On 1/26/2011 5:49 PM, Jim Brown wrote:
> On 1/26/2011 12:29 PM, KD7JYK DM09 wrote:
>> I have taken photos of our electrical "grounds".
> You need to study the fundamental principles of AC power distribution. A
> GROUND conductor does NOT carry load current -- it's sole function is
> SAFETY. The current carrying conductors are the Phase and Neutral
> conductors. A 240V single phase system has two Phase conductors and a
> Neutral conductor, fed from a center tapped transformer. The two Phase
> conductors are connected to the two ends of the transformer, and the
> neutral is connected to the center tap. There is NO ground conductor
> between the power company and your home. Rather, the Neutral conductor
> is grounded FOR LIGHTNING SAFETY both at the transformer and at the
> point where it enters the building. A separate Equipment Ground
> conductor (the Green wire) is carried with the phase and neutral
> conductors to every outlet, and to every piece of equipment wired
> directly into the power system. The purpose of that Green wire is
> SAFETY. It's function is to cause a fuse (or breaker) to blow if
> something goes wrong.
I'd add that the green wire is needed for ground fault breakers and
outlets to work.
Also it causes the fuse or breaker (GFI) to go if there is a short (or
leakage current to ground), but over current (as in a shorted
transformer or tube) is likely to trip the breaker without ever making
use of ground.
As you say the green wire is for safety and should never take part in
carrying power for the actual operation of the equipment.
I once had a vertical in my West yard about 50 feet from the West end
of the house. (when I lived South of Breckenridge MI). The "shack" was
in the basement on the South East end of the Basement. The equipment was
grounded directly behind the rig about 40 to 50 feet East of the
service ground. The rig at the time was a Yaesu FT101B and IIRC only
used a 2 wire power cord. There was an overly generous solder
connection on the power socket (from the factory) at the back of the
rig. Moving the power cable would cause the pins to more around enough
for that solder to contact the chassis of the FT101B with no apparent
effects in the basement, or to the operator. There was also an 8'
ground rod under the 33' 40 meter vertical with the coax braid and
radials tied to it. That meant when the 120VAC went to chassis ground
it was divided between the 8' ground rod just outside the basement wall
and the one under the vertical with the service ground being *about*
half way between them. No, most of us didn't even know what a single
point ground was back then.
It was well into spring, the snow had melted and the yards were wet, but
no standing water. A number of the radials for the vertical had come
loose and coiled up at the base of the antenna. I took each radial,
pulled it straight and stuck about 6" of it into the ground. The ends
of the radials had a 90 degree bend and they'd lay nice and straight.
It was that last radial... I pulled it straight, but my back was getting
tired so I knelt down to stick the wire into the ground. As soon as my
knees hit that wet ground it had me. In my hands and out my knees. I
couldn't let go. Fortunately being balanced on the balls of my feet, I
fell over backwards. As soon as my knees broke contact I threw that wire.
The point being that even though the rig was grounded through two 8'
ground rods about 50 feet either side of the service ground, there
wasn't enough current to trip the 20A breaker. There was however enough
voltage present that it could paralyze a person and that HURTS! Boy does
that hurt and you can't make a sound. Had I not fallen or fallen
forward instead of backwards it's likely I'd not be here today. Had
there been a single point ground, or the station ground tied into the
house ground it would have popped the breaker as soon as the line to
chassis occurred.
> The EARTH is a lousy conductor, and should NEVER carry current in a
> power system. A connection to earth is almost never part of a solution
> to an RF or noise problem.
I'm a believer!
73
Roger (K8RI)
> There is a tutorial discussion of power systems on my website.
>
> http://audiosystemsgroup.com/publish.htm
>
> 73, Jim Brown K9YC
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