Outside plant: Grounding for safety

A look at safety issues behind the TIA/EIA 758 and 607 standards.

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A look at safety issues behind the TIA/EIA 758 and 607 standards.

Aznixter Inc.'s (www.anixter.com) Technical Support Team, a part of Anixter's Levels Lab, probably fields more inquiries concerning customer-owned outside plant (CO-OSP) than any other category of interest. Grounding for safety is essential to CO-OSP applications. While the Customer-Owned Outside Plant Telecommunications Cabling Standard (TIA/EIA-758) of April 1999, addresses some of the essential details, it cannot provide a complete blueprint. The BICSI Telecommunication Distribution Methods Manual Chapters 8 to 12 and Customer-Owned Outside Plant Design Manual provide additional useful information. These documents are recommended as useful sources. Also recommended is TIA/EIA-607 of 1994, previously discussed in a recent issue of Cabling Installation & Maintenance (see "Taking the mystery out of grounding and bonding," July 2001, page 41).

This brief article is summarized from a heavily illustrated 22,000-word Anixter OSP training manual. The drawing of a CO-OSP (see page 32) is designed to assist customers, contractors, and sales staff in the identification of common elements required for aerial and buried CO-OSP routes.

The emphasis on grounding and bonding of all metallic elements is an absolute requirement applicable to all outside plant equipment. Lightning strikes are a frequent occurrence. A lightning bolt has the potential to be more than 10 million V discharged at an average 60,000 A and lasting about 0.1 milliseconds. Less frequent, but just as deadly, are power line breaks and transformer malfunction, both of which can impart high voltages to outside plant cables and equipment, due to their close proximity.

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This drawing is an overview of outside plant routes, materials, and equipment, as well as grounding considerations. The drawing is detailed by Nick Logisz, an electrical engineer and network administrator who provides CAD-technical drawing support to the Levels Lab.
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The campus CO-OSP drawing illustrates aerial and buried routing of 100-pair voice cable. For aerial routes, a stranded wire messenger supports the cable. Figure-8 cable, which includes a messenger molded onto the jacket, is usually the simplest solution, as no lashing is required. The messenger is strung between two anchor points, which could be a tangent support clamp on the pole and a wall strap or "thimbleye" bolt at the wall. A strand vise is installed to serve as a specialized turnbuckle. A ground clamp is installed on the steel messenger, and a #6 ground wire runs down to a buried ground rod or to the grounding and bonding busbar system.

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Any shielding or armor in the cable construction must similarly be grounded with a shield-bond connector and #6 ground wire that runs to the busbar network. Within the aerial splice closure, shield-bond connectors and bonding braid are used as a bridge to provide continuity across the splice.

Where cables enter the building above ground, a U-shaped drip loop is formed to provide water run-off. From the exterior, the cable enters through an upward angled hole, which should be sealed. As soon as possible and no more than 50 feet from building penetration, the flammable CO-OSP cable is cut, stripped back, cleaned of any gel-fill, and spliced to the building entrance protection (BEP) stub within a riser-rated splice closure (shown on the left side of drawing) or terminated directly to the BEP blocks (shown on the right side).

Like other metallic elements, the conductor pairs can carry high voltage into the building and onto the phone or data network. Building entrance protection shunts higher than normal voltages (more than 300 V) to ground via a protector module (one per pair) and busbar network. The BEP is designed to protect people. Equipment requires secondary protection modules that trip at lower, double-digit voltages.

For buried routes, the same grounding and bonding principles apply. Lightning can strike the earth or an exposed pedestal, or it can be carried onto buried cable from an aerial segment of the network. Every appearance of the cable above ground, such as the illustrated pedestal, requires a ground rod, clamp, ground wire, and protector modules. Shielding and armor elements of buried cable are continuity-bridged with bonding braid and shield-bond connectors within any buried splice cases. Since buried cable often enters the building below grade, the conduit tunnel through the foundation is sealed. An epoxy end dam kit can be used to seal the end of CO-OSP cable to prevent the leakage of gel-fill, a Vaseline-like material designed to provide water blocking.

Serving as an example

Because of the variety of uses for the products described in this publication, those responsible for the application and use of these products must be sure that all necessary steps have been taken to assure that each application and use meets all performance and safety requirements, including any applicable laws, regulations, codes, and standards.

The illustrations, charts, sample materials lists, and layouts shown here are intended solely for purposes of example, since there are many variables and requirements associated with any particular installation.


Frank Dickmanis an RCDD manager of technology for Anixter Inc. (www.anixter.com). He is the voting member for Anixter on behalf of customer interests at several standards development organizations.Ron Harrisis a senior technical support engineer covering technical support, OSP engineering, and product management for both copper and fiber OSP and central office products.

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