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