WEEK 1 Flashcards
What is the purpose of a track circuit?
To detect the absence of a train.
What voltage is A.C main supplied at?
600V AC
What are the voltages for:
- Track winding?
- 7-way bay?
- 100V and 125V
- 100V and 110V
Why is the BX selection a requirement of signalling circuits fed from the 7 way fuse bay?
- If we had an Earth fault, it would blow the fuse.
- This would be a self-announcing, right-side failure, protecting the circuit.
- If we did not do this and the NX was selected, and an Earth fault occurred, it would be undetected.
- If 2 Earth faults occurred, it could bypass safety critical equipment, causing a wrong-side failure.
What frequencies are the A.C main supplied at, and would colour relays would be in these areas?
33 1/3Hz = Red
125Hz = Silver
Why are there 2 pigtails connected to the rail?
Redundancy (back-up in case there was a fault with one).
Why are there 2 bonds for non-insulated rail joints?
Capacity (the continuous rail must always be next to the positive traction rail, to dissipate fault current).
What size cable is track wire?
7 strand, 0.85mm
List the 4 main functions of a capacitor?
- Acts of a load, is a current limiting device.
- Variable - we can adjust the capacitance depending on the rail conditions.
- Creates a 90° phase angle, for optimum relay operation.
- Blocks DC, only allowing AC.
How far into the green area of the vane indicator should be for:
1. Tunnel
2. Outside
3. Both?
- 1/3
- 2/3
- 2/3
What is the maximum capacitance for:
1. 33 1/3Hz?
2. 125Hz?
- 40 µF
- 10 µF
What are 3 reasons for a floating failure, and how would you test for them?
- Lack of capacitance - a clip and lift test would read 100V across R3 and R1. Low volts would be read at the output of the capacitor. Adjust capacitance.
- Partial short or earth - a clip and lift would read 0V. Current clamp is used throughout circuit to detect.
- High resistance in series - a clip and lift would read 100V. High volts would be read at output of capacitor. Voltmeter is used throughout circuit to detect.
What can be done to increase current flow in the circuit, without overloading the trackside transformer?
The t0 - t1 (100V), could be changed to t0 - t2, which would now feed 125V, which would increase current flow.
What 3 things must be done when any adjustment of the track circuit is made?
- Obtain AWC prior to any adjustment.
- 1Ω shunt the track at the T-end, middle, and TR-end, and every metre across points and crossings. Must ensure relay de-energises and front contacts break.
- The energisation levels must be correct.
What is the track circuit limitation?
10m - 600m
What is the purpose of an isolating transformer?
To remove the Earth from the supply.
What loads are fed from the 7 way fuse bay?
• Signal selection
• Q coils on relay
• Repeater circuitry
• Lighting
What is a berth track?
The track before a signal, which is a vulnerable place for a train, as it it the only place where a train can be stationary. We provide extra protection through berth track diversity: TR-GR and TR-TR.
What is TR-GR?
The R coil of the berth track relay is wired in series with the R coil of the signal relay in the rear. In the event of the berth track relay becoming mechanically stuck up, the signal relay would still de-energise and go to danger.
What are 3 reasons for NOT using TR-GR?
- The distance between TR and GR is too far, so would result in too much loss.
- If it was not a DEV relay.
- If there is more than 1 signal in the rear.
What is TR -TR?
The berth track has 2 track relays: the R coils are wired in series with each other. Both relays are detected in the signal selection for the signal in the rear.
