Page 414 - IRSEM_Main Book

18.2.4 Single Line Token Block Instrument in RE Area

(a) Where earth return is used, the rest contact of the instrument shall be
substituted by the contacts of a slow release relay. The arrangement shall
be such that the relay is energized during the operation of plunger but is
dropped only after the condenser has discharged to a safe limit. The circuit
details are illustrated in Drg. No. 18-D5.

(b) Whenever stray DC is present in the earth, metallic return is provided for
block circuit. Metallic return should be used with a modified filter circuit
eliminating the condenser. The chokes and surge arrestors shall be
retained and shunt resistors shall be provided across the Galvo (150 ohms)
and Polarised Relay (77 ohms). The resistors shall be of substantial rating
so that they do not get open circuited. The circuit details are illustrated in
Drg. No. 18-D6. In this case, slow to release relay is not required.

18.2.5 Double Line Block Instrument in RE Area

(a) IRS type Block Instrument with Block filter shall be used for Double Line
section electrified with 25 KV AC. The circuit details are illustrated in Drg.
No. 18-D7.

(b) The voltage of the Battery shall be sufficient to ensure not less than 18 mA
and not more than 25 mA current through the polarized relay. Needle coil
resistance shall be modified to obtain this level of current.

Section 3: Installation of Block Instruments

18.3.1 General Fixture

(a) All block instruments shall be of approved type only.

(b) The Table or other fixture on which instruments are placed must be
substantial and the Instruments are securely fixed there on.

(c) Turn-table arrangement may be provided for fixing the Token and Handle
type instruments to enable easier maintenance but the plate on which the
instrument turns shall be securely fixed to the table.

(d) Block Instruments, Distinction of: Where two or more block instruments
are located in the same room, they shall be fitted with bell units/bell buzzers
of distinctive tones.

Note: Painting of Block Instruments shall be in accordance with the painting scheme in
Annexure: 18-A1.

Chapter 18: Block Instruments, BPAC & IBS Working Page 362 of 535