Sectioning and Paralleling:
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The portion of the OHE between a feeding post and the next neutral zone on one side is called a section of the traction supply. A substation normally supplies power to two sections, these sections consisting of the up and down OHE portions between the feeding post and the neutral section on either side. A sectioning and paralleling post (SP) (or sometimes sectioning post) is provided near the neutral zone, that has two paralleling interruptors to keep the two portions of the OHE (one in each direction) supplied in parallel. Bridging interruptors are with undervoltage relays are provided to allow feeding of a section that is normally supplied by an adjacent feeding post, in an emergency. When this is done, the portion of the catenary near the...
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next feeding post has to be treated as a phase gap as there could be different phases supplying the sections on either side, and drivers have to lower their locos' pantographs when crossing that feeding post. Note that on IR neutral sections are not provided immediately at the feeding post as is done in some countries, but only at the sectioning posts. This is because bridging adjacent sections is done only as a manual measure in case of an emergency and the risk of short-circuiting adjacent sections supplied by different phases is considered to be low.
A section may be divided into subsections by the provision of subsectioning and paralleling posts (SSP) every 10-15km. Each such SSP has a two bridging interruptors for bridging adjacent subsections, and a paralleling interruptor to parallel the up and down tracks. A subsectioning post is a variant where no paralleling interruptor is provided (rare).
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(IRFCA)