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Bonding & grounding de-mystified

One of the most notable additions to the TIA 607-B standard compared to the previous edition is activity in computer rooms.


March 1, 2012  


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TIA has recently published a new ANSI/TIA-607-B-2011 “Bonding and Grounding (Earthing) Standard for Customer Premises.” This is a major revision of the standard and contains a lot of new material. For this month’s article, I wanted to provide a brief overview of its main components as well as the main differences compared to the previous edition of the standard.

The primary purpose of telecommunications bonding and grounding infrastructure is to create a low resistance (low impedance) path to ground and thereby to equalize ground potentials and to reduce the problems associated with ground potential differences. In order to understand the structure of a bonding and grounding system for telecommunications, it is first necessary to learn the terminology of the components that comprise the system, as follows:

TMGB — Telecommunications Main Grounding Busbar is connected to the building’s main ground, which is also referred to as a building grounding electrode system. The TMGB is the central attachment point for the TBB(s) and equipment.

TBB — Telecommunications Bonding Backbone is a conductor that originates at the TMGB and follows the backbone pathways to connect to the TGBs in various distributors (telecommunications rooms) throughout the building. The TBB conductor size depends on the length and shall be a minimum of 6 AWG for lengths up to 4 meters and should increase in size at 2 kcmil per linear foot of conductor all the way up to 750* kcmil for lengths greater than 91 meters. (*Note: This is new. The maximum conductor size for the TBB in the J-STD-607-A standard was a 3/0 conductor.)

TGB — Telecommunications Grounding Busbar is the grounding connection point for telecommunications systems and equipment in the area served by a distributor.

GE — Grounding Equalizer is used within a multistory building to interconnect multiple TBBs at the top floor and at a minimum of every third floor in between to the lowest floor level. As a minimum, the GE shall be the same size as the largest TBB.

BCT — Bonding Conductor for Telecommunications is used to bond the TMGB to the service equipment (power) ground. As a minimum, the BCT shall be the same size as the largest TBB.

Clause 6 of the TIA 607-B standard provides more detailed information on the materials and the construction of the TMGB and the TGB. The TIA 607-B standard now allows the use of copper alloys having a minimum of 95% conductivity in addition to copper or electrotin-plated copper for the TMGB and TGB.

Clause 7 of the TIA 607-B standard provides information on how to design a telecommunications bonding and grounding infrastructure for bonding of metallic pathways, cable shields, racks, enclosures and equipment in telecommunications rooms, equipment rooms, and entrance facilities.

The TIA 607-B standard also provides guidance in several Annexes on the design of Grounding Electrodes, Grounding Systems for Towers and Antennas and Electrical Protection of Telecommunications Circuits.

One of the most notable additions to the TIA 607-B standard compared to the previous edition is the bonding and grounding of equipment in computer rooms.

For example, the standard states that a computer room should also contain a supplementary bonding network, such as a mesh-BN, that is bonded (and thus becomes grounded) to the TGB or TMGB. A mesh-BN as the name implies consists of a bonding grid of conductors, either flat conductors or bare round wire, joined together via proper welding, brazing, listed compression connectors, or listed grounding clamps at each of the crossing points.

Metallic enclosures, including telecommunications cabinets and racks, shall be bonded to the mesh-BN, TGB, or TMGB using a minimum sized conductor of No. 6 AWG.

A new term “Telecommunications Equipment Bonding Conductor (TEBC)” is used for the conductor that connects the equipment racks and cabinets to the TMGB/TGB.

The standard also provides information on how to establish the connections to a TEBC and how to bond the equipment in a rack or enclosure.

In summary, the main difference in the new TIA 607-B Bonding & Grounding Standard is that it does not stop at the busbar (TGB), but extends all the way out to the equipment.

It provides design requirements on how to build and test a bonding and grounding infrastructure to ensure a reliable, low resistance path to ground.

The recommended maximum value for resistance between any point in the telecommunications bonding and grounding system and the building’s electrical grounding electrode system is 100 milliohms.

There is a lot more to bonding and grounding for telecommunications than meets the eye. There are a lot of nuances and procedures that are not well understood. The reader is encouraged to consult the TIA 607-B standard on how to properly design and implement a telecommunications bonding and grounding system for customer premises.

CNS

Paul Kish is Director, Systems and Standards at Belden.
The information presented is the author’s view and
is not official TIA correspondence.