2004 Track Circuit Calculations

[jenni.joseph9]

Hi,

I have attempted 2004 Track Circuit calculations question.

The last bit of the question relates to the advantages of additional Relay end resistance.

Feed End resistance is used to operate the track in various ballast conditions but am not sure why the additional relay resistance is used for.

Please help and provide your valuable feedback/suggestion.

Thanks in advance.

Regards,
JJ

[jenni.joseph9]

Hi,

Can anyone look in to my post and provide feedback, please.

Thanks & Regards,

JJ

[Peter]

Hi,

I have attempted 2004 Track Circuit calculations question.

The last bit of the question relates to the advantages of additional Relay end resistance.

Feed End resistance is used to operate the track in various ballast conditions but am not sure why the additional relay resistance is used for.

Please help and provide your valuable feedback/suggestion.

Thanks in advance.

Regards,
JJ

I am not quite sure if there are some bits that you have carried over into these pages that you have calcualted elsewhere because I cannot see where you have got some of the numbers from. For instance, at the bottom of the first page, you say "rail to rail voltage = relay voltage + tail voltage" and go on to say "0.7 + 0 = 0.7". Where does the 0.7 come from. At pick up, relay voltage would be 0.04*9 = 0.36v and at DA, it would be 65% of this.

I don't have the question to hand to see if there is anything else given to help on this bit so if you can let me know, I'll be able to look into it a bit more for you.

Peter

[jenni.joseph9]

Hi,

Thanks for the reply.

As per the bullet no.1 mentioned in the question, I have taken the value as 0.7v. Please let me know if am wrong while considering the values.

Here is the question, for your reference please.

A d.c. track circuit is provided with an additional resistance at the relay end, connected in series with the relay coil. The characteristics of the track circuit are as follows.
Feed transformer rectifier output: 5.0 Volt
Relay coil resistance: 9.0 Ohm
Relay pickup current: 40.0 mA
Ballast resistance: 2.5 Ohm km
Length of track circuit 600 m
1. Calculate the value of the additional relay end resistance that will ensure a minimum rail to rail voltage of 0.7V when the track circuit is clear of trains.
2. Calculate the value of the feed resistance that will ensure reliable operation of this track circuit.
3. Calculate the drop shunt when the feed and relay end resistances have the values you have calculated.
4. Explain briefly the advantage of the additional relay end resistance.

Thanks in advance.

Regards,
Jenni.

[Peter]

Hi,

Thanks for the reply.

As per the bullet no.1 mentioned in the question, I have taken the value as 0.7v. Please let me know if am wrong while considering the values.

Here is the question, for your reference please.

A d.c. track circuit is provided with an additional resistance at the relay end, connected in series with the relay coil. The characteristics of the track circuit are as follows.
Feed transformer rectifier output: 5.0 Volt
Relay coil resistance: 9.0 Ohm
Relay pickup current: 40.0 mA
Ballast resistance: 2.5 Ohm km
Length of track circuit 600 m
1. Calculate the value of the additional relay end resistance that will ensure a minimum rail to rail voltage of 0.7V when the track circuit is clear of trains.
2. Calculate the value of the feed resistance that will ensure reliable operation of this track circuit.
3. Calculate the drop shunt when the feed and relay end resistances have the values you have calculated.
4. Explain briefly the advantage of the additional relay end resistance.

Thanks in advance.

Regards,
Jenni.

That helps a lot. Sorry, I don't seem to have a copy of 2004 paper. What you put down numerically is very good. You have used the correct numbers and appropriate formulae and get a sensible set of values.

One thing, note that 2. asks for the value of the feed reisitor that will give reliable operation. The value that you worked out was to give you the critical voltage on the rails that will just pick the relay when the TC goes clear. This would not really count as reliable so you should have either maded a qualitative statement like "for reliable operation, need ot make sure there is a bit more current, therefore pick a value x% lower than this" or build a margin into the calcs by saying that you want critical rail volts + (say) 20% for reliable operation = 0.84v.

