Many experienced cable installers claim that more than 50% of premature fiber-optic cable failure is caused by improper cable pulling. Here are some important guidelines and techniques for overhead, indoor, and underground installations.
Is installing cables menial work? You certainly shouldn't think of it this way. Yes, it's an everyday task, but all tasks are important. You should always install cables correctly; short cuts inevitably don't work. This is especially true with delicate fiber-optic cables. Even outdoor armored fiber-optic cable suitable for direct burial has a crush resistance of only 500 lbs to 1000 lbs. So, how do you install fiber-optic cable and not damage it?
Running fiber-optic cable overhead between buildings requires fastening the cable to the buildings or stringing between poles. Whichever way you choose, you must install something to take the strain off the cable. Stringing a messenger cable between the buildings or poles and lashing the cable to it is one option. Another is to use a "Figure 8" cable, which has the messenger as part of the cable. NEC Art. 321 outlines the requirements for cables fastened to messenger wire. Normally, these cables will be round, loose tube type. To allow for thermal expansion and contraction, you should install outdoor messenger-supported cables with a sag of at least 1.7 ft (450 cm) per 100 ft. Yet another option is to use a self-supporting cable that uses Kevlar (Aramid yarn) as the support with the Kevlar fastened at both ends. Most self-supporting cables have limitations as to span length: usually about 500 ft, depending on loading conditions.
There are many different ways to install fiber-optic cable indoors In plenum areas, the cable must have the proper flame test rating, per NEC Sec. 300-22 or Sec. 56 of the Canadian Electrical Code. Otherwise, you have to install the cable in noncombustible conduit. The days of throwing a bundle of cable across the drop ceiling are gone forever. Now, you have to hang the cable on approved hangers spaced at no more than 3 ft to 4 ft (1 meter 1), except when the cable is large, armored, or stiff.
Be careful not to cinch the cable too tightly, and you have to cut the free tab off to prevent over tightening. The rule for tie wraps is "use them, but don't over tighten them." Oh yes-these cable ties might also require a fire rating.
Installing fiber-optic cable in an innerduct can prevent future damage ...and is often a wise investment. If you're using cable tray, you must place fiber-optic cables away from power cables and in a flexible, nonmetallic innerduct. If you're running the cable in a vertical tray, you must support it evenly. You must use loops of at least 1 ft dia at the top and bottom of the run (depending on the cable type and diameter) and every 100 ft. Depending on the jurisdiction you're working in, there can be variations to the electrical codes. You probably can't run optical cables in a tray with cables carrying more than 600V, but you should check with the local authority having jurisdiction (AHJ) to be sure. Fiber-optic cables present no hazard to power or control cables but if they burn, these 600V cables can destroy the fiber-optic cables.
If you're using conduit, make sure its bend radius is not less than the minimum required for the cable.
Many people build new homes with optical fiber installed. Most inspectors require you to observe flame rating requirements and install the cable with protection, whether innerduct, running boards, or pulled through drilled holes in the building structural members. Make sure you leave 3-ft (1 meter) tails at each outlet per TIA/EIA-568A. If not hooked up, you should coil the cable carefully and place it in a 4-in.-square box (2 and 1/8 deep). When connecting the cable, use a proper receptacle cover to suit the type of connectors you're using. Remember: Running fiber in a house is a two-person job, if you don't want damaged or kinked cable.
Running fiber-optic cable underground can involve direct burial or installation ...in underground ducts between maintenance holes. You have to install direct buried cable below the frost line. You also have to surround it with sand because rocks move horizontally and vertically due to freezing pressures, possibly pushing against the cable, causing bumps and small losses in signal strength. Many of these small losses can add up to an unacceptable large loss. Don't forget to mark the cable with a marker tape buried below the surface; this will act as a warning to anyone digging in the area. If you're using a cable construction not containing metal, use a metal impregnated marking tape so the cable can be located with appropriate equipment.
If you're pulling fiber-optic cable through ducts between manholes, you'll need to plan ahead: * First, rod the duct to clean out any dirt, bits of cement, empty bottles, etc. A variety of devices that clean the duct, remove any sandy dirt, and add lubricant are readily available. * Second, verify the maximum tension you can place on the cable. You can monitor your pulling tension in several ways. One is the use of break-away swivels (shown in Fig. 2), which attach to the cable. They're designed to break at a pre-set tension. Another is the use of a slip-clutch capstan. Many installers use a chart recorder to record the amount of applied tension when they're guaranteeing the installation. Still another method is to install a gauge in the pulling line. * Make sure you have enough manpower for the pull because you'll be spreading large Figure 8s at the manholes. Also handle the cable carefully and protect it from sharp bends while it's out of the manhole.
Pulling lubricants are especially important for fiber-optic cables. Make sure the lubricant is compatible with the cable's jacket. It should have a low coefficient of friction and should leave a slippery film when dried. This will help when starting the cable moving again during retrofit or removal.
These lubricants may leave a hazardous mess on the floor of the manhole. Have lots of wipes handy and garbage bags for their disposal. Pulling lubricant is also available in wipes, which are great for lubricating small cables without making a mess.
There is a multitude of fiber-optic pulling grips on the market. It doesn't really matter what you use, as long as you ensure the pulling strain is on the strength member of the cable, intended to take the strain. Fasten the cable securely and seal its end with rubber splicing compound or a shrink sleeve to keep out moisture and dirt.
Aside from the regular work hazards of cuts, bruises, falls, etc., you must consider additional hazards. Check for hazardous gases before anyone enters a manhole. In fact, you might need a separate air supply. You'll have considerable forces on the puller, sheaves, and/or pulleys. And, you may be using pulleys 5 ft in dia. If one of these lets go, serious injury can result. Your best bet is to use chain or steel cable (chokers) rather than rope to tie pulleys and pulling equipment in place. Above all else: Examine each situation carefully before you start work.
Guard the manhole A combination guard and emergency manlift is ideal. If the manhole is in a high traffic area, you'll need a warning light. If the cable installation takes more than one day and you leave it idle overnight, protect it during that time. The cable is valuable enough, along with the labor invested in installation, that it might pay to hire someone to guard the cable from damage or theft during off-hours.
