Notes On Daido Single Line Block Instrument

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TEACHING NOTES ON DAIDO BLOCK INSTRUMENT

SIGNAL & TELECOMMUNICATION TRAINING CENTRE, BYCULLA, MUMBAI ( I S O 9001-2000 CERTIFIED )

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CONTENTS 1. 2. 3. 4. 5.

Introduction Special requirements Coding principle Different parts Functions of relays: a) Internal b) External

6. Special features of DAIDO block. 7. Position and specifications of relays 8. Explanation of circuits 9. Modification in RE area 10. Safety checks 11. Overhauling

PAGE NO. 03 03 04 05 07 08 09 10 12 18 19 20

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HANDLE TYPE TOKENLESS BLOCK INSTRUMETNT FOR SINGLE LINE INTRODCUTION: It is a single line Block Instrument (modified instrument) designed to work on Modulated frequency (Carrier frequency 1800 Hz or 2700 Hz) and modulating frequency (65 Hz and 85 Hz) having so many advantage over Token block Instrument to increase section capacity in single line. A pair of block Instruments are connected electrically through overhead or cable to control a single line Block section. Advantages over token Block Instrument: 1. Operating Time less. 2. No token exchanging time that required in Token Instrument. 3. Physical strain less. 4. Problem like token missing, token balancing, and census of token recoupment or lost token or damaged token. 5. Avoidable detention is minimized. 6. No question of maintaining normal polarity /Reverse Polarity instrument. SPECIAL REQUIREMENTS On sections where token less block working is to be introduced, the following equipments must be provided: a) One pair of instruments for each block section, fitted with visual indicators giving the following indications:1. When the instruments are normal and there is no train in the block section “Line Closed” at both stations. 2.

When permission to approach for train to leave block station has been

received from block station ahead “Train going To” at the sending station.

4 3. When permission to approach for train to leave a block station in rear has been given “Train coming from” at the receiving station. 4. When a train has entered the block section “Train on Line” at both sending and receiving stations, in addition to “Train Going To” or “Train coming from” indications. b) A control to ensure that the last stop signal at the sending station cannot be taken ‘OFF’ until the receiving station instrument has to be operated to the “Train coming From” position / condition and the sending station instrument to the “Train Going to” position/condition. c) A control to ensue that the last stop signal of the block station is replaced to “ON” by the entry of the train into the block section and is maintained in that position until the train has cleared the block section and the instruments are put back to the “Line Closed” condition and again set as in clause (b) above. d) A control to ensure that the instruments are set to the “train on line” condition at both sending and receiving stations when the train passes the last stop signal and enters the block section. e) A control to ensue that the Outer and Home or the Distant and Home Signals, as the case may be, and the opposing last stop signal are proved to “ON” to receive and dispatch any code except automatic “Train on Line” code and the bell signals. f) A control to ensure that the opposing last stop signal of the block section can not be taken “OFF” at one and the same time. CODING PRINCIPLES: 1. D.C. (-) ve on L1 – Bell. 2. 1800 or 2700 Hz modulated with 85Hz & DC (+)ve on L1 To operate the block handle from line closed to receiving ®, ‘R’ to line closed and leaving (L) to line closed. 3. 1800 OR 2700 Hz. Modulated with 65 Hz. only to set the other instrument at TOL.

5 4. 1800 OR 2700 Hz. Modulated with 65Hz. & DC (+) ve on L1 to operated the Block handle from line closed to leaving (L) position. DIFFERENT PARTS OF THE INSTRUMENT: 1. Block Handle: Block Handle is locked in three positions: in ‘Normal’, ‘R’ (Receiving) and ‘L’ (Leaving) position - It is free to turn between (X-Y), (R-D) & (L-B). For turning from ‘Normal’ to TCF, check is effective in (X) position. Initial Position F.M. & D.C. Of B/H Voltage required On L1

Lock Magnet

Final Position Of B/H

‘N’

85 Hz. & (+) ve

Energized

‘R’

‘R’

--do--

--do--

‘N’

‘N’

65 Hz. & (+) ve

--do--

‘L’

‘L’

85 Hz & (+) ve

--do--

‘N’

2. Galvo: Indicates incoming and outgoing DC current. 3. Push Button PB1: When only PB1 is pressed i)

DC (-) ve on L1 for Bell

ii)

Dc (+) ve on L1 for TOL acknowledgement.

