Q: At a Veterans Administration Hospital in Helena, MT, we ran a 24-AWG, 1800-pair copper backbone shielded cable. Should both ends on the cable be bonded to ground? This riser cable is installed in a 4-inch steel conduit from closet to closet, and the conduit is grounded at both ends. We grounded only one end of the cable shield.
Edward A. Hartman
DBA Phone Factory
Helena, MT
A: ANSI/TIA/EIA-607, Section 6.3.2, says, "In buildings where the backbone cable incorporates a shield or metallic member, this shield or metallic member shall be bonded to the tmgb/tgb [telecommunications main grounding busbar/telecommunications grounding busbar]."
In an intrabuilding backbone cable with an overall cable shield, the shield acts as the coupled bonding conductor (CBC). The CBC, like a telecommunications bonding backbone, not only provides equalization of potential but also, because of the close proximity to the conductors, provides protection through electromagnetic coupling with the telecommunications cable. To work properly, the CBC must be directly connected to the ground at the protector end (the TMGB) and to the ground at the equipment end (the TGB). Many equipment manufacturers require a CBC between their equipment and exposed circuit protectors.
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: ballast@utexas.edu.
