All metal towers are part of the transmission system. The transmission system serves distribution substations and very large customers. Duke Power has about 70,000 miles of transmission and distribution lines. In the substation, transformers reduce transmission voltage to distribution voltage. In addition, there are breakers that protect the transmission system from problems originating from the distribution system.
Breakers minimize the number of customers affected by a problem.
By tripping, they help to minimize the number of customers affected by an outage. Many circuits are fed from each substation. Most wooden utility poles are part of the distribution system. Typically the distribution system operates at either 12,000 or 24,000 volts. Duke Power has about 70,000 miles of transmission and distribution lines.