4. Push Button PB2: PB2 is pressed always along with PB1 to pick up PBPR in turn. I) Transmitter is connected with local battery to transmit modulated frequency. II) For sending (+) ve on L1. 5.

Single Stroke Bell:

It sounds when BLR or NR picks up and used for

exchanging Bell Code. 6. Transmitter: Transmits a frequency-modulated output (1800 or 2700 Hz) it gives DC output to pick up CR1 or CR2 when connected with local battery.

6 7. Receiver: Receivers modulated frequency (1800 or 2700 with 65 or 85 Hz) it gives DC output to pick up CR1 or CR2 when connected with local battery. 8. TOL Indicator: Display white indication when the Block section is clear and display Red indication with writing “Train on Line” when the train entered into the Block section and it holds magnetically. 9. Buzzer: BZ1 – Sounds at both stations when the train entered into the Block Section and stops after acknowledgement by the pressing PB1 at the Receiving Station. BZ2- Sounds when the train arrived completely with proper signal and sequence at the Receiving station and stops by normalizing the Home signal SM’s slide/lever. 10. Switch-1 (S1) – With counter: S1 is reversed for normal cancellation and counter offers the next higher number. 11. Time Release Indictor: Normally shows ‘white’ with writing ‘LOCKED’ and when S1 is reversed for normal cancellation, it changes to ‘green’ with writing ‘FREE’ after 120 seconds. 12. Switch 2 – (S2) with counter:

Push Back cancellation is performed

immediately after reversing the S2 and the counter offers the higher number. 13. SM’S KEY: When it is taken out the instrument becomes inoperative except for: I) Reception of Bell code. II) Reception or transmission of TOL code. 14. Shunting Key: I) It can be removed only when instrument is in i)

‘N’

ii)

‘L’

7 II) When the shunting key is out the Block handle becomes mechanically inoperative. III) Extraction and Insertion of this key is only possible when SM’s key is IN and turned. 15. Level Adjust Switch: It is provided in the back or Transmitter to adjust the level low, medium & high. 16. Impedance switch: It is also a three position switch provided on the back of the transmitter to suit the proper impedance matching. i)

600 ohms for overhead.

ii)

1300 ohms for cable.

iii)

1120 ohms for optic fiber.

17. Attenuator: One Tough switch and another fine switch are provided on the back of receiver for adjusting the incoming db loss up to 28 db. Function of the following Relays housed within the Block Instrument 1. BLR - It is a biased relay and picks up when ‘+’ ve on L2 and –ve on L1 is received from the line and the single stroke Bell sounds through the pick up contacts of BLR. 2. NR – It is also a biased Relay and pick up when ‘+’ve on L1 and ‘-‘ve on L2 is received from the line, single stroke Bell sounds and lock magnet energized through its pick up contact. 3. PBPR – Picks up when PB1 & PB2 pressed simultaneously or only PB1 with TOLR pick up condition during TOL code transmission. It connects DC (+)ve on L1 and (-)ve on L2 and transmitter to the local battery. 4.

CR1 (Coding Relay):

Picks up when the receiver received FM85 Hz

modulated with 1800 Hz/2700 Hz carrier frequency, it helps to energize lock magnet for turning the Block Handle from N to R, R to N & L to N.

8 5.

