Q: I`ve seen a chart that listed sizes of grounding conductors to telecommunications systems by length. I cannot seem to locate this chart anymore. I`ve searched BICSI`s Telecommunications Distribution Methods Manual, TIA/EIA standards, and the National Electrical Code (NEC). None of these documents seems to deal with sizing of the conductor by length.
I`m planning to redo the grounding system of my main telecommunications equipment room and the campus entrance system at our administration building. This building will have five separate cable entrances and protector locations in the basement. The reason for the various locations is there is very little room in the equipment room for an outside-plant cabling upgrade. The protectors will be located close to the cable entrance, then inside-wire cable will be used to transfer the circuits from the protectors to the central crossconnect in the main equipment room. The building is 220 feet long, and the main building ground system is in one corner of the building. I need to be able to ground all these systems properly, so I need either a formula or chart to determine the proper grounding conductor and busbar sizes.
Robert S. Ely
New Jersey Dept. of Human Services
New Lisbon, NJ
A: For more than 100 years, electricians and electrical engineers have been directed to create and use low-resistance ground connections to earth for lightning protection and power-transmission fault control. This procedure is appropriate for lightning and transmission-line ground faults--since part of their path is through the earth--but not for 120-, 240, and 208-volt utilization circuits. At these voltages, a system of interconnected or bonded conductors acting as a voltage reference network can equalize voltage difference throughout the network much more effectively than multiple low-impedance earth contacts. Such an interconnected or bonded network can serve as both a power and signal reference regardless of its voltage with respect to earth ground. But connection to earth ground is required by the NEC to avoid shock hazards within a building structure.
The same day I received Robert`s question (day 1), I began digging into the recent meeting notes from Work Group TR-41.7.2 of the Telecommunications Industry Association and the Electronic Industries Alliance (TIA/EIA--Arlington, VA). We in the group are busy revising the TIA/EIA-607 standard and are discussing the "how to" and minimum sizing of the telecommunications bonding backbone (TBB). Just as I located the meeting notes, along came another e-mail from Robert (day 2) with an attached chart using NEC Table 8 and a maximum conductor resistance of 0.1 ohm as a basis for sizing the TBB.
Our worst enemy is noise. In gardening, a weed is anything growing where it should not be growing. Noise signals are the "weeds" of telecommunications transmission, with frequencies that can vary from direct current to gigahertz. Electronic systems are inherently noise-sensitive. Adequate grounding can control these noise levels and minimize interference.
But what constitutes "adequate?" There are numerous national codes, installation guidelines, recommended practices, and military standards available regarding the installation of power distribution systems, grounding systems, and environmental control systems. Unfortunately for us, these standards do not always agree on what procedure should be followed for the design and installation of the power and grounding systems. Although not everyone is calling it the same thing, sizing of the TBB is addressed in a number of reference sources. The following is a short list of those that immediately came to mind when I received Robert`s e-mail:
IEEE Std 241-1990 "Grey Book," page 601, requires at least 2kcm (2000 circular mils) per linear foot of conductor.
IEEE Std 142-1991 "Green Book," Section 5.9.5, gives an example of a 500kcm copper riser for a building.
ANSI/TIA/EIA-607, Section 184.108.40.206, requires a minimum of a No. 6 and in typical standards weasel words--"consideration should be given"--suggests a conductor as large as No. 3/0. The first draft of what will be published as TIA/EIA-607a suggests consideration be given to 2kcm per linear foot of conductor.
The abc of the Telephone, volume 13, chapter 5, page 56, indicates 2kcm per linear foot of conductor or a maximum of 0.005 ohm in any ground lead with the minimum size (TBB) conductor being a No. 4/0.
The BICSI Telecommunications Cabl-ing Installation Manual, Chapter 9--Troubleshooting, instructs you to "use a multimeter to test the connections for a maximum value of 0.1 ohm." Please note that this is for troubleshooting, not design.
The common thread in the design references is 2kcm per linear foot of conductor. So using NEC Table 8 for conductor size in circular mils and at least 2000 circular mils of cross-section area for every running foot of grounding conductor, I created the table above.
Why use a fixed cross-sectional area per linear foot? Because it is easy to calculate and it works. In one example given in The abc of the Telephone, a 16-foot length of No. 1 was replaced with a No. 2/0, reducing the noise by 26 decibels. When sizing grounding conductor, bigger is better.
As for the busbars, follow the tia/ eia-607 requirements. The required width and depth of the telecommunications main grounding busbar (TMGB) and telecommunications grounding busbar (TGB) specified are readily available in variable lengths from several manufacturers.
One problem with Robert`s approach was that 0.1 ohm maximum should be 0.005 ohm for design. Another problem is the state of New Jersey`s requirement that only a professional engineer (PE) design telecommunications infrastructure. I recommend that all the fieldwork and the table be sent to a PE for design. Grounding is, first and foremost, a life safety issue (see "Basics of grounding and bonding infrastructure,").
Donna Ballast is a communications analyst at the University of Texas at Austin and a bicsi registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, the University of Texas, Austin, TX 78713; tel: (512) 471-0112, fax: (512) 471-8883, e-mail: email@example.com.