2004 Track Circuit Calculations

[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