CR2 (Coding Relay):

Picks up when the Receiver received FM 65 Hz

modulated with 1800hz/2700 Hz. It helps to energize the lock magnet for turning the Block Handle from N-L. 6. TRSR: Train sending Relay: Picks up when the Block handle is turned to ‘L’ position. It maintains the “one line clear one train”. 7. IR: It proves the normal condition of reception signal and last stop signal levers and SM’s slide. 8. ITPR: This is a repeater of LSS Track Relay (ITR). 9. TOLR: Picks up when ITPR drops i.e, the train occupies the LSS tracks provided TRSR should be pick up. 10. 2R: Picks up after compete arrival of the train with proper sequence. 11. 3R: Cancellation Relay: When S1 is turned for normal cancellation, after a specified time (120 sec.) delay it picks up and the ‘FREE’ indicator energized through its pick up. 12. TER: Once S1 is turned for normal cancellation, the electronic timer relay gives output of 24 v after a specified time delay (120 sec.) which in turn picks up the TEPR. 13. Timer Relay: It is an electronic type relay used for pick up of TEPR relay after 120 seconds time delay in case of normal cancellation. 14. TELR: Telephone Relay type for Resistance 70 ohms, picks up when switch of hand micro telephone is pressed and in turn it connects the telephone on line and also disconnect the Tx & Rx from the frequency line. External Relays: 1. ASR: Advance Starter Relay: It picks up with the picking up of TRSR. 2. HSR: Home Signal Control relay. 3. TAR: Train Arrival Relay: Picks up after complete arrival of the train with proper sequence. Through its picks up contact 2R picks up.

9 4. SNR: Signal Normal Relay: Proving only signal levers normal condition.

SPEICIAL FEATURE OF DAIDO TOKENLESS BLOCK INSTRUMENT 1. To ensure that the relays “NR” and “BLR” are not picked up simultaneously, back contact of NR relay is proved in BLR circuit. Similarly in the PBPR circuit back contact of CR1 and CR2 relays have been provided to ensure that PBPR relay will not be energized while receiving modulated frequencies. 2.

Front contact of PBPR Relay has been proved in the DC feed circuit of

transmitter, whereas back contact has been proved to the receiver circuit to guard against receiving its own F.M. output modulated frequency transmitted. Similarly, back contact of CR1 and CR2 relay have been proved in the DC feed circuit transmitted to ensure that no code except the code of bell signals can be generated unless the code relays are de-energized. 3.

Since TOL code has to be transmitted automatically as soon as a train

occupies FVT. Front Contact of PBPR Relay is not proved in DC feed circuit to the transmitter but the same is taken in TOLR Front contact NR Relay back contact with the block handle at L position. 4. Similarly for the receiver to be in readiness. To receive the TOL code, the Dc feed circuit is taken via 2R Relay back contact with the block handle “R” position. 5. Pressed contact of the PB2 button in the D. C. feed circuit of the transmitter proves the positive action taken to energize PBPR Relay for transmitting modulated frequency along with DC. 6. Block handle contacts (BX) and (DY) are included in the DC feed circuit to the receiver so that the DC feed to the receiver is switched on only. When the block handle operation is initiated. Thus minimizing battery consumption. 7. SM’s key contact has not been proved in the DC feed circuit of Transmitter for transmitting and receiving of automatic TOL code to ensure that the TOL indicator will display, immediately the block sections occupied irrespective of the position of the SM’s key.

10 8. To ensure that the relay CR1 and CR2 are not be energized at the same time, back contact of CR1 relay is proved to energize CR2 relay and similarly back contact of CR2 relay is proved to energize CR1 relay. 9. Cross protection to the lock Magnet is given through the CR1 and CR2 back contacts. 10. “Transmitter” and “Receiver” are connected to line through the back contact of TELR relay, which ensures that during conversation on telephone, no code is transmitted or received, by the transmitter and receiver respectively at either end. Similarly, the telephone set is connected to the lines through the front contact of TELR relay. Feed is taken through the back contact of CR1 and CR2 relays to ensure that during Transmission and reception of code the telephone is disconnected. The back contact of TOLR is included in the TELR pick up circuit to ensure that the telephone circuit is disconnected the moment TOLR pick up to transmit T.O.L. code. 11. TOLR relay is made slow to release since its energizing circuit is through the front contact of TRSR. And released stick circuit as through the back contact of TRSR relay. POSITION OF RELAYS IN RELAY RACK T E P R

