Editing Overhead line (section)

== Breaks ==
Lines are divided into sections to limit the scope of an outage and to allow maintenance.

=== 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.

=== Neutral section (phase break) ===
[[File:Neutral Gap Sign.svg|thumb|Neutral Section Indication Board used on railways in the [[UK]]. Six of these would be required at [[#Crossings|crossings]]]]
[[File:ED9M-pantograph passing neutral section of 25 kV catenary.webm|thumb|A [[Pantograph (transport)|pantograph]] of EMU passes neutral section of [[25 kV AC railway electrification|25 kV 50 Hz AC overhead line]] without lowering but with switching off a circuit breaker]]

Sometimes on a larger electrified railway, tramway or trolleybus system, it is necessary to power different areas of track from different power grids, without guaranteeing synchronisation of the phases. Long lines may be connected to the country's national grid at various points and different phases. (Sometimes the sections are powered with different voltages or frequencies.) The grids may be synchronised on a normal basis, but events may interrupt synchronisation. This is not a problem for [[Direct current|DC]] systems. [[Alternating current|AC]] systems have a particular safety implication in that the railway electrification system would act as a "Backdoor" connection between different parts, resulting in, amongst other things, a section of the grid de-energised for maintenance being re-energised from the railway substation creating danger.

For these reasons, Neutral sections are placed in the electrification between the sections fed from different points in a national grid, or different phases, or grids that are not synchronized. It is highly undesirable to connect unsynchronized grids. A simple section break is insufficient to guard against this as the pantograph briefly connects both sections.<ref>{{Cite journal |last1=Morais |first1=Vítor A. |last2=Martins |first2=António P. |date=2022-03-01 |title=Traction power substation balance and losses estimation in AC railways using a power transfer device through Monte Carlo analysis |journal=Railway Engineering Science |language=en |volume=30 |issue=1 |pages=71–95 |doi=10.1007/s40534-021-00261-y |s2cid=256402259 |issn=2662-4753|doi-access=free |bibcode=2022RailE..30...71M }}</ref>

In countries such as France, South Africa, Australia and the United Kingdom, a pair of permanent magnets beside the rails at either side of the neutral section operate a bogie-mounted [[transducer]] on the train which causes a large electrical circuit-breaker to open and close when the locomotive or the pantograph vehicle of a multiple unit passes over them.<ref>{{cite web|url=http://www.vortok.com/signalling/train-control-magnets/automatic-power-control-apc-magnet|title=Vortok Automatic Power Control Magnet|access-date=25 July 2018|archive-date=25 July 2018|archive-url=https://web.archive.org/web/20180725183756/http://www.vortok.com/signalling/train-control-magnets/automatic-power-control-apc-magnet|url-status=dead}}</ref> In the United Kingdom equipment similar to [[Automatic Warning System]] (AWS) is used, but with pairs of magnets placed {{em|outside}} the running rails (as opposed to the AWS magnets placed midway between the rails). Lineside signs on the approach to the neutral section warn the driver to shut off traction power and coast through the dead section.

A neutral section or phase break consists of two insulated breaks back-to-back with a short section of line that belongs to neither grid. Some systems increase the level of safety by the midpoint of the neutral section being earthed. The presence of the earthed section in the middle is to ensure that should the transducer controlled apparatus fail, and the driver also fail to shut off power, the energy in the arc struck by the pantograph as it passes to the neutral section is conducted to earth, operating substation circuit breakers, rather than the arc either bridging the insulators into a section made dead for maintenance, a section fed from a different phase, or setting up a Backdoor connection between different parts of the country's national grid.

[[File:Neutral zone of overhead catenary.jpg|thumb|25&nbsp;kV AC neutral zone in Romania]]

On the [[Pennsylvania Railroad]], phase breaks were indicated by a position light signal face with all eight radial positions with lenses and no center light. When the phase break was active (the catenary sections out of phase), all lights were lit. The position light signal aspect was originally devised by the Pennsylvania Railroad and was continued by [[Amtrak]] and adopted by [[Metro-North Railroad|Metro North]]. Metal signs were hung from the catenary supports with the letters "PB" created by a pattern of drilled holes.

=== Dead section ===
A special category of phase break was developed in America, primarily by the Pennsylvania Railroad. Since its traction power network was centrally supplied and only segmented by abnormal conditions, normal phase breaks were generally not active. Phase breaks that were always activated were known as "Dead Sections": they were often used to separate power systems (for example, the Hell's Gate Bridge boundary between Amtrak and [[Metro-North Railroad|Metro North]]'s electrifications) that would never be in-phase. Since a dead section is always dead, no special signal aspect was developed to warn drivers of its presence, and a metal sign with "DS" in drilled-hole letters was hung from the catenary supports.

=== Gaps ===
[[File:Spoorbrug Meppel - Buitenvaart, Jaagpad - 04.JPG|thumb|A [[swing bridge]] near [[Meppel]], the Netherlands. There is no overhead line on the bridge; the train coasts through with raised pantograph.]]

Occasionally gaps may be present in the overhead lines, when switching from one voltage to another or to provide clearance for ships at moveable bridges, as a simpler alternative for moveable overhead power rails. Electric trains coast across the gaps. To prevent arcing, power must be switched off before reaching the gap and usually the pantograph would be lowered.