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As might be expected.the capital investment cost of distributing power through underground circuits is appreciably higher than by overhead circuits.underground circuits have many advantages such as freedom from sleet and icing of conductors ,tree limbs,lightning surges,damage by vehicles, and improved appearance. On the other hand,there are substantial disadvantages in the from of higher investment coast and longer time required for repair.
· There are many factors that lead to the use of underground systems.Historically the distribution systems in the downtown areas of cities have been the first to go to underground construction.The substation and associated feeder breakers that supply the primary feeders are surface mounted equipments.
The primary feeder cables are underground circuits that supply the network transformers.The network transformers,network protectors,and low voltage secondary circuits are underground.The network transformers and network protectors,when located in street or sidewalk vaults,are capable of withstanding complete submersion in water.
The primary switch on the transformer the associated primary feeder circuits and the secondary circuits are also able to withstand submersion.
· In some locations in some cities,vault are subject to very frequens floodingflooding.Almost invariable vault are designed to withstand .