Page 415 - IRSEM_Main Book

18.3.2 Transmission Media and Wiring

(a) Line Wires: The line wires from the point where the lines of two adjacent
block sections meet to the terminating point on or near the building shall
either be insulated or be so erected as to be not less than 150 mm from any
other wire. The provision of insulated wires should be preferred. The
insulated line wires must be terminated on pot-head insulators. The line
wires must not be easily accessible. Alternatively, a cable, suitably
protected, may be used, but a separate cable should be provided for each
instrument circuit. The insulated line wires must be led in sloping upward to
the building, otherwise a drip loop should be provided.

(b) Leading in Wires: The wires leading into the building from the terminating
point shall be adequately separated from the wires of other circuits and
shall be single, braided or metal sheathed, run in one length from the pot-
head insulator to the test panel. Alternatively, a cable, suitably protected,
may be used, but a separate cable should be provided for each block
instrument circuit.

(c) Indoor Wires: Indoor wires must be run in troughing or other suitable form
of trunking which provides protection from interference and separation from
the wires of any other circuit.

(d) Radio/Optical Fibre Cable: Where radio channel or optical fibre cable is
used in lieu of line wires, approved type block interface shall be provided.

Note: All wires or cables shall be in accordance with the Indian Railway Standard
Specification or other approved specification.

18.3.3 Protective Measures against Power Line Parallelism

(a) Lightning Dischargers shall not be located in the Block Instrument but
installed separately and shall be the demarcating point for test purposes.

(b) It shall be ensured that induced voltage due to power line parallelism does
not exceed 150 V and the short circuit fault current does not exceed 430 V
as prescribed by CCITT.

(c) If the induced voltage goes beyond these limits, overhead block circuit shall
be transferred to an underground telecom PET quad cable/OFC/Radio, so
as to bring the induced voltage within the permissible limits.

(d) Since the immunity of PR relay is only up to 10 V, block filters shall be used
in all block instruments using this relay where this limit is exceeded.

(e) Block filters shall be used with Handle type Block instruments using
frequency coded signal.

(f) Use of single line push button tokenless block instruments is permitted in
sections where length of parallelism does not exceed 1.5 km, so as to
ensure that the DC coding used in these block instruments is not distorted.

Chapter 18: Block Instruments, BPAC & IBS Working Page 363 of 535