C R 1

C

R 2

P B P R

B L R

N

R

T O L R

T R S R, I T P R

I

2 T

R

3 R

RELAY SPECIFICATION 1. T E P R 2. C R 1

Style QN1 – 8F – 8B

3. C R

Code - 002

2

4. P B P R

SPEC. NO. 930

5. T O L R

IRS NO. S-34, 5-23

6. T R S R

R1 - 345 ohms

7. 2 R

11 8. 3

R

1. B L R

]

Style QBA1 – 8F - 8B

2. N R

]

Code – 026, spec; BR No. 932 AC immunized biased IRS No. S-34, R-215 ohms

1. T P R ]

Style QNA1, 8F/8B, Code No. 022, SPEC BR NO. 931, AC Immunized, IRS No. S-348, S-23

2. I R

]

Resistance: 215 ohms

TERs -

24V DC, 1 Hot & 1 Cold contacts, Electronic Timer.

TELRs-

Plug in miniature telephone type Relay, 8F/B R: - 350 ohms.

SOURCE OF POWER: B24VDC

= Battery for token less internal circuit

B24VDC

= Battery for IR, ITPR

Bx2]

Voltage depends on line resistance

B 2 ] Line Battery

and adjustment for circulating rated

N 2 ]

current of NR relay at other end.

DIFFERENT COLOUR CODE FOR INTRNAL WIRING COLOUR

-

DESCRIPTION

RED

-

Local Source (+)

BLUE

-

local Source (-)

BLACK

-

Block Line Circuit

BROWN

-

Relays Circuit for Reverser Control

YELLOW

-

Transmitter and Receiver Circuit

GREEN

-

Relays and Lock Magnet Circuit

WHITE

-

Other Circuit

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RESISTANCE AND CURRENT PARTS

Resistance in Ohms Working Current Working Voltage in mA In Volt GALVO 18.2 110 2 310 77 24 BELL COIL L.M.COIL 310 77 24 TER 200 120 24 TOLK 1) 1st coil 200 + 200 ohms Resistor

60

24

2) 2nd Coil 500 + 470 ohms Resistor

24

24

EXPLANATION OF CIRCUITS A) NORMAL OPERATION: Normally, the block handle is in intermediate position, the SM key, shunting (SH), cancellation key (S1) and push back key (S2) are all in normal position, the TOL and time release indicator are white and the Needle indicator indicates zero position. In this case, all the relays except for ITPR and IR are dropped away, with no current flowing in the block line and the concerned signals and levers are in normal position (Fig. 1). II. When a train is leaving from A station to B station. 1) Turning the SM key at A station completes a circuit. Push Button PB1 is pressed to give an audio call signal to B station. Then a direct current (-) flows to B station to pick up the BLR at B station. With the BLR picking up, a current flows through a circuit, (see fig. 4) the bell rings even if the SM key at ‘B’ station is not “OFF” position. 2) Upon receipt of a call signals, the SM key is turned on at B station. Then the PB1 at B station is pressed to issue an answering call to A station. The same circuit as described operates to ring the bell at ‘A’ station. 3) Arrangements are made between A station and B station over the telephone.

