2007 Track circuit calculation, Q3

[Peter]

I do not think you would have got many marks for that part of the question because you were not going down the right lines but just getting the numbers wrong, you were doing some good numerical sums but not for the right reason. Don't be put off though. I have found with many things in engineering and maths, once you figure out what it is that you are supposed to be working out, it gets a lot easier, and it is far better to learn by trying to work it out with some guidance than to be just told the answer. If that was your first ever attempt, you have some good foundations for future tries.

Have you looked at any of the attempts for some of the other questions to try to get an understanding of the way to approach things?

If you can draw out a sketch of the circuit and mark things on it and post that as a scan, we might be able to give you some pointers on where your thoughts are going wrong.

Peter

[Archie]

I have had a look at others, just trying to jump into what is going on is a little tough, but given time I am sure I will be fine.
As you have suggested about going down the line of drawing a sketch I will attempt, and try to finish this 'drop shunt' calculation off. I don't mind making the mistakes if it brings me closer to where I need to be.
I will try and post at some point, probably won't be able to do so till early next week due to work.
Many thanks Peter

I do not think you would have got many marks for that part of the question because you were not going down the right lines but just getting the numbers wrong, you were doing some good numerical sums but not for the right reason. Don't be put off though. I have found with many things in engineering and maths, once you figure out what it is that you are supposed to be working out, it gets a lot easier, and it is far better to learn by trying to work it out with some guidance than to be just told the answer. If that was your first ever attempt, you have some good foundations for future tries.

Have you looked at any of the attempts for some of the other questions to try to get an understanding of the way to approach things?

If you can draw out a sketch of the circuit and mark things on it and post that as a scan, we might be able to give you some pointers on where your thoughts are going wrong.

Peter

[Archie]

P.S.- one thing I forgot to ask is that I have applied to study the Mod 5 exam, but what is the content of the study pack, does it go over in detail of how to approach such calculations as track circuits etc?

I have had a look at others, just trying to jump into what is going on is a little tough, but given time I am sure I will be fine.

Have you looked at any of the attempts for some of the other questions to try to get an understanding of the way to approach things?

Peter

[PJW]

P.S.- one thing I forgot to ask is that I have applied to study the Mod 5 exam, but what is the content of the study pack, does it go over in detail of how to approach such calculations as track circuits etc?

I have had a look at others, just trying to jump into what is going on is a little tough, but given time I am sure I will be fine.

Have you looked at any of the attempts for some of the other questions to try to get an understanding of the way to approach things?

Peter

I think thisis one of the better Study Packs, but don't think of any of them as text books / "how to pass the examination" step-by-step instructions. They are not generally designed to teach the subject; rather than to give examples of previous questions and including activities designed to get you to think about things that you may not have otherwise thought about.

In this case there is one worked example as a model answer [1996 Question 6] and a selection of other past questions, a few of which are supplemented by some specific comments. However you'll find more on this website- the examples here aren't perfect but you can actually learn from other people's mistakes!

[Archie]

P.S.- one thing I forgot to ask is that I have applied to study the Mod 5 exam, but what is the content of the study pack, does it go over in detail of how to approach such calculations as track circuits etc?

I have had a look at others, just trying to jump into what is going on is a little tough, but given time I am sure I will be fine.

Have you looked at any of the attempts for some of the other questions to try to get an understanding of the way to approach things?

Peter

I think thisis one of the better Study Packs, but don't think of any of them as text books / "how to pass the examination" step-by-step instructions. They are not generally designed to teach the subject; rather than to give examples of previous questions and including activities designed to get you to think about things that you may not have otherwise thought about.

In this case there is one worked example as a model answer [1996 Question 6] and a selection of other past questions, a few of which are supplemented by some specific comments. However you'll find more on this website- the examples here aren't perfect but you can actually learn from other people's mistakes!

Right hello again, I am back as promised, with a revised attempt at the 'drop shunt' calculation. Please see what you think, and comments greatly received!!!

-PU current plus 10% for reliability=121mA
-DA current = PU current x 68% of PU current
121x0.68=82.28mA
-DA voltage = DA current x relay resistance
0.08228x20=1.645 volts
-Current supplied by feed = feed voltage-DA voltage/relay resistance
(10-1.645)/7=1.193A
-Ballast current = rail voltage/TC length
2.2/0.69=3.188A
-Current through 'drop shunt' = ballast current-current supplied by feed-DA current
3.188-1.193-0.08228=1.91272A
-Value of 'drop shunt'=DA voltage/current through shunt
1.645/1.912=0.86 Ohms

As quoted from the 'SMS, Signalling Maintenance Specifications,' minimum drop shunt value=0.5 Ohms, Preferred drop shunt value 0.8-1.0 Ohm

So this answer given falls into the 'preferred' drop shunt value.

Fingers crossed!!!

[Peter]

Right hello again, I am back as promised, with a revised attempt at the 'drop shunt' calculation. Please see what you think, and comments greatly received!!!

OK. One thing that would help a lot is if you could write down the initial conditions from the question so that we know whether or not you have realised that something is included or not. The reason being that the question at the start of this thread refers to a feed end relay being fitted. Your calculations have not included this which, numerically, does not make things wrong, but in terms of answering the question, it does not quite fit the bill. Anyway, on with what you did do.

-PU current plus 10% for reliability=121mA
-DA current = PU current x 68% of PU current
121x0.68=82.28mA
-DA voltage = DA current x relay resistance
0.08228x20=1.645 volts

OK so far.

-Current supplied by feed = feed voltage-DA voltage/relay resistance
(10-1.645)/7=1.193A

Numbers correct but you mean "Feed resistance" not "relay resistance" in your description. Apart from that all OK

-Ballast current = rail voltage/TC length
2.2/0.69=3.188A

Oops.
Several alarm bells here.
1. Dimensions: you are seeking to calculate a current - you won't get amps by dividing a voltage by metres.
2. The value you have used for rail volts is the value that would be present at pick up given the numbers in the original question (110mA through 20ohm relay). You have already said that we are looking at the drop away point and you calculated the rail volts at this point as 1.645v above.
3. Where did you get the 0.69 from?
4. Magnitude of the answer. Think about what you are calculating. From your words, it is the element of the current that leaks through the ballast. How, therefore, can this be bigger than the total current supplied from the feed.

-Current through 'drop shunt' = ballast current-current supplied by feed-DA current
3.188-1.193-0.08228=1.91272A

Right sort of words, but wrong numbers (and remember the point I made above about the feed end relay)

-Value of 'drop shunt'=DA voltage/current through shunt
1.645/1.912=0.86 Ohms

As quoted from the 'SMS, Signalling Maintenance Specifications,' minimum drop shunt value=0.5 Ohms, Preferred drop shunt value 0.8-1.0 Ohm

So this answer given falls into the 'preferred' drop shunt value.

Fingers crossed!!!

Have a look at those bits and try to annotate your steps with a bit more of an explanation to we can understand your thinking a bit more. Don't lose heart, you are starting off well.

Peter

[dorothy.pipet]

FYI