Firming up the Framing Why and How to Stiffen the Telecommunications Cabling Infrastructure

摘要

The intent of this paper is to show the need for adequate bracing and framing of cable trays in the central office environment, particularly cable racks bearing power cables. Power busbars and cabling are the densest conductors in the telecommunications environment and everything in the central office is dependant on power. The paper will cover seismic influences on the cabling structures and the ways in which seismic displacement could result in systemic failure of the telecommunications systems if the mechanical support for the cabling or buswork is inadequate. The paper also will cover bracing and framing for busbars and busduct systems with text and illustrations. The paper will cover bracing and framing for cable trays including primary and secondary power cables, communication cables (often called "Switchboard cable) and optical fiber cable trays. Most telecommunications huts and CEVs are equipped with adequate cableways and so this paper is intended to speak mainly to central office cabling whether for wireline or wireless use and for data centers with dc powered systems. Finally, the paper will socialize an idea in the telecom community. To date, no cable manufacturer makes a plenum-rated dc power cable. Telcos with raised floors would gain more robust systems if the power cables were run beneath that floor. Unfortunately, the way the National Fire Protection Association (NFPA) has written the codes, only plenum rated cables may be run beneath the floor if that floor also is used as a cooling air plenum. The requirement is because toxic gasses from burning cabling insulation being pushed into the working space occupied by employees were a fire to ignite beneath the floor. However, telcos now have a twenty year history of successfully using Very Early Smoke Detection Alarm (VESDA) systems. Many of these systems are designed to sample room air through PVC pipes placed under partial vacuum and then route that sampled air past a particle counter. The substance perceived as smoke is a cloud of soot and other particles formed during combustion. By counting these particles, the VESDA system is able to detect overheating equipment or cabling in the incipient stage of a fire, long before visible smoke is present. Most VESDA systems are capable of several levels of alarm state depending on the particle-count (smoke density) of the sampled air. By using one alarm level to automatically shutdown the fans forcing air into the plenum, smoke that might be generated by the non-plenum rated dc power cable for the most part would remain in the plenum while the dc power was disconnected. The advantage of using this arrangement is that the central office gains the seismic reliability associated with under-floor cabling while using a known reliable smoke detection system to maintain a safe environment by shutting down the pressurization airflow if fire broke out. This author intends to approach the NFPA with this concept for the next code cycle and wishes to see the telecom industry's reaction to the idea.