Mock Test Questions Flashcards
Why is track opposition employed within capacitor fed track circuits ?
- In the event of a block joint failure at least one track circuit will fail, this is due to the tracks being in opposition and if the IBJ fails.
- The two oppositions will cancel each other out which will starve the TR. This will result in the signal changing to danger.
- If a block joint failure occurs between and a berth and replacing track it can result in the train stop raising and signal changing to danger, this means the driver will not have enough time to decrease the speed of the train which will cause a high speed SPAD.
What does fail safe mean ?
When a component fails leaving the signalling system in a safe state.
List 3 failing conditions that will leave T1R up and T2R down ?
- Short across T2R R Coil
- Open circuit in a Q coil of T2R
- Earth in the middle potential of T1R & T2R
How does the system protect against its consequences of a berth track relay becoming mechanically stuck up ?
- Berth track means the track will be wired in series with the signal in the rear or the track before.
- We will either have a TR-TR or TR-GR.
- Train at the most vulnerable position
- R coil wired in series
How is automatic train stop protection provided in a conventionally signalled area ?
Train stop & tripcock
What is the electrical characteristics of an anti opposition track ?
An Anti Opposition track does not have a phase relationship with the adjacent track
List 3 places you will find an anti opposition track ?
- Platforms - maintainers break
- Substation boundary - To mitigate phase relationship of current between substations
- +VE traction rail is transposed - continuous rail must always be with the +VE traction rail, to ensure continuity and dissipate fault currents.
Why are there 2 pigtails connecting the track wire to the rail ?
For redundancy
What size cable is track wire ?
0.85mm & 7 strands
How many bonds are required over a non-insulated rail joint on the continuous rail ?
For redundancy & capacity
What are the three reasons that can cause a floating failure
- Partial short/Earth
- Resistance in series
- Lack of capacitance
What action may be taken to increase current flow in the circuit without overloading the circuit the trackside transformer ?
Get authority (AWC) to change the t0-t1 to t0-t2 which will change the supply voltage from 100v to 125v
What documentation is required prior to any adjustment of the track circuit, and what test/checks must be carried out following any adjustment?
You will need an AWC to make any adjustments of the track circuit. You must then 1ohm shunt the track and test from T, Middle & TR end to make the relay de energise and energise.
What must be done when track conditions change ?
You must re monitor track relay energisation
List 5 pre installation checks you will undertake before replacing a D.E.V track relay ?
- Check impact detector to make sure it is not dislodged
- Do a visual check of the relay for any damage such as broken glass.
- Like for like - type of relay, frequency and lid contact arrangements.
- Overhauled within 3 years
- Check serial number or part number.
Why is the NX100 busbar connected to earth in a 7 way fuse bay ?
We earth the NX so an earth in the BX can become known by blowing a fuse which will make it self announcing. Without an earth in the NX, 1 earth would go unnoticed however 2 earths in the BX leg would bypass safety signalling which can cause a wrong side failure.
What checks must you do when installing a relay ?
- Ensure relay energisation is correct
- Lift Q2 and observe relay immediately to ensure it de energises and front contacts break.
- 1ohm shunt the track, observe relay to de energise.
- If TR/GR - lift R coil on GR and observe TR drop
Where are three places where a check rail must be used ?
- Sharp bends/curves
- Tripcock testers
- Points and crossings
What unsafe condition could potentially occur if a fan track was bonded incorrectly ?
- a train entering the fan track which passes over an incorrect bonding could cause the relay to stay energised.
- this can allow another train through and if the first train is not fully clear of the fan track it can result in a collision.
- it is important bonding is wired in series with the relay, this means if the bonding fails it will drop the relay. This is why we have two bonds for redundancy.
What is the effect of an Earth fault developing between 1F 315TR and ON contact 317V ?
- If an Earth fault developed it will blow the fuse as it would by pass the load (TR) as there is excess current in the circuitry.
- The TR would loose its Q Supply and de energise the relay.
What is the purpose of a train stop TQ circuit ?
- Trainstop proving TQ circuit proves the trainstop has come on after the passage of every train.
- If it does not come on, the circuit will prevent the track relay to energise as the track relay is in the signal selection of L.O.C of 2 signals in the rear.
- Both signals will be at danger keeping 1 working trainstop between trains.
