Nathan tells us: We are a Wireless Internet Service provider, WISP. We have about 118 towers. We have Cat 5 running up the towers to radios at the top. We are currently looking for the best way to earth the Cat 5 cables. We think we are creating earthing loop. We are earthing the Shielded Cat 5 cable to the tower in multiple spots and in the building. This year has been very bad for equipment lose so we are evaluating how we are earthing things. Based on the research I have done, there does not seem to be a standard or 1 way to earth our equipment. We are looking for some guidance.
Thank you for your question regarding the earthing and bonding of Cat 5 cables. It is out pleasure to help.
When dealing with data lines, including Cat 5, you really have no choice but to set up some form of a earth loop as you must bond both ends of the cable to the equipment it is being attached too. The real trick is in the equipment earthing (or chassis earthing) system. This is the copper wire system that is connected to your earth bars and to the buried earth loop outside your structure. Often at cell sites we see the equipment earthing systems comprised of tinned #2 solid conductor (bare).
Your cell sites equipment earthing system must be able to achieve a number of engineering goals in order to effectively earth your system. These factors include the famous resistance-to-earth; but possibly more importantly for data lines, the Ground Potential Difference (GPD). When one end of your Cat 5 data line is tied to the tower, and the other side is tied to some gear in the equipment shelter some distance away, there can be a significant difference in potential between the two points. As the shield wire in that cable is quite thin, it will have a relatively high resistance for DC currents, but an even higher impedance for AC currents (transients, harmonics, etc.). This high resistance/impedance of the shield wire will allow significant voltages to form thereby damaging your sensitive electronic equipment. This is called a voltage differential of a difference in potential. It is the goal of your chassis/equipment earthing system to eliminate this difference in potential and provide an alternate conductor path (to earth and not through your equipment) for the unwanted stray currents to flow.
Often the problem for many cell sites is that the chassis earthing system itself has a significant voltage differential. You can actually measure this quite easily by attaching one end of a standard volt meter to the earth bar at the top of your tower, and the other end to the earth bar in your equipment shelter. Our experience tells us that you should read something less than 0.1-ohms if you have a good system. A reading over 0.5-ohms you should consider bad.
When designing an earthing system, it is not just the resistance-to-earth that a good engineer should be worried about. In your case where Cat 5 data lines are a concern, the Earth Potential Difference (EPD) is paramount. It would be very expensive to simply start stringing earth wires around. We would recommend that you get a good computer model developed. It will tell you what size and how many conductors you need in order to get the Earth Potential Difference (EPD) of your site to acceptable levels. Of course, lowering the resistance-to-earth of your site and providing a clean (noise free) and testable electrode will make things even better. Again a computer model is the way to go for this as well.
As far as standards go, we recommend you take a look at the Motorola R56 Standard Guidelines for Communications Sites. This excellent manual has two sections dedicated to earthing (one for above-grade and one for below-grade earthing) complete with illustrations, instructions, and test procedures. It is kind of hard to locate this manual on the web, so if you are trying to find it, you can order the book (ID# 6881089E50 $85) or the CD version (ID# 9880384V83 $64) at 800-422-4210.
You may also want to check out ANSI/EIA/TIA-J-STD-607-A for additional information regarding server room earthing. While this standard does not relate directly to cell sites, many of the earthing principles do apply.
E&S Earthing Solutions is of course more than happy to help you with the testing of your cell sites and developing new earthing designs / work plans to achieve your goals. But if not us, please get someone with the proper measurement tools, engineering software and experience to get the job done right. Please feel free to call one of our engineers directly and they will be glad to speak with you about your project free of charge.
The Engineering Team at E&S Earthing Solutions