13 4) Push-button PB1 and PB2 are pressed simultaneously at A station to send a code of signal with both buttons being maintained in pressed down position. A current flows to the following circuit to pick up the PBPR (SEE FIG. 5) with the PBPR picking up. The NR at B station pick up through the following circuit in the line circuit (See Fig. 5 to 8). In this instance, the BLR at B station does not pick up as the current is in the opposite directions on the other hand with the PBPR at station picking up, a source voltage is applied to the transmitter (Tx). In the keying circuit of Tx a modulated current F1 flows to terminals 34, 37 to the line side isolation transformer. Upon the receipt of a modulated current F1 the receiving relay CR1 at B station picks up (Fig. II). As the bell rings one time at B station because by the NR picking up as above mentioned turning the block handle at Y releases said Block handle, which can be turned to right position (TCF position) (Fig. 10). After confirming that the pointer of Galvanometer has turned to zero position (indicating that A station has stopped sending a code of signal) PB1 and PB2 are simultaneously pushed down at B station to send a code of signal to A station (Fig. II). Local and outgoing circuit at ‘A’ (Fig. 12, 13 & 14) local and incoming circuit at ‘B’ (Fig. 15 to 16). Upon receipt of a direct current (+) and a modulated current F2 the NR and CR2 at A station pick up with the bell ringing one time because of the NR picking up turning the block handle as far as the X point at this time causes the TRSR to pick up (fig. 17 & 18). With the TRSR picking up the handle can be released and turned to left position (TGT position) (Fig. 19). Putting the SM slide in reverse position after turning the block handle to left position cause a relay IR to drop away.

An

advance starter control relay ASR picks up with the ASR picking up a circuit outside the instrument is completed make it possible to take “OFF” the advance starter (Fig. 20).

14 When a train leaves station and outer, 1T then 1TR and 1TPR drop away, releasing the stick circuit of TRSR drop away and bring the advance starter signal to normal (ON) position automatically. With 1TPR drops away, the TRSR also drops. The TRSR having slow to release characteristics, TOLR picks up. (Fig. 21). With the TOLR picking up, a source voltage is applied to the Tx with a keying circuit being completed a modulated current F2 flows to the line (a direct current does not flows in this instance) (Fig. 22). With a modulated current F2 being received at ‘B’ station, the CR2 picks up because the Rx power source is already available. TOLR and Bz, operates through the same circuit as in A station to give an indication of train departure. Local and incoming circuit at ‘B’ (Fig. 23). With a push button PB1, being pressed to issue a call signal from A station and a push button PB1 being pressed down to answer said call signal at ‘B’ station picks up through the following circuit. With the PBPR picking (fig. 25) up a direct current (+) flows to A station through the same circuit as described in (Fig.6) causing the NR at A station to pick up (Fig. 8). With the NR at A station picking up a stick circuit for the TOLR is broken with the TOLR dropping away and a buzzer Bz stops buzz ring (Fig. 21). As the TOLK is a magnetic stick type, however it is not restored to normal position. On the other hand, with the TOLR dropping away, a modulated circuit for Tx is broken (Fig.22) and no modulated current F2 flows to B station thereby deenergizing CR2 at B (fig. 23) and causing the TOLR to drop away at B with the TOLR dropping away Bz stop. Buzz ring (Fig. 24). The home signal is put to Reverse position at ‘B’ station with the home SM slide being in reverse position and the IR dropping away (IR back contact is looped in instrument in HSR and ASR circuit if SM’s normal contact is not proved in IR relay circuit) the HSR picks up through the following circuit (Fig. 26). When a train reaches ‘B’ station the BTR picks up and a relay 2R picks up through the following circuit. With the relay 2R picking up the HSR drops away and the home signal is automatically restored to Normal position. At the same

