Final answer:
Transmission lines can be sectionalized by opening circuit breakers found in substations. These protective devices interrupt electric current flow, allowing isolation and maintenance of specific sections of the transmission line.
Step-by-step explanation:
Transmission lines can be sectionalized by opening circuit breakers, which are mostly found in substations. Circuit breakers are protective devices that can interrupt the flow of electric current in a circuit, allowing for the isolation and sectionalization of different parts of the transmission line. Opening a circuit breaker breaks the electrical connection and can isolate a specific section of the transmission line for maintenance or in the event of a fault.