Find out why placing data cables underground can save you time and money in the long run.
In the first two installments of this 3-part series, we presented a present-day view of the Datacom market and the basics of data cabling, installation, and testing. In this final article, we’ll discuss the benefits of placing cables underground and review some installation techniques. As a wrap-up, we’ll explain the basics of telephone networks, which are still the main cross-country conduit for data signals.
Underground data cable. There are several advantages to placing data cables underground versus locating them overhead (which is frequently a less-expensive option). First of all, a direct burial cable is virtually free from storm damage and has lower maintenance costs than aerial cable. In addition, aerial installations often lack aesthetic appeal. In some communities, they are even prohibited.
High-density polyethylene jacketed cable is well equipped for direct burial because it can withstand the compressive forces of the media in which it is buried. High-density polyethylene is nonporous and non-contaminating, and provides complete protection against normal moisture and alkaline conditions.
Most direct-buried cables have an additional moisture barrier of water-blocking gel under the jacket. If water should penetrate the jacket, it can’t travel down the cable and create further damage. The damage is localized and thus more readily repaired. Since the earth thermally insulates buried cable, its year-round temperature varies by only a few degrees.
The tools used to bury cable consist of trenchers, boring machines, backhoes and shovels.
Installation techniques. The outer jacket is the cable’s first line of defense. Any steps you can take to prevent damage will go a long way toward maintaining the integrity of the cable. The following techniques cover many situations you’ll may encounter in the field.
• Cable should be buried in sand or finely pulverized dirt—free of sharp stones, cinders or rubble. If the soil in the trench does not meet these requirements, tamp 4 to 6 in. of sand into the trench, lay the cable, and tamp another six to eleven inches of sand above it. A creosote- or pressure-treated board placed in the trench just above the top layer of sand, will provide some protection against subsequent damage that could be caused by digging or driving stakes. In particularly difficult installations, such as in rubble or coral, or where paving is to be installed over the cable, PVC may be used as a conduit. This pipe protects the cable and usually makes it possible to replace cable that has failed without digging up the area.
• Examine the cable as it is being installed to be sure the jacket has not been damaged. The outer jacket can sustain damage while in storage or as it is being dragged over sharp edges on the payoff equipment.
• Lay the cable in the trench with some slack. A tightly stretched cable is likely to be damaged as the fill material is tamped.
• Bury the cable below the frost line to avoid damage by the expansion and contraction of the earth during freezing and thawing.
• The National Electrical Code (NEC) states specific requirements for cables to be buried underground. The NEC specifies 24 in. as the minimum burial depth for 0V to 600V nominal applications.
• If an installation must meet all NEC requirements, consult with a local inspector during the planning phase of the project. Electrical inspectors frequently don’t look at Datacom cabling during inspections.
Underground raceways. An alternative to burying cables directly in the earth is to place them in an underground raceway. A raceway offers superior protection and the ability to add conductors at a future date with a minimum of expense. Obviously, it is far more expensive to install a raceway than it is to direct bury a cable.
When installing underground conduits, it is important to install more raceway than you need at the time of installation. This allows you to add additional install more cables in the future at very little expense. Pulling more cables in a conduit that is already partially filled is not generally a viable plan. Another option is to install multiple inner ducts inside one large conduit. This allows you to have three or four separate mini-raceways inside one large raceway. This method often eliminates the need for pull rope and reduces the amount of lubricant required.
The U.S. telephone network. It is important to understand the differences between analog and digital circuits. Analog signals vary continuously and represent particular values, such as the volume and pitch of a voice or the color and brightness of a section of an image. Digital signals have meaning only at discrete levels, that is, the signal is either on or off, present or absent, 1 or 0.
Analog telephone lines are the legacy (old, traditional) systems of the telephone industry. Most residential telephone lines are still analog. Analog lines makeup what is known as a local loop-the connection between your home telephone jack and the telephone company’s central office. Since a typical local loop is about 2.5 miles in length, the central office is most often an inconspicuous building in or near your neighborhood. (See the Figure, on page 64Z.)
At the central office, the analog signal is converted to digital so it can be switched across the telephone network. Aside from a few remote areas, the U.S. telephone network that interconnects central offices uses digital signaling.
Dedicated versus switched lines. Telephone lines can be dedicated circuits you lease or switched services you buy. If you order a T-1 line, you are renting a dedicated point-to-point circuit with a 1.544 megabits per second (Mbps) capacity from the telephone company. These virtual circuits allow you (the customer) to specify the end points of the circuit, whereas the actual communication route is determined by traffic volume on the telephone company’s network. Switched services, such as residential analog telephone service, are services purchased from the telephone company. You can select any destination on the telephone network and connect to it through the network of public switches. You generally pay for connect time or actual traffic volume, so unlike a dedicated line, the bill will be low if usage is low.
If you have access to a digital circuit, you don’t need a modem to provide digital-to-analog conversion services between your terminal equipment and telephone. Nonetheless, customer premises equipment must present the correct electrical termination to the local loop, transmit traffic properly, and support phone company diagnostic procedures.
New technologies. A new family of telephone technologies called Digital Subscriber Line (DSL) looks promising, but may never see full implementation. There are several types of DSL services, such as ADSL, HDSL, GLite and others. They are collectively referred to as xDSL. All are similar in that they use electronic boxes on each end of a standard telephone line to achieve transmission speeds of 1.5 Mbps or higher. For this technology to gain widespread acceptance the telephone companies (Telcos) must implement it. Unfortunately, it is not in their best interest to do so. The Telcos make far more money on T-1 lines. The predecessor of DSL - ISDN - was also left to “die on the vine” by the Telcos. The same fate probably awaits DSL.