15 time a Buzzer BZ2 produces a buzz ring sound to tell the train arrival restoring the HS, SM slide to normal position causes the BZ2 to stop buzz ring (fig. 27). B station transmits a message to A station by pressing down the PB1 and PB2 simultaneously. In this instance the PBPR picks up at ‘B’ station (fig.5). A direct current (+) and modulated current F1 flow to A station. The NR picks up causing a bell to ring (fig. 8 to 10). Returning the handle to the B point supplies in source voltage to the receiver through the following circuit (Fig. 29). With the NR and CR1 picking up an electromagnet is energized making it possible to restore the handle to normal position the TOLR in turn energized and restored to Normal (Fig. 31) Pressing down the PB1 and PB2 at A station causes a direct current (+) modulated current F1 to flow to B station. The circuit is same as described in Fig. 5,6 &7. The NR picks up at ‘B’ station causing a bell to ring (fig. 8 to 10). Returning the Handle to : a. Point applies a source voltage to the receiver Rx causing the CR1 to pick up (Fig. 9) with the NR and CR1 picking up it is possible to restore the handle to normal position through the same circuit as described in Fig.11 (B) CANCELLATION OF “LINE CLEAR “ BEFORE A TRAIN ENTERS THE BLOCK SECTION: The cancellation switch S1 is put to reverse position at “A” station. Putting the S1 to reverse position energizes the timer through the following circuit because the TRSR has already picked up (Fig.32). The counter S1 is operated simultaneously. Upon the lapse of predetermined time, the TEPR is energized through the output load of the timer and 5 made to stick. With the TEPR picking up the 3R is energized and made to stick and disconnect the timer circuit. It being so arranged that the time clapsing before the energisation of the 3R is two minute, i.e. it takes two minute for relay 3R to pick up after operated of switch S1 (fig. 32).

16 A cancellation signal is issued from A station to B station. Push button PB1 and PB2 are pushed down at A station to send a signal of cancellations. With 3R picking up, a modulated current F1 flows through the keying circuit (Fig. 33). The performance of open circuit is as described here in before within a direct current (+) and modulated current F1 flowing to B station. Upon receipt of the above-mentioned signal, the block handle is turned from right position to normal position. Push button PB1 and PB2 are pushed down at B station to issue a code signal to A station (fig.32). The signal send in this case which is the same as above mentioned is by means of a direct current (+) and modulated current F1. Upon receipt of the signal at A station after the cancellation switch S1 has been restored to normal position, the block handle is restored to normal position at ‘A’ station.

The circuit operates in the same

manner as described earlier. (C) PUSH BACK OPERATION: When a train in the block section push back to departure station A in this instance, the block handle at A station and ‘B’ station are in left position and right position respectively. With the TOL buzzer stopping following are confirmed. Putting the push back switch S2 to reverse position at ‘A’ station actuates the counter S2. The home signal lever is put to reverse position at ‘A’ station. In this instance, the Home signal is put to reverse position through the control circuit of HSR (fig. 26). Upon the arrival of the pushing back train to ‘A’ station the home signal is restored to normal position automatically through the same circuit as described earlier with 2R picking up and the arrival buzzer BZ2 producing a buzzing sound. Putting the SM slide to Normal position causes the buzzer to stop buzzing. The signal indicating train arrival is issued from A station to B station and push button PB1 and PB2 are pushed down. Then a modulated current F1 flows from ‘A’ station through the keying circuit of Transmitter at B station (fig.35). Upon receipt of a direct current (+) and modulated current F1, the Block handle

17 can be turned from right position to normal position, at B station this is done through the same circuit as in the case cancellation. Push button PB1 and PB2 are pushed down at B station to send a direct current (+) and modulated current F to A station. After restoring the switch S2 to normal position, the block handle is turned to normal position from left position at A station. Then the signal for train arrival is issued to B station. (D) WHEN A TRAIN SHUNTING (AT ‘A’ STATION) The block handles are put to normal position both at ‘A’ station and ‘B’ station. A station calls ‘B’ station to obtain authorization over the telephone. After the SH key has been turned and taken out, it is handed over to the driver of a shunting train. This locks the block handle. Upon finishing a shunting operation, the SH key is returned to the station master, who after inserting and turning the ‘SH’ key to normal position, notifying ‘B” station over the telephone to the effect. (E) TELEPHONE CIRCUIT: While modulated current F1 to F2 is being transmitted or received, telephone circuit is isolated from power line. While telephone is used, transmitter and receiver are isolated from line. To explain in detail, telephone relay TELR is inserted in series connection with relay. CR1 (B), CR2 (B) and TOLR (B) in the circuit (Fig.36). Also TELR (F) is connected in series in the telephone circuit, Thus telephone circuit is isolated from signal circuit. Where modulated current is transmitted and received by TELR, which is inserted between signal circuit and Transmitter/Receiver. Transmitter/Receiver is isolated from signal line where telephone is used.

