Theory Questions Flashcards
What is the purpose of a Capacitor fed Track Circuit?
To detect the absence of a train.
At what Voltage is A.C Main supplied?
600V A.C Single Phase
What frequencies is the A.C. Main supplied at and what colours would the track relays be in these areas?
Red = 331/3 Hz
Silver = 125 Hz
When would it be allowable to work live on the AC Main?
Never
Why is BX selection a requirement of signalling circuits fed from the 7-way fuse bay?
To detect and protect against earth faults as BX Selection contains critical safety signalling contacts.
As the NX busbar of the 7-Way fuse bay is referenced to earth, any earth fault in the BX Leg would reveal itself by causing the supply fuse to blow as a result of overcurrent. This fail safe prevents vital signalling contacts to be by-passed.
If the signal selection contacts were placed in the NX an earth fault would cause safety signalling contacts to be bypassed causing a wrong side failure.
An earth fault in the NX without any safety contacts will not affect the circuit.
Why are there 2 pigtails connecting the track wire to the rail?
Redundancy – If one pigtail fails current can still pass through the other one
What size of cable is track wire?
7 / o.85mm – 7 strands of copper with 0.85mm diameter
What are the main functions of a capacitor within a track circuit?
- A means of adjusting current flow through the circuit
- Acts as a load - Current limiting device (when train shunts track)
- Prevents D.C to pass through protecting the Transformer
- Creates a 90° Phase angle relationship between current and voltage for optimum relay operation
How far into the green area of the Vane Indicator should the Vane Pointer of the TR be if the track circuit is half outdoors and half in a tunnel?
Cut and Cover – 2/3rds into the green
What maximum capacitance is a TO authorised to put into a track circuit?
a) 33 1/3 Hz
b) 125 Hz
33 1/3 Hz = 40
125 Hz = 10
You attend a track circuit failure in a 33 1/3 Hz area and identify the track relay is floating forward. You test across R3-R1 and read a potential difference of 3V.
a) What are three reasons that could cause this failure?
- Lack of Capacitance
- Partial short or earth
- High resistance in series
What testing would you conduct to identify each type of floating failure?
- Lack of Capacitance – C+L = 100V – Go to Capacitor – If Output = Low V – Get AWC and Adjust Capacitance
- High resistance in series – C+L = 100V – Go to Capacitor – If Output = High V – Use Volt Meter to find Fault
- Partial short or earth – C+L = Low V – Use Current Clamp to find fault
After your testing, it becomes apparent that the track failure is caused by wet beds.
Even with the maximum available capacitance in the circuit, the track relay vane pointer remains mid-way between the contact pressure line and the green section. Signals are still failing
a) What action may be taken to increase current flow in the circuit without any additional capacitance being introduced to the circuit?
Increase the voltage to 125V by changing the Secondary windings of Transformer from t0 – t1 to t0 – t2
What documentation is required prior to any adjustment of the track circuit, and what tests or checks must be carried out following any adjustment?
Prior:
- AWC + old and new capacitance
Following:
- 1 ohm shunt (T End – Middle – TR End) (1 metre over points and crossings) + front contacts break
- Relay Energisation to correct level
Why would the R coil of the TR be wired in series with the R coil of a GR and what does this protect against?
For Berth Track Diversity.
A berth track is the only place a train is expected to be stationary making it vulnerable. In the rare event of the TR of a berth track becoming mechanically stuck up the train would become invisible to the signalling system.
Berth track diversity is designed to protect against this. By wiring the R coils of TR and rear GR coils in series, if the TR was to become mechanically stuck up, although the GR ‘Q’ would be supplied via the selection circuit, its ‘R’ supply would not be restored whilst the train is shunting the berth track of the signal ahead.
Describe different means of achieving Berth Track Diversity?
- Series TR-GR: Having the R coil of the Berth TR wired in series with the R coil of GR in the rear
- Series TR-TR: Having 2 x berth TR with R coils wired in series which both need to be detected in signal selection of GR in rear.
- Having the length of the Berth Track shorter than the smallest train causing it to always shunt two tracks creating diversity of 2 signal selection contacts for GR in rear.
What is the purpose of a Trainstop Proving Circuit?
The Trainstop proving circuit ensures that a Trainstop comes to the fully on position after the passage of every train.
If it failed to come to the fully on position the circuit will not restore the Q supply to the replacing TR keeping it down even after the train has left.
As this replacing track is in the L.O.C of 2 signals in the rear both will be kept at danger as the signal selection will not be made, thus ensuring at least 1 Trainstop is protecting the train ahead.
Why is the NX100 busbar connected to earth in a 7 way fuse bay?
To Detect and Protect against earth faults.
With the NX100 busbar referenced to earth, any earth fault in the BX Leg would reveal itself bypassing the load causing the supply fuse to blow as a result of overcurrent. This fail safe prevents vital signalling contacts to be by-passed causing a right-side failure.
If the NX100 busbar was not referenced to earth, 1 earth fault in BX would go undetected (latent failure) however if 2 earth faults occurred this would bypass critical safety signalling contacts creating a wrong side failure.
Why is a 1:1 isolating transformer required when track circuits are fed from busbars in a relay room?
As the busbar is referenced to earth an isolating transformer is used to remove the earth for an earth free supply to the track circuit which uses the continuous rail.
List the pre-installation checks you would undertake before replacing a D.E.V. Track Relay?
- Overhauled within 3 Years
- Note part and serial numbers
- Transit String Removed and Grub Screws installed
- Check NO Continuity across all Armatures
- Correct (like for like):
a. Type (DEV / SEV)
b. Colour / Frequency (33 1/3 / 125 Hz)
c. Contact arrangement - Damage (Physical):
a. Impact detector not dislodged
b. No cracks on housing / frame
c. Glass not cracked / broken
d. Spindle straight / Vane not touching coils
e. No loose wires (internally)
f. No condensation
Why is track circuit opposition employed within capacitor fed track circuits?
To detect against an IBJ failure.
By switching adjacent track supplies by 180° and creating opposition an IBJ failure will reveal itself by ensuring at least one track will fail.
Without opposition an IBJ Failure may go undetected as it will cause 2 tracks to become 1.
This is especially dangerous between berth and replacing tracks as it would cause a train to trip at high speed.
What are the characteristics of an anti-opposition track?
- Jumper cables
- IBJ on both running rails
- No phase relationship with adjacent tracks
List 3 places you would find an anti-opposition track?
- Platform Areas (for Maintainer’s Break)
- Substation Gaps / Boundaries (to Mitigate Phase Drift)
- Transposition of +VE Rail (as continuous rail will also need to be Transposed next to +VE Rail)
Give three reasons why T1R would be UP and T2R would be DOWN?
- Open circuit Q coil
- Short or Earth in R coil
- Mid Potential Earth
What is the effect to the system of 1-1:1 contact within 317 trainstop becoming an open circuit and then a train passes through the site?
- Train will shunt 315 track causing TR to deenergise. (breaking stick path)
- Red signal will feed Trainstop to come on however because of open circuit this will not make the pick path in 315 TQ circuit.
- With the no way of BX supply to get to Q coil of 315 TR it will remain down.
- As 315 TR is in signal selection of A317 and A315 both will remain at danger.
Explain what testing is required when installing an additional transformer into the circuit?
- Test Meter + Get AWC
- Isolate Floating Track
- Connect a test wire from t0 – t0
- Re-Instate Floating Track
- Connect voltmeter from t1 – t1
0V = IN PHASE (Parallel) – Replace meter leads with Wire.
200V = OUT OF PHASE (Series) – Move wire to-t1 – recheck with meter.
Why is a 1:1 transformer required when connecting a transformer from the 7-way fuse bay?
To isolate from the earth reference in the 7-way NX bar for an earth free supply to feed the Track Circuit.
What checks are required after adjusting capacitance on a track circuit?
- Note new capacitance.
- Relay energisation to correct level.
- 1 ohm shunt ( T – Mid – TR ) – Ensuring relay deenergises and front contacts break
Why is the NX at Earth potential on conventional signalling circuits?
To Detect and Protect against earth faults.
With the NX referenced to earth, any earth fault in the BX Leg would reveal itself bypassing the load causing the supply fuse to blow as a result of overcurrent. This fail safe prevents vital signalling contacts to be by-passed causing a right-side failure.
If the NX was not referenced to earth, 1 earth fault in BX would go undetected (latent failure) however if 2 earth faults occurred this would bypass critical safety signalling contacts creating a wrong side failure.
What unsafe condition could potentially occur if a fan track was bonded incorrectly?
Incorrect bonding such as in parallel rather than in series would result in a broken bond to be undetected. This latent failure will allow the relay to energise and remain energised when the train wheels shunt the track in the area with the broken bond making it invisible to the system, clearing the signals in the rear allowing trains to travel into an occupied area causing a collision.
Why are track circuit interrupters installed at terminal stations and sidings?
To detect illegal train moves.
Without a TCI, if a train travelled too far where it shouldn’t and derailed the track would pick up clearing the signal allowing another train into an occupied area causing a collision.
Without a TCI if a train passed its stop point it would damage end of line equipment such as F.R.L potentially knocking it down. Once train has left there will be no indication of the damage.
With a TCI any illegal move past the stop point will cause an open circuit on the track deenergising the TR keeping the signal in the rear at danger.
List the reasons why there might be an anti-opposition track?
- For Maintainer’s Break
- To Mitigate Phase Drift
- Where continuous rail has to be Transposed next to +VE Rail
How does the system protect against the consequences of a berth track relay becoming mechanically stuck up?
By implementing berth track diversity in 1 of 3 ways.
TR-GR: By wiring the R coils of the berth TR and rear GR in series, if the TR was to become mechanically stuck up, although the GR ‘Q’ would be supplied via the selection circuit, its ‘R’ supply would not be restored whilst the train is shunting the berth track of the signal ahead.
TR-TR: Having 2 x berth TR with R coils wired in series which both need to be detected in the signal selection of the rear GR.
Having the length of the Berth Track shorter than the smallest train causing it to always shunt two tracks creating diversity of 2 signal selection contacts for GR in rear.
What is the purpose of the Trainstop proving circuit?
The Trainstop proving circuit ensures that a Trainstop comes to the fully on position after the passage of every train.
If it failed to come to the fully on position the circuit will not restore the Q supply to the replacing TR keeping it down even after the train has left.
As this replacing track is in the L.O.C of 2 signals in the rear both will be kept at danger as the signal selection will not be made, thus ensuring at least 1 Trainstop is protecting the train ahead.
What tracks would have a Trainstop proving circuit?
Trainstop proving TQ circuitry will only be found on track circuits which are replacing tracks of signals which have a following move.
You find a broken crimp that is the cause of your open circuit failure.
Explain the steps you will take to bring the track circuit back into safe operation?
- STEP BACK – State you need to ‘ENACT WIRE OFF PROCEDURE’
a. Take PHOTO of:
Wire
Host
Context
Prints - Inform D-SIM (You have found a wire off) and Request:
a. AWC
b. 2nd Competent Person (To attend site) - ISOLATE (Preferably after the Capacitor)
- Gather sufficient EVIDENCE for AWC:
a. Hand Trace (highest form of proving)
b. Identify Physical Attributes:
Cable Type
Size / Length
Colour
Labelling
Numbering - Remove Isolation (REINSTATE Power)
- ELECTRICALLY PROVE:
a. Test both sides to Earth (1 = 100v / 1 = 0v)
b. Test Wire to Host (To prove Host is NX Potential)
Break circuit BX SIDE to lose Voltage proving wire is part of circuit (Closest to wire)
Break circuit NX side to lose Voltage Proving host is part of circuit (Closest to Host) - PRE – Wire Count (With 2nd Competent Person)
a. Must be INDEPENDENT / Diverse checking (Do not lead)
b. YOU = Check Prints - HE/SHE = Counts at Terminal
c. Confirm Discrepancies (Ask to Repeat) - Confirm AWC issued with D-Sim (Request AWC number)
- ISOLATE
- RECONNECT Wire to Host (Take picture)
a. Clearly LABEL:
Date
Name - POST – Wire Count (HE/SHE checks work)
- REINSTATE Power (Observe Track Pick-up)
- Check Correct energisation of Relay – Witness train shunt track – 1-ohm Shunt
- Clear AWC – REQUEST NIGHT CHECK
A Siemens wheel detector sensor has been temporarily removed due to a re-rail.
How should it be remounted (in relation to the crown of the rail)?
a. It should be mounted in the exact same place as before
b. For a new rail it should be fitted 42mm from the crown (to allow for wear)
What are the minimum tests to be carried out when replacing a Siemens wheel detector?
