Editing Overhead line (section)

=== Section break ===
[[File:SectionInsulator.png|thumb|A section insulator at a section break in Amtrak's 12&nbsp;kV catenary]]

To allow maintenance to the overhead line without having to turn off the entire system, the line is broken into electrically separated portions known as "sections". Sections often correspond with tension lengths. The transition from section to section is known as a "section break" and is set up so that the vehicle's pantograph is in continuous contact with one wire or the other.

For bow collectors and pantographs, this is done by having two contact wires run side by side over the length between 2 or 4 wire supports. A new one drops down and the old one rises up, allowing the pantograph to smoothly transfer from one to the other. The two wires do not touch (although the bow collector or pantograph is briefly in contact with both wires). In normal service, the two sections are electrically connected; depending on the system this might be an isolator, fixed contact or a Booster Transformer. The isolator allows the current to the section to be interrupted for maintenance.

On overhead wires designed for trolley poles, this is done by having a neutral section between the wires, requiring an insulator. The driver of the [[tram]] or [[trolleybus]] must temporarily reduce the power draw before the trolley pole passes through, to prevent arc damage to the insulator.

Pantograph-equipped locomotives must not run through a section break when one side is de-energized. The locomotive would become trapped, but as it passes the section break the pantograph briefly shorts the two catenary lines. If the opposite line is de-energized, this voltage transient may trip supply breakers. If the line is under maintenance, an injury may occur as the catenary is suddenly energized. Even if the catenary is properly grounded to protect the personnel, the [[Electric arc|arc]] generated across the pantograph can damage the pantograph, the catenary insulator or both.