Being the cruel tormentor that I am, I'll not answer your question directly on the purpose of the relay end resistance, but suggest that you run through the calculations for operating the TC without the extra resistor fitted (eg calculate rail volts and hence feed R then DS with just the relay (hint, the rail volts then equal relay volts)) and then try to explain what difference putting the relay resistor in there does and why this might be beneficial.

Peter

[jenni.joseph9]

Hi,

Thank you for the feedback.

I appreciate your mentorship and analysing the condition of the Relay end resistor is quite a good idea. I will do that and get back with the result.

I have a doubt regarding the feed end resistor.

Can the margin of 20% be included in every track circuit calculation, initially before starting the calculations?

Please help.

Regards,

Jenni

[Peter]

Hi,

Thank you for the feedback.

I appreciate your mentorship and analysing the condition of the Relay end resistor is quite a good idea. I will do that and get back with the result.

I have a doubt regarding the feed end resistor.

Can the margin of 20% be included in every track circuit calculation, initially before starting the calculations?

Please help.

Regards,

Jenni

My approach tends to be to work out the critical number and then comment, rounding the number to something sensible. 20% is arbitrary and you can argue for more or less. The important thing is to understand the magnitude - ie don't suggest a tiny fraction and don't suggest something that is too big. Get comfortable with what is a reasonable value and put words down to support that.

[jenni.joseph9]

Hi,

I have done the calculation with out the additional Relay end resistance.

I observed that the feed end resistance and drop shunt value increased and these may be result in a condition where the TC may not pick up automatically after the passage of train due to the high resistance.

The above condition can be avoided by including the Relay end resistance.

Please correct me if am wrong. Sorry for my delayed response.

Thanks in Advance.

Regards,
Jenni.

[Peter]

Hi,

I have done the calculation with out the additional Relay end resistance.

I observed that the feed end resistance and drop shunt value increased and these may be result in a condition where the TC may not pick up automatically after the passage of train due to the high resistance.

The above condition can be avoided by including the Relay end resistance.

Please correct me if am wrong. Sorry for my delayed response.

Thanks in Advance.

Regards,
Jenni.

I am slightly surprised that both the feed resistor and the DS increased when there is no relay end resistor. The former I would expect but the latter I would expect to be lower. (Please feel free to post copy of calcs for comment if you are unsure where things went awry). If you came up with such numbers, your conclusion would have been reasonable, but as that should not have been the case, the reasons I know of for relay end resistors are given below.

The provision of an extra resistor at the relay end on the face of things appears to degrade the operation of the TC but it used to be used where there were problems with lightweight vehicles not giving a good train shunt (ie you want the relay to drop with a higher value of drop shunt resistor in order to detect trains which do not make good contact with the rails). There is of course the trade off that doing this makes the track more susceptible to right side failures and also reduces the maximum length that the TC can be (the same effect as having long tail cables at the relay end).

The other reason I have seen quoted for relay end resistors is to avoid problems with residual voltage (a problem where a voltage is still present on the track when the track feed is removed which could hold up the relay). Putting the additional resistor in place effectively de-sensitises the relay because any voltage present on the rail is divided between the relay and its resistor.

In the UK, this arrangement does not tend to be used now, so if there is any other practice that others know about, perhaps they would like to share it.

Please don't beat yourself up about taking time to respond. This forum is for you to use at you own pace as you move through your studies.

Peter

[sugavanam nagarajan]

Purpose of adding adding Relay end Resistance:
-Ability to detect on poor rail surface & Reduce significance of interference voltage is it correct?

Hi,
I observed that the feed end resistance and drop shunt value increased and these may be result in a condition where the TC may not pick up automatically after the passage of train due to the high resistance.

Regards,
Jenni.

I am slightly surprised that both the feed resistor and the DS increased when there is no relay end resistor. The former I would expect but the latter I would expect to be lower.
Peter