OVERVIEW

Underground transmission and distribution are installed when:

1. Space is not available for overhead lines, as in the congested downtown areas of large cities.

2. The hazard of high-voltage overhead lines is too great, as in heavily built-up areas of large cities.

3. The appearance of numerous heavy overhead lines would be unsightly, as in dense downtown areas or in new or redeveloped districts of cities.

4. Community ordinances, administrative codes, or franchise agreements require the installation of underground electric facilities in defined areas.

Underground lines cost more than the equivalent overhead lines. Underground lines are therefore used only when necessity demands them, as stated above.

Parts of Underground System. An underground system may consist of six parts:

1. Conduits or ducts

2. Manholes

3. Cables

4. Transformer vaults

5. Risers

6. Transformers

The first three parts listed above are illustrated in Fig. 29.1. The ducts are the hollow tubes in the conduit runs connecting the manholes, one with another. The cable is the electrical power circuit placed in the conduit through which power flows, and the manhole is the chamber where cable ends are spliced together to make a continuous circuit. Transformer vaults (Fig. 29.2) are underground rooms in which the power transformers, network protectors, voltage regulators, circuit breakers, meters, etc. are housed. Cables terminate in transformer vaults or customers' substations or connect with overhead lines in potheads, as illustrated in Fig. 29.3. The last termination is called a riser.