Roger,
Thank you very much for sharing your very scary experience with the list.
It's been my observation over many years that all too many hams either do
not practice good grounding or even if they do know what to do and how to
do it, their particular system may not be as it should be. I say the
latter because your situation appeared to be one in which parts of the
ground (or, more correctly, "grounding system") had developed high
resistance current paths. These can happen over time, or even with new
systems, if soil conductivity is poor.
I just want to add 3 comments to what you and Jim Brown, K9YC, have said in
this thread:
1. Totally correct about NO current should flow through the GREEN safety
wire of connected devices. Thankfully, that is now true for everything I
can think of, but up until 10 or 15 years ago, certain appliances were
allowed to operate with current on the green wire. The devices that come
most to mind are electric clothes dryers. For decades , those dryers used
240 VAC power for the heating elements and maybe the motors, but the timers
often ran on 120 VAC. The dryers had only a 3 wire power cord: 2 hots and
1 ground, NO neutral (as defined in the US National Electrical Code). So,
the timer motor (and sometimes the motor that turned the dryer drum) were
connected between 1 hot and chassis ground. Be very careful if you have an
electric dryer with only a 3 wire cord in your home.
2. Because of the very likely possibility of severe shock, as well as for
lightning protection, any grounding system should be checked to verify that
it is a low resistance system (for safety) between the equipment and your
power entrance ground. (After all, if the power line is the source of
current, then fault current MUST be able to return flow to the source.)
However, all power must be disconnected/turned off prior to attempting to
check safety ground resistance. Use of a standard V-O-M is not acceptable
for checking such grounding systems - special test instruments are made for
that purpose and they impose some real current in order to make certain
that the grounding conductors and their connections can handle current.
3. A grounding system for radios should also be designed to be low
impedance if it is going to be useful for lightning protection. See
MIL-STD-419A (available on-line) for the design of facility grounding
schemes, as well as a wealth of tech info on grounding issues. Pay
attention to the use of perimeter ground systems (the practice of using a
ring ground around a tower base or around a building foundation). I use
both concepts in my installation and am glad I have done so - 2 direct hits
so far, no lost equipment inside the house. (Lots blown on the tower,
though!) My arm was connected to the mic cord of a radio during the first
hit, and I'm still here because the ground system did its thing.
Roger, in my many years in the EMC biz, I have spoken with many commercial
and industrial customers whose grounding systems became suspect over time.
That often left these folks with equipment that had AC line power applied
without the benefit of adequate safety grounds. Thanks again for bringing
this important topic forward.
73, Dale
WA9ENA
Sr EMC Engineer
> [Original Message]
> From: K8RI (Roger) <K8RI@rogerhalstead.com>
> To: <rfi@contesting.com>
> Date: 1/26/2011 6:09:18
> Subject: Re: [RFI] CHECK your grounds.
>
> 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
> > _______________________________________________
> > RFI mailing list
> > RFI@contesting.com
> > http://lists.contesting.com/mailman/listinfo/rfi
>
>
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