[TowerTalk] Ground system

[William Sheh]

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.

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.

If the issue is high frequency noise, then you would need to consider the
overall impedance of ground path to earth. Maybe even take into
consideration the "skin effect" of high frequency wave propagation in
different shape conductors.

Often times problems are created why one haphazardly adds grounding
electrodes and wires to an existing system, creating hazardous ground loops.
Proper bonding techniques are extremely important.

As is the case with any project, use the right tool for the right job. Also
use the right materials for the job.

One must also consider the prevailing environmental conditions. Is the area
subject to seasonal freezing, drying, flooding? Are you near any high
voltage/current sources? Does your equipment normally create any leakage
current? 

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


Will Sheh
Www.atitec.com




> Date: Fri, 30 Apr 2004 14:56:08 -0700
> From: Jim Lux <jimlux at earthlink.net>
> Subject: Re: [TowerTalk] Ground system
> To: K8fu at aol.com, towertalk at contesting.com
> Message-ID: <5.2.0.9.2.20040430122312.00ac1150 at mail.earthlink.net>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> 
> At 01:43 PM 4/30/2004 -0400, K8fu at aol.com wrote:
>> Well the unanimous opinion was that horizontal installation of ground rod is
>> right up there with trying to get pet rocks to generate offspring.
> 
> 
> Except that the regulatory agencies, who do actually analyze this stuff,
> think that horizontal ground rods are just fine.
> 
> For instance, page 131 of IEEE Std-142(1991) "IEEE Recommended Practice for
> Grounding of Industrial and Commercial Power Systems" has a table showing
> ground resistances for a variety of configurations, including "buried
> horizontal wire". Later, on pages 133-135, it discusses the advantanges of
> horizontal cable grounds, particularly where the soil is hard and/or of low
> conductivity. Mention is made that several short horizontal conductors may
> be better than a single deep rod because the inductance is lower.
> 
> NBS (now NIST) did tests which indicated that the resistance decreased only
> 5% when changing burial depth of horizontal rods and strips from 18 inches
> to 36 inches.
> 
> There are some recommendations as to depth of burial in places where it
> freezes.
> 
> Consider that  UFER ground ("Concrete Encased Electrode) is considered
> fine, and has almost no vertical penetration.