----- Original Message -----
From: "William Sheh" <wsheh@atitec.com>
To: <jimlux@earthlink.net>; <towertalk@contesting.com>
Sent: Friday, April 30, 2004 6:28 PM
Subject: Re: [TowerTalk] Ground system
> I've been reading with interest what fellow members have been posting
> regarding the depth and orientation of ground rods.
>
> One must keep in mind that there are many different types of grounding
> systems and that an effective grounding system consists of more than just
> burying ground rods in the soil. The key factor to keep in mind is what
one
> is trying to protect against. Is it direct lightning strikes, induced
> transient voltages/currents, side flashes or signal noise etc? Each type
of
> problem has its specific characteristics and strategies.
I fully agree.
Separate the RF grounding problem from the electrical safety grounding
problem from the lightning dissipation grounding problem, and separate
direct strike on your equipment from a) transients conducted from outside
(i.e. coming from the power line) and b) potentials induced by a nearby
strike.
>
> A horizontal conductor at a shallow depth (i.e. less than 3 feet) is
> perfectly allowed under the NEC and may be sufficient for electrical
safety
> of normal household circuits, but it is definitely not suitable for a
direct
> lightning strike. Under high current loads (a few KA) a horizontal rod can
> easily "jump" out of just a few feet of shallow soil coverage. A vertical
> rod is better restrained in the soil. Current magnitude under fault
dictates
> how much electro-mechanical energy is created. This is often an overlooked
> aspect in designing a grounding system.
I believe that the IEEE recommendation is at variance with your opinion.
They are specifically talking about grounding for lightning in the pages I
was citing. Indeed, the electromagnetic forces would need to be addressed,
and are, in things like substation bus bars (for that matter the forces from
just the normal kA load currents have to be dealt with). However, in the bus
bar case, the magnetic fields are much stronger than the earth's field, and
they're continuously vibrating (leading to fatigue failures, even with small
amplitude, on materials such as Aluminum).
On the other hand, one would think that in the extensive literature cited in
and forming the basis for the IEEE recommendation, such an event as rods
jumping out of the ground would have been mentioned as significant.
They do address the problem referred to as "smoking rods": boiling the water
in the soil due to the fault current.
As a practical matter, in soil of even poor conductivity, the current comes
out of the rod and into the soil pretty quickly, both from induction and
conduction.
Consider also... using the standard formula F=B*I*L (B=magnetic field
(Tesla, I= Current (Amps), L=Length(Meters), F=Force (Newtons)) Assume you
have a current of 10kA. In a field of 1 Gauss (1e-4 Tesla) you'll have a
force of 1 Newton per meter. The Earth's magnetic field is about 1/2 Gauss.
4.5N=1 lbf. That's only a few ounces. Hardly enough to make the wire jump
out of the ground, although sufficient to make a conductor hanging in air
move around.
> >
> Here are some references for those who are interested.
> IEEE Std 80-2000, Substation Grounding
> IEEE Std 142-1991, Grounding of commercial power systems
> IEEE Std 1100-1991, Grounding of Sensitive Electronics
> Mil Hnbk. 419-A, Grounding Protection
> NFPA 780, Lightning Protection
> UL Std 96A, Lightning Protection
> UL Std 1449, Transient Voltage Surge Suppression
>
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