Neutral Section

                            

Neutral Section Indication Board used on railways in the UK

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

For bow collectors and pantographs, this is done by having two contact wires run next to each other over a length about four wire supports: a new one dropping down and the old one rising up until the pantograph smoothly transfers from one to the next. The two wires never touch (although the bow collector/pantograph is briefly in contact with both wires). In normal service, the two sections are electrically connected (to different substations if at or near the halfway mark between them) but this can be broken for servicing.

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 turn off the power when the trolley pole passes through, to prevent arc damage to the insulator.

Sometimes on a larger electrified railway, tramway or trolleybus system, it is necessary to power different areas of track from different power grids, the synchronisation of the phases of which cannot be guaranteed. (Sometimes the sections are powered with different voltages or frequencies.) There may be mechanisms for having the grids synchronised on a normal basis but events may cause desynchronisation. This is no problem for DC systems but, for AC systems, it is highly undesirable to connect two unsynchronised grids. A normal section break is insufficient to guard against this, since the pantograph briefly connects both sections.

Instead, a phase break or neutral section is used. This consists of two section breaks back-to-back so that there is a short section of overhead line that belongs to neither grid. If the two grids are synchronized, this stretch of line is energized (by either supply) and trains run through it normally. If the two supplies are not synchronized, the short isolating section is disconnected from the supplies, leaving it electrically dead, ensuring that the two grids cannot be connected to each other.

The sudden loss of power over the phase break would jar the train if the locomotive was at full throttle, so special signals are set up to warn the crew. When synchronization is lost and

the phase break is deenergized, the train's operator must put the controller (throttle) into neutral and coast through an isolated phase break section.

On the Pennsylvania Railroad, phase breaks were indicated to train crews by a metal sign hung from the overhead with the letters PB on it, created by drilled holes. When the phase break was "dead", a signal with eight lights in a circular pattern indicated so.

Transnet Freight Rail in South Africa has permanent magnets between the rails at both sides of the neutral section where two phases are separated. These are detected by equipment on the locomotive, which disconnect and reconnect power from the pantographs.

The neutral section used in mumbai is between kalyan-shahad and kalyan-vitthalwadi to seperate the AC and DC line. Before Kalyan towards CST DC overhead line is used and after kalyan towards khopoli and kasara AC overhead line is used so between them the neutral section is provided to seperate the AC and DC lines.