only one point. That point is the portion of the service
entrance grounded conductor that is between the splices that connect it
to the utility company's conductors and the SDM.

A practice that was very popular in the computer industry was to ground
the computer equipment power supplies of main frame computers to a new
and separate grounding electrode at the computers location. Connecting
that ground back to the power service ground was carefully avoided on
the belief that doing so would cause ground loops on the signal circuits
between the main frame and the associated terminal equipment. Some self
qualifying authorities recommended the same approach for radio
equipment. That left the earth between the two electrode systems as the
only return path for current faulting to ground at the improperly
isolated ground equipment. With the resistance to earth of most driven
rod electrodes being over fifty ohms the series resistance of such a
fault current pathway was often over a hundred ohms. Such a high
impedance would limit the fault current flowing back to the utility
transformer from whence it came to a flow too small to trip the Over
Current Protective Device (OCPD) protecting the faulted circuit. You
then have a situation in which all of the exposed conductive parts of
the radio or computer equipment is energized at 120 volts relative to
earth or any body that will behave as the earth does electrically.
Since it only takes about a third of an ampere to kill a human being the
user could be electrocuted without ever tripping the OCPD. Because the
danger to life was of a greater concern to code authorities then the
difficulties caused by ground loops on signal lines the electronic
engineers lost that war with the electrical engineers and the rule
forbidding separate grounding electrode systems survived the attempts to
remove it from the nations electrical codes.

The good news for us as amateur radio practitioners is that what is safe
is also good operating practice. By bonding all of our grounding
electrode systems together into a single system we get a better ground
for most purposes and we greatly decrease the likelihood of equipment
damage or operator injury / death.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison