Disengaged by in control table

[oxfordjack1]

Can someone help explain what function of "disengaged by" in track circuits of signal controls?
What is relationship of "disengaged by" to entrance signal?
What relationship between "disengaged by" and wheel replacement?

Thank you.

[PJW]

In circuit terms it means "GSR de-energised"; in SSI terms it means "Signal stick unset".

For a controlled signal (which is not operating in auto mode) the signal is replaced to danger by the occupancy of any track in its route, but the first track section in addition "disengages" it. This means that subsequent clearance of that track does not permit re-clearance of the signal aspect. This means that after the passage of one train, then the signal is held to danger until the signaller takes deliberate action to permit clearance for a following train. This avoids a second train inadvertantly following the first when in fact it should have been routed elsewhere.

So the disengagement tracks are generally the berth track of the entrance signal and the track beyond that signal being simulataneously occupied, thus showing that a train is actually in act of passing it.

It is also usually the case that the occupancy of the first track beyond replaces the signal; however this is not always the case and the aspect replacement is in some situations deliberately delayed. Note the signal still becomes "disengaged" as above, but the aspect is maintained "off" for longer. Particularly the case with shunt signals and occasionally other signals where there is a specific need (far less frequently nowadays than in the past when double headed trains and/ or an assisting engine pushing at the back of the train were more common). This is what is called "last wheel replacement" contrasting with the much more common "first wheel replacement".

Can also be the case where there is regular use by steam locomotives (there is often a lot of length between the first wheels of the loco and the position of the driver) or where TPWS TSS loop has to be placed well beyond a signal due to site constraints (or else there would be self reversion and the train be stopped as a result of itself putting its signal to danger as it passes it)- often though this is just achieved by a small time delay within the controls.

Typically the entrance signal continues to display a proceed aspect despite having been disengaged whilst its berth track is still occupied (i.e. not all the train yet past it) and also tracks further along its route are clear (i.e. not maintained off indefinitely for a rediculous length of train- as a mitigation against the apparent occupancy of the berth track actually being a rightside track failure).
This is where the $40 referenece is used on the control tables to depict which of the tracks in the route are to be ignored whilst the "delayed replacement" condition is being satisfied.

Can someone help explain what function of "disengaged by" in track circuits of signal controls?
What is relationship of "disengaged by" to entrance signal?
What relationship between "disengaged by" and wheel replacement?

Thank you.

[oxfordjack1]

thank you for your help.
May I know the referrence book name?

[PJW]

I am sorry- I do not know to which "reference book" you refer.

If you mean what is the source of the information I have written then there is no one source, just an amalgam of stuff accumulated in my head over the past 35 years.

However if you read the information you had been studying that prevoked the question, you ought to be able now to understand it better and see that it makes sense.

thank you for your help.
May I know the referrence book name?

[oxfordjack1]

Thank you. I will do more exercises and read prevoked the question. Thank you very much.

[maliha]

Thank you for explanation.