4.2. SR 06-15-1998 - Laserfiche WebLink

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It is much more difficult to repair underground cable. With overhead, you can generally see the problem. With underground, it is much more difficult to locate, dig it up and make repairs. Four months of the year, the ground is frozen and difficult to dig especially with other utilities (gas, telephone, cable TV) close by. Often with failed cable in the winter, a temporary overhead line is constructed or cable is temporarily laid on top of the frozen ground. What could be a 15 minute power outage in an overhead system can become a 2 to 3 day affair in an underground system. What causes underground cable to fail ? Nicked conductor from sharp rocks, gophers, installation equipment, third party dig-ins, over voltage stress from lightening, or breakdown of cable insulation or jacket. Gopher State One Calls helps prevent third party dig-ins, but it still happens too often. Mishandling of cable during installation from bending cable too much, stretching or nicking from dragging on street or over rough ground. Underground splices become critical. Any moistures permeating into the cable causes cable failures. Fusing electrical systems or sectionalizing is done to minimize the areas that are effected during a power outage and also identifies more accurately geographically where the trouble may be. Sectionalizing an underground system is much more difficult and much more expensive to do. ($50 for O.H. to $5,000 for U.G.) The fuse coordination is also more difficult. Underground systems will have more customers without power for longer periods of time. Problem from animal interference is about a toss-up. Gophers cause problems for underground systems as where raccoons, squirrels and large birds cause problems for overhead systems. Underground systems do not solve the entire aesthetic problem. Customers do not like transformers, junction boxes or fuse cabinets in the front yards or boulevards. Overhead electrical systems provide poles to attach Street Lights. Underground systems must use additional street light standards. A good case can be made that overhead systems are safer than underground systems. In overhead systems, the conductor and equipment are high in the air and are inaccessible. If a vehicle hits a pole and breaks it off, the fuses should de-energize the line in a matter of milliseconds. With an underground system, the equipment is more accessible to the public because it is on the ground and the fusing is not as good if a car runs into a transformer, junction box or fuse cabinet. Single phase or three phase taps of the main feeder line are generally buried. The cost disparity between underground and overhead decreases and approaches zero. The sectionalization problems are also minimized with an overhead fuse at the tap. The other arguments still apply. In general, overhead lines are less aesthetic, less costly, and more easily maintained and quicker to repair. In the case of School Street, this is a main feeder. If this line were to be put underground, all of the above arguments would apply. From an engineering perspective, it is not desirable to bury an electrical line with an overhead line attached on each side. Installation of lightening arrestors then becomes critical. Generally the cable will fail prematurely due to voltage stress from lightening.