Grounding Shielded Cable

Q: We are installing an Ethernet system in a hospital using the following Category 5 products: 4-pair braided shielded cable, shielded RJ-45 wall jacks, shielded RJ-45 patch panels and patch cords (Eia/tia-568 standard specification). Although it is a 10Base-T Ethernet system now, it could grow to be 100Base-T in the future. Do we need to ground both ends of the shielded twisted-pair cable?

Q: We are installing an Ethernet system in a hospital using the following Category 5 products: 4-pair braided shielded cable, shielded RJ-45 wall jacks, shielded RJ-45 patch panels and patch cords (Eia/tia-568 standard specification). Although it is a 10Base-T Ethernet system now, it could grow to be 100Base-T in the future. Do we need to ground both ends of the shielded twisted-pair cable?

Our cable and RJ-45 suppliers say we need to ground only at the patch panel, and not at the wall outlet. Also, a senior engineer at the company that supplied my Category 5 wire tester agrees that we do not need to ground at both ends--even though the wire tester measures shield continuity.

However, the European directive Iso/iec 11801 states that the shield must be continuous. In addition, recent articles by the chairman of the Telecommunications Industry Association (TIA) committee project PN3193 suggest that the cable be grounded at both ends.

What standard should we use? We plan to ground the shield at the patch panel, but do we also need to ground it at the wall outlet?

John Ginty

Siemens

Atlanta, GA

A: The nice thing about free "expert" advice is that there are so many versions from which to choose. So, for what it`s worth, here`s mine.

A generic cabling system is made of basic links, not channels. You are installing a generic, 100-ohm screened twisted-pair cabling system--a lot of basic links; therefore, you should ground the shield of the cable in the telecommunications closet (TC). Typically, the cable shield is grounded through the connector to the patch panel. Then the panel is grounded to the rack, which is grounded to the telecommunications grounding busbar in the TC. Do not ground the cable shield at the work-area outlet.

To install a 10Base-T connection in a specific work area in a 100-ohm screened twisted-pair cabling system, the technician will need a hub and port assignment, a shielded twisted-pair equipment cable, a shielded twisted-pair basic link, a shielded twisted-pair work-area cable, and a network interface controller (NIC) in the work-area computer, with a shielded connector that is grounded to the equipment chassis and chassis ground. This meets the two-point grounding that Ned Sigmon of AMP Inc. (Harrisburg, PA) and chair of TIA PN3193 referred to in the April 1996 issue of Cabling Installation & Maintenance (see "Screened 100-ohm cabling gains acceptance in high-speed networks," page 37). If the cable shield at the work-area outlet were also grounded, you would have three points of grounding.

Iso/iec 11801, section 10.1, states that "shielding has to be continuous for the complete channel." This means that the equipment cable from the hub to the patch panel, the connector on the patch panel, the cable terminated on the patch panel and on the connector in the work area, the work-area cable from the connector to the NIC in the computer, and the connector on the NIC must all be shielded.

Iso/iec 11801, section 10.2, states that "all shields of the cables should be bonded at each TC. Normally, the shields are bonded to the equipment racks, which, in turn, are bonded to building ground." No mention is made of the connector in the work area.

Note also that for your field tester to perform a shield-continuity test, the tester must have a shielded connector and shielded test cords.

(See also "Ask Donna" June 1996, page 57, regarding intermateablity between manufacturer products. Until the TIA Telecommunications Systems Bulletin [(TSB)] is published and manufacturers begin shipping compliant products, you have little assurance that a jack from manufacturer A and a plug from manufacturer B will provide shield continuity.)

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