Interconnecting the various IT resources in a data center requires large amounts of cabling (whether fiber, copper or a combination of the two). Perhaps you’ve seen the mess of wires and cables that can quickly build behind your desk, especially if you have a number of connected gadgets; the situation in the data center can be much worse if not carefully controlled. Finding a good place for cabling that permits good airflow, accessibility for maintenance and expansion, and safety (for personnel and equipment) is critical
Moving Away From the Under-Floor Plenum
Although raised floors remain common in data centers, they are generally considered a less than optimal solution with respect to energy efficiency. One of a raised floor’s main benefits is it creates a space under the equipment that can contain the multitude of cables (power and data) required to feed servers and other IT equipment. Even assuming a raised floor isn’t an energy-efficiency hindrance in itself, using the under-floor plenum for cabling can hamper airflow, creating hot spots that necessitate running the cooling system at a higher capacity. Furthermore, the cable holes in tiles (or, worse, complete removal of tiles) results in air leakage, another drag on efficiency. From a cabling perspective, running cables under the floor makes them much less accessible at maintenance time; lifting floor tiles on a raised floor must be done carefully, and finding a particular cable can still be difficult or impossible, particularly if cables are not well marked or if many “dead” cables are left under the floor (another cause of clutter).
As a good chunk of the data center industry has moved away from raised floors, instead implementing hot aisle/cold aisle techniques, the matter of cable distribution has also come into focus. Obviously, if cabling cannot be placed under the floor, it must be placed above it.
Benefits of Overhead Cabling
From an energy efficiency standpoint, overhead cabling eliminates one major source of airflow obstruction, helping reduce the likelihood of hot spots. According to an APC by Schneider Electric white paper (“How Overhead Cabling Saves Energy in Data Centers”), “The decision to place network data and power cabling into overhead cable trays can lower cooling fan and pump power consumption by 24%.”
But another major benefit is accessibility. Instead of being under the floor—and possibly all but inaccessible owing to the arrangement of equipment above the floor or the hassles of lifting floor tiles—overhead cabling can be entirely accessible, easing the process of maintaining existing cables or adding new ones. A TechTarget.com article (“Using overhead cables to tidy your data center: Ask the Expert podcast”) cites Robert McFarlane, a principal at consulting and technology design firm Shen Milsom and Wilke, as identifying another tremendous advantage: “avoiding the need to comply with article 645 of the National Electrical Code (NEC) and the dangerous Emergency Power Off (EPO) button that article requires.” The EPO button is a perennial source of headaches for data center operators: it has been mistaken for a variety of purposes, including a door opener, to the catastrophic detriment of data center uptime. Of course, McFarlane is referring to the use of overhead cabling for power cables in this context. But it is worth noting that the overhead cabling concept can also apply to power cables, delivering the same airflow and maintenance benefits on the facilities side as it does on the IT side.
Thus, if implemented properly, overhead cabling can improve both data center efficiency and uptime—a dual win. But the key is doing so in a way that avoids some common pitfalls.
Cable trays are a means of running cables above the floor while still keeping them accessible for maintenance. A common form of cable tray is a wire mesh or “basket” style. But without proper planning, cable trays can still result in the same types of problems that plague under-floor cabling. For instance, unless “dead” cables are removed regularly when they fail or otherwise are unused, cable clutter can build to the point that identifying a faulty cable for maintenance purposes becomes nearly impossible. Furthermore, the buildup of weight can cause cable trays to sag—if nothing else, creating an unsightly appearance in the facility.
Thus, part of implementing cable trays properly is simply performing necessary maintenance: the trays cannot remove dead cables for you. To aid organization (and thus simplify maintenance tasks), modular cable tray systems are a good option. A Schneider Electric blog (“Overhead Cabling Can Reduce Data Center Energy Costs”) notes, “Creating a modular overhead system provides a solution to this potential problem. Data center personnel can more easily sort and plan cable location, integration and removal with multi-level cable tray organization. In addition, the system facilitates the removal of a ‘dead’ cable, since it will not be tangled or buried among a multitude of other cables.” Modular designs for cable trays (as in almost any other situation) may require some more planning to implement properly, but the return on this investment can accrue quickly in both less maintenance effort and less downtime.
Another important consideration for cable trays is how to install them properly, particularly in existing data centers. As cables accumulate, these trays must bear a significant (although not necessarily huge) weight. Thus, they must be secured to an existing structure—but without causing hazards. According to McFarlane via TechTarget.com, “You really want to avoid…suspending the tray from overhead in an existing facility. Drilling into overhead concrete or removing insulation from beams so you can attach anchors creates dust and contaminates…So in an existing facility you’re usually better off mounting the tray to the tops of the cabinets and racks. Many are made to accept the accessory stand-offs that are made for this purpose.” Installing cable trays in new facilities is generally simpler since the arrangement is planned and can be integrated with the building’s structure, if appropriate.
Yet another concern with cable trays is the possibility of creating zinc whiskers, which are tiny filaments of zinc metal that can “grow” from zinc-galvanized steel as a result of mechanical stress. Zinc whiskers that break off (simple contact with a surface can free zinc whiskers) can be caught by the air handling system and eventually land in sensitive IT equipment, possibly causing faults (unexpected system restarts, short circuits and so on) whose source is difficult to identify. Cable trays that are galvanized with zinc are susceptible to this problem.
A range of cable tray styles and features are available from numerous vendors. The different products, however, generally aim to highlight and build on the particular benefits of overhead cabling: easing maintenance, improving organization and improving airflow. Of course, cable trays are not a solution by themselves: data center operators must implement and follow organization and maintenance procedures to avoid the same clutter and build-up of dead cabling that plagues cabling in under-floor plenums. But the benefits of overhead cabling in energy savings alone (to say nothing of organization, neatness, uptime and maintenance) can quickly yield a return on the investment in cable trays. Furthermore, when hot aisle/cold aisle techniques are implemented, the use of overhead cabling eliminates entirely the need for a raised floor, which is a significant expense in the data center.
Photo courtesy of alexhutnik
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