- Place dummy wheel over detector
- Check evaluator card for light off (extinguished)
- Remove dummy wheel
- Check evaluator card for light on (illuminated)
List the different types of Trainstops?
OUTSIDE:
- J Type (3 strips - 6 bolts – Oil pot)
- HO Type (NO strips – 4 pineapples – NO oil)
TUNNEL:
- HT Type (2 strips – 4 pineapples – NO oil)
- K Type (3 strip – 6 bolts – Oil pot)
EMERGENCY:
- Flatbottom (Silver)
- Bullhead (yellow or black)
List the different types of block joints?
Benkler:
1. Bullhead
2. Flatbottom
Nylon 666:
1. 666 Mk2 (Bullhead)
2. 666 (Flatbottom)
How would you test block joints after re-rail?
With adjacent track R fuses removed and Meter set to Ohms for continuity.
Benklar:
Test point on IBJ (inside Rail) – To each Bolt and Plate
Test point on IBJ (outside Rail) – To each Bolt and Plate
Nylon 666:
Rail 1 – To each Bolt and Plate
Rail 2 – To each Bolt and Plate
What is the Track gauge measurement?
4 Foot 81/2 Inches – 1435mm
Why are TCIs installed on the railway?
To detect illegal train moves.
Without a TCI, if a train travelled too far where it shouldn’t and derailed the track would pick up clearing the signal allowing another train into an occupied area causing a collision.
Without a TCI if a train passed its stop point it would damage end of line equipment such as F.R.L potentially knocking it down. Once train has left there will be no indication of the damage.
With a TCI any illegal move past the stop point will cause an open circuit on the track de-energising the TR keeping the signal in the rear at danger.
What would be the possible outcome if a TCI was installed in the incorrect position after a re-rail?
(i.e within 3 meters of the FRLS)
Because of the clearance between the front of the train and its wheels, If a train passed its stop point and damaged the end of line equipment such as F.R.L potentially knocking them down, there will be no indication of the damage after the train has left.
List all places where check rails may be used?
- Bends and Curves
- Adjacent to Tripcock Testers
- Bridges
- Around Points and Crossings
What effect could rusty or contaminated rails have on a track circuit?
It would prevent the train wheels getting a good connection with the rails causing it not to shunt the track effectively making it undetected and invisible to the signalling system causing the signals in the rear to clear when track ahead is actually occupied causing a wrong side failure and potentially a full speed crash.
How many bonds are required over a non-insulated rail joint on the continuous rail and why?
2 Bonds – For capacity in the case of a traction short fault.
Which engineering standard provides guidance on irregular connections/adjustments?
Engineering Standard S2530
What must be completed after adding capacitance?
- Note old + new capacitance.
- Check relay energisation to correct level
- 1 ohm shunt ( T – Mid – TR ) – Ensuring relay deenergises and front contacts break
- Clearing of AWC
Why do some tracks have two TRs?
For series TR-TR Berth Track Diversity.
A berth track is the only place a train is expected to be stationary making it vulnerable. In the rare event of the TR of a berth track becoming mechanically stuck up the train would become invisible to the signalling system.
Having 2 x berth TR with R coils wired in series even if one was to become mechanically stuck the other would react
to the shunt of the train.
As both TR is in the signal selection of GR in rear both will need to be detected before the signal can clear.
What secondary output voltages are available from a trackside transformer and what is the VA rating?
a. Track winding = 100V – 125V (100 – 125 VA)
b. 7-Way fuse bay= 100V – 110V (400 – 440 VA)
What must be done when Track conditions change?
What is the sequence of events when a train has passed a Tripcock Tester under the following 3 circumstances (you must include if the train will trip and if the light will remain on or off):
a. Too Low
b. Too Tight
c. Too high
Why would the R coil of the TR be wired in series with the R coil of another TR and what does this protect against?
For series TR-TR Berth Track Diversity.
A berth track is the only place a train is expected to be stationary making it vulnerable. In the rare event of the TR of a berth track becoming mechanically stuck up the train would become invisible to the signalling system.
Having 2 x berth TR with R coils wired in series even if one was to become mechanically stuck the other would react to the shunt of the train.
