This section is equipped with four tracks, and the
contact rail for each track, together with the direct current feeders
which supply it from sub-stations Nos. 11 and 12, are electrically
insulated from all other circuits. Of each pair of track rails one is
used for the automatic block signaling system, and, therefore, is not
used as a part of the negative or return side of the direct current
system. The other four track rails, however, are bonded, and together
with the negative feeders constitute the track return or negative side
of the direct current system.
The diagram on page 109 illustrates the connections of the contact
rails, track rails and the positive and negative feeders. All negative
as well as positive feeders are cables of 2,000,000 c. m. section and
lead sheathed. In emergency, as, for example, in the case of the
destruction of a number of the cables in a manhole, they are,
therefore, interchangeable. The connections are such as to minimize
"track drop," as will be evident upon examination of the diagram. The
electrical separation of the several contact rails and the positive
feeders connected thereto secures a further important advantage in
permitting the use at sub-stations of direct-current circuit-breakers
of moderate size and capacity, which can be set to open automatically
at much lower currents than would be practicable were all contact
rails electrically connected, thus reducing the limiting current and
consequently the intensity of the arcs which might occur in the subway
in case of short-circuit between contact rail and earth.
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