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Single Point Ground

Jack,

A true single point ground system means just that, everything is referenced to a single point and that is were everything within your shelter is tied to the external earth ground system. The external ground system could include the utility ground, a facility ring ground, tower ground and your antenna and radial ground systems. The single point is truly a single point, typically a single bus bar or equal that is typically 4 or 6 inches by 24 inches. All system grounds terminate to this single bus bar connecting point. In reality each component and external source is effectively bonded to a single point, which is then effectively bonded to the facility or site external ground system.

The reference paragraph below explains that between any two points on a conductor, or lets say ground system, you may have significant potential differences as current resulting from lighting activity is introduced into this conductor or the conductors of the system. With DC current we merely consider the true circuit resistance. With AC and RF we must consider the impedance of the circuit and that is where the 600 kV of the example below comes into play.

By having all equipment, including antenna transmission lines, power, telephone, cable TV, etc, all entering the facility at nearly the same point and all being bonded to a single common ground point, little current will flow between these systems and therefore there will be essentially no difference of potential between the components of the system (everything within the shelter) during lightning activity. The environment is safer for personnel and the equipment has a much higher probability of surviving significant lightning activity. We must also consider that there may be inductively coupled energy from a nearby lightning strike that would be coupled into the facility structure or circuit conductors within the facility. This however is not part of and is not directly related to the single point ground. You must also consider suitable surge suppressors for ungrounded circuit conductors entering/exiting the facility.

On the other hand if you have antenna transmission lines entering the facility at one point and power, telephone, cable TV, etc circuit conductors entering the facility at different points you will have current flowing through the connected equipment as energy attempts to equalize during lightning activity. This is true even if all of these systems and the equipment are connected to the site ground system you describe. The problem is they would be connected to the system at different points and as the example below points out the voltage between these different points may be many kV during lightning activity. Consider that your internal equipment is also a part of the circuit path. The transmission line from one direction and the power etc. circuits from another direction. This constitutes a series-parallel circuit between opposite ends of the shelter or different parts of the system. You have the interior ground conductors, exterior ground conductors in addition to the conductors interconnecting the equipment making up the series-parallel circuit. This could result in personal injury, equipment damage and system failure.

Lets assume that the power enters one end of the facility and the antenna transmission lines enter the opposite end of the facility. Each is bonded effectively to an external (site) ground system that is complete around the facility and includes the utility grounds and the antenna system. At DC the circuit resistance is very low, say in the milliohms, but at AC or RF frequencies the impedance may be several hundred or even several thousand ohms. The voltage developed across the several hundred or several thousand ohms will be significant during lightning activity.

Consider RF transmission line theory. A quarter wave length or odd multiple of a quarter wavelength of transmission line at its resonate frequency will appear "open" at the source end even if the opposite (load) end of the transmission line is effectively "short" circuited. Consider that lightning activity covers a broad segment of the spectrum. Yes, there is a DC component but the actual spectrum is quite broad and it may be "tuned" by antenna systems or parameters contained in the circuit and associated components. Therefore, you may have a very solid conductor between two points but because of the RF properties of lightning it may not be an effective bonding conductor. Thousands of volts may develop across its length during lighting activity.

You may desire to review IEEE Standard 1100-1999 (Emerald Book of the IEEE Color Book Series), which is an IEEE Recommended Practice for Powering and Grounding Electronic Equipment. I believe you will find detail relevant to your question and this discussion in that document. As you will learn there are many factors to site, power and ground system design that go far beyond the scope of an Email message. Just remember that safety of personnel is of utmost importance. Equipment is secondary unless this is a Public Safety system and safety of personnel is dependent on system performance.

I hope I have answered your question and furnished some food for thought.

Good Luck and let me know if I may be of further assistance.

Bruce Carpenter Consulting, LLC
http://www.bruce-carpenter-consulting.com/

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Grounding and Bonding Book — 2002

 

Grounding and Bonding textbook is all new and printed in full color. Loaded with detailed, color coded graphics, this text gets to the root of all problems associated with grounding and bonding.

Product Code: 02NCT2
Pages: 94
Illustrations: 206

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