As both TRs are in the signal selection of GR in rear both will need to be detected as energised before the signal can clear.
Why would you use TR-TR instead of TR-GR?
- If the GR is too far
- If the GR is not a DEV relay
- If there is more than 1 signal in the rear
- If there is no signal in the rear
How does TR-TR achieve Berth track diversity?
By having 2 x berth TR with R coils wired in series even if one was to become mechanically stuck the other would react to the shunt of the train.
As both TRs are in the signal selection of GR in rear both will need to be detected as energised before the signal can clear.
How does TR-GR achieve Berth track diversity?
By wiring the R coils of the berth TR and rear GR coils in series, if the TR was to become mechanically stuck up, although the GR ‘Q’ would be supplied via the selection circuit, its ‘R’ supply would not be restored whilst the train is shunting the berth track of the signal ahead.
List the post-installation checks you would undertake when replacing a D.E.V. Track Relay?
a. Line up lid and tighten Thumbscrew
b. Check relay energised (correct level)
c. Lift Q2 (observe immediate de-energisation of relay + front contacts break)
d. For TR-GR (lift R coil on GR + observe TR de-energise)
e. Smooth operation of vane + doesn’t hit stop spring
f. 1 ohm shunt (T – Mid – TR)
What are the fundamental principles of signalling?
- Safe distance between trains.
- Safe passage of trains over points and crossings.
How would you connect an additional spare track transformer?
In parallel:
t1 – t1
t0 – t0
How would you wire in an additional capacitor?
In Parallel:
Input – Input
Output- Output
Which Engineering Standard provides guidance on Track Circuit Maintenance?
S2541
State the measurements including tolerances required to gauge a Trainstop?
a. Height = 3” ± 1/8
b. Distance = 6” ± 1/16
c. Head = 51/2” Minimum
What are the different types of wire degradation?
- Green Spots (Conductive)
- Liquefaction (Strong Smell)
- Shrinkage (Bare Conductors)
- Red Spots
- Twiglets
- Embrittlement
- PCB liquid from capacitors and transformers
What action do you take when you come across wire degradation such as Green Spots?
- Do not touch it (as dangerous to person and circuit)
- Take picture and immediately report to DSIM or Materials Engineer
- Check entire length of cable or loom for similar degradation
- Sleeve with authority / escalation as soon as possible
why is the Continuous Rail double bonded?
Continuous rail is double-bonded to provide sufficient current carrying capacity in the event of a traction fault.
What does ‘Fail Safe’ mean?
A right-side failure - where a fault is self-revealing and defaults the signalling system to its safest state of signals displaying danger aspect.
What is an example of a wrong side failure?
Any failure that does not reveal itself so will not default the signalling system to its safest state of signals at danger aspect:
- Mechanically stuck TR vane with no berth track diversity
- Blockjoint failure with no phase relationship with adjacent track
- Rusty or contaminated rails
How is automatic train protection provided in a conventionally signalled area?
With the Trainstop and its engagement with the Tripcock on the train in the event of a S.P.A.D, applying the trains emergency breaks.
What is the effect of an earth fault developing in the pick path of a Trainstop proving circuit?
Earth fault would by-pass the load, causing overcurrent resulting in the fuse to blow.
With no BX supply to the Q coils the TR would remain de-energised. As the TR is in the signal selection of 2 signals in the rear they will be kept at danger.
What are the fundamental principles of signalling?
- Safe distance between trains
- safe passage of trains over points and crossings
What is the immediate effect of an earth fault in the track local coil?
- TQ fuse blows
- TR de-energises with no Q supply breaking front contacts
- As TR in signal selection of 2 signals in rear both will display danger aspect
How would you determine the cause of the fault with a TQ blown fuse fault?
- Get AWC
- Remove NX leg from circuit
- Reinstate fuse:
If blows = Earth
If holds = Short
How would you locate an earth fault in the Track local coil?
- Remove fuse – Reconnect NX
- Individually isolate parts of the circuit where possible (V head – 1 armature – Q1 – Q2)
- For each isolation reinstate fuse. When fuse blows it indicates fault lies between this and last isolation point.
What are position detectors used for?
- Route Exit Device: Train length clear from fouling point (set route ready before train has left L.O.C)
- Speed Control: 2 PD measuring speed