18 MODIFICATION REQUIRED FOR USING IN 25 KV AC RE AREAS Normally the transmitter/receiver circuit (FM signals) is super imposed on the same pair of line wires of DC circuit. In RE area it is worked on a pair of conductor of PET quad of main telecom. Cable and the DC circuit works in two phantoms derived from the PET quad. Since diodes are used in the BLR and NR circuits, induced EMF under earth fault conditions will get rectified and effect their relays. Hence to protect the BLR and NR from AC induced voltages, a block filter unit is connected between the block instrument and the line. External relays XR and YR are also used, as in the case of Neale’s type token block instrument, to prevent energisation of the line relays due to momentary discharge of condenser in the filter unit. The block telephone is connected to the other physical pair of the PET quad. RDSO has examined different ways of reducing the operating voltage of handle type token less block instrument, to be used in RE area and suggested the following vide their letter no. STS/E/S-6 dated 2.4.91. 1. REMOVAL OF CHOKE CH-1: Choke CH1 is provided (in non Re area) to prevent FM signal getting affected by the local battery, since both DC and FM signals are super imposed on the same pair of the line wires. However, in RE area, since a physical pair is used for DC, the choke CH1 does not serve any purpose. Removal of the choke will result in a reduction of 4 volts in the power supply in view of 100mA current flowing through the circuit. 2)

REMOVAL OF RESISTNACE R2: Short circuit protection resistant R2 (20 ohms) may be dispensed with,

since this protection is not required and is not, in any case, provided in other block instruments. This will means, consequently a reduction of 2 volts in the power supply.

19 3) ELIMINATION OF DIIODES AND FILTER UNIT: In the existing circuit, both NR and BLR are of QBA1 type, each requiring about 100mA current. They are connected in parallel and blocking diodes D1 and D2 have been used to only one relay at a time. The use of these diodes has, however necessitated the need for use of filter units, which are otherwise not required. In absence of filter unit the induced A.C. voltage might result in rectified DC voltage due to the presence of the blocking diodes. This problem is now ought to be eliminated by the use of Ac immunized 1000 ohms neutral relay instead of QBA1 relay for BLR. This will however, result in increase of line current from 100mA to 112 mA. In such case, filter unit can be dispensed with, resulting reduction in the power supply. Replacement of two numbers of QBA1 relays for BLR and NR with one number of polar relay and relaxation for use of earth return circuit are also being considered by RDSO. SAFETY CHECKS IN TLBI – HANDLE TYPE: 1. Ensure that the lock and seal are intact. 2. Operate the instrument without co-operation and see whether the handle goes to receiving or leaving position. The block handle should not go to ‘L’ or ‘R’. 3.

Both the instrument is in line-closed position. Try to operate any one

instrument with co-operation and see whether the handle goes to ‘L’ position. The lock handle should not go to “L’ position. 4. Operate the instrument with co-operation and turn to “L” with a momentary feed, see that the “L” lock is force dropped in check lock notch and the block handle cannot be turned to “L” position. 5. Try to close the instrument with co-operation with a train in block section and see whether the handle goes to line closed position. The Block handle should not go to line closed position.

20 WITH LAST STOP SIGNAL: 1. Try to take ‘off’ the LSS without line clear (“L” position of block handle), the signal will not clear. 2. Shunt the LSS track; the LSS shall go back to danger automatically. 3. The signal will not re-clear again unless a fresh line clear is received. OVERHAULING: 1. Ensure overhauling is not due – once in 7 years. 2. Failure analysis, observations and suggestion as per Letter No. SIG/465/182/Pt. 7 of dtd .01.95 at BSP DIVN.

21

22

23

24

25

26

27

28

29

30

31

32