Reduced overlap

[Nandhakumar]

Hi PJW,

Could you please explain the meaning of Reduced overlap supported with circuits?

[PJW]

(09-09-2010 07:13 AM)Nandhakumar Wrote:  Hi PJW,

Could you please explain the meaning of Reduced overlap supported with circuits?

The modern use of the term reduced overlap is merely an overlap shorter than a full overlap (generally 180m but on certain higher speed lines 225m) that can be justified as suitable in the circumstances. This is generally a function of permissible speed but also takes account of the site conditions and the effectivelness of train protection. There are no special circuits; it is just that the overlap track circuit is not as long as otherwise it might be and points lying outside the reduced overlap are not locked. There would not be a full overlap for that signal.

I suspect you meant what is nowadays called a restricted overlap. This is ADDITIONAL generally to a full overlap and is selected for use when the full overlap would be restrictive to other traffic. The signal in rear authorising moves up to the signal having the ROL, is provided with a Warning Route; this just proves a sub-set of the full overlap controls but imposes quite stringemt approache release of the aspect to ensure that the train passes it slowly. The only special circuits affect the subsequent "stepping up" of the Warning route to the Main route in circumstances in which the forward route from the exit signal is latter set and thus commits the full overlap for the signal in rear.
Basically the (W)RLR has to be dropped the (W)NLR picked then the (M)RLR picked. The important thing is that the aspect isn't replaced during this; the reason why
a) have to prove that the full overlap ASPECT conditions are satisfied
b) In RRI only: the signal hasn't actually yet cleared (in SSI the change of the class of route can be exerchanged in a single instruction before the message to the TFM is generated and thus "invisible").

[Alasdair]

We were discussing the stepping up of warner routes today.
Is the only way of stepping up a warner to main by setting a route from the exit signal, or can the signaller overset the main route on top of the warner?

[PJW]

(09-09-2010 08:54 PM)Alasdair Wrote:  We were discussing the stepping up of warner routes today.
Is the only way of stepping up a warner to main by setting a route from the exit signal, or can the signaller overset the main route on top of the warner?

The quick answer is only by forward route setting.
If the Warning route is set then the Main is prevented from setting (think about it; on the Control Table for the (M) woute one of the entries is he associated (W) and (C ) are route normal.
Of course if the signal has not yet cleared the (W) route won't be approach locked and thus if signaller wanted to chnge their mind then could pull entrance button then push again and select the Main exit and achieves their aim.

If signal has already cleared then (in RRI at least) the step-up has to be prevented to avoid a brief aspect reversion; it is really of little significance anyway gven that the train will be almost at a stand about to pass the signal in rear- too late to be of any benefit.

The Western Region traditionally only had one class of route with the selection such as it was at aspect level. In this system the "Main" with a full overlap is the default, but a "Delayed Yellow" broadly equivalent to a "Warning" results if the full overlap is not available but the shorter overlap is. The signalleer can do this deliberately by keying some points to the conflicting position; they then always have the choice to move them to the full overlap position and they then become locked in that position and the aspect clears and effectively has become a "Main". Hence in this case it is a short of equivalent to step-up.

[Nandhakumar]

(09-09-2010 01:28 PM)PJW Wrote:  

(09-09-2010 07:13 AM)Nandhakumar Wrote:  Hi PJW,

Could you please explain the meaning of Reduced overlap supported with circuits?

Thanks Mr.PJW.

[PJW]

(13-09-2010 05:53 AM)Nandhakumar Wrote:  

(09-09-2010 01:28 PM)PJW Wrote:  

(09-09-2010 07:13 AM)Nandhakumar Wrote:  Hi PJW,

Could you please explain the meaning of Reduced overlap supported with circuits?

Thanks Mr.PJW.

Thought I'd answered this before, but can't find it- think may have lost internet connection to website when posting previously, so apologies for the wait.

A reduced overlap is only an overlap of less than a full overlap length (typically 180m, but to modern standards and particularly when over 100mph 225m) justified by virtue of a risk assessment due to relatively slow approach speed, site conditions etc meaning that likelihood of SPAD and consquences of SPAD are both regarded as acceptable. There are no special circuits. it is provided INSTEAD OF a full overlap.


I expect that you are really thinking of what is nowadays called a restricted overlap - confusingly this was called a reduced overlap in the past (and those of my generation and older always have to do a "double take" to ensure we use the right terminology).
This is provided IN ADDITION TO a full overlap.
At aspect level the circuits are the same except for the fact that a shorter length of overlap applies an d therefore it proves fewer track sections and probably a lower number of points than are within the full overlap. Typically but not always there will be one set of triling points within the full overlap which are excluded from the restricted overlap.
A restricted overlap is associated with a WARNING class route at the signal9s) n rear that read up to the exit signal having the ROL.

The "special circuits" are:
a) the (W) NLR/RLR which are basically similar to the normal (M)NLR/RLR (apart from the smaller extent of what is proved)
b) the (W) NLR/RLR is cross-proved against the (M)NLR/RLR and vice versa
c) various controls which make the (W) step-up to a (M) when the forward route is set, the full overlap aspect conditions are satisfied and the signal itself is not approach locked. Actually this condition is only strictly needed for RRI [to prevent the aspect reverting briefly- since it takes a finite time for the route relays to change state and the cross-proving prevents the (M)NLR being unlatched until the (W)NLR has been latched up again]. In SSI the class of route can be exchanged within the execution of a single instruction which is executed between the output telegram being calculated for the next cycle of the interlocking- the signal remains off as the TFM only gets periodic updating on each major cycle of the SSI and remains displaying an aspect as previously calculated for a duration not exceeding 1 second and hence the reversion prblem would not occur.

I am not sure that providing circuits would actually be very useful to aid understanding- they are obviously site specific and I hope that my explanation above assists. If however you find a diagram more useful than a wordy explanation let me know and I'll try to look out / construct a typical when I have more time

[Nandhakumar]

(26-09-2010 12:10 AM)PJW Wrote:  

(13-09-2010 05:53 AM)Nandhakumar Wrote:  

(09-09-2010 01:28 PM)PJW Wrote:  

(09-09-2010 07:13 AM)Nandhakumar Wrote:  Hi PJW,

Could you please explain the meaning of Reduced overlap supported with circuits?

Thanks Mr.PJW.

Thought I'd answered this before, but can't find it- think may have lost internet connection to website when posting previously, so apologies for the wait.

A reduced overlap is only an overlap of less than a full overlap length (typically 180m, but to modern standards and particularly when over 100mph 225m) justified by virtue of a risk assessment due to relatively slow approach speed, site conditions etc meaning that likelihood of SPAD and consquences of SPAD are both regarded as acceptable. There are no special circuits. it is provided INSTEAD OF a full overlap.


I expect that you are really thinking of what is nowadays called a restricted overlap - confusingly this was called a reduced overlap in the past (and those of my generation and older always have to do a "double take" to ensure we use the right terminology).
This is provided IN ADDITION TO a full overlap.
At aspect level the circuits are the same except for the fact that a shorter length of overlap applies an d therefore it proves fewer track sections and probably a lower number of points than are within the full overlap. Typically but not always there will be one set of triling points within the full overlap which are excluded from the restricted overlap.
A restricted overlap is associated with a WARNING class route at the signal9s) n rear that read up to the exit signal having the ROL.

The "special circuits" are:
a) the (W) NLR/RLR which are basically similar to the normal (M)NLR/RLR (apart from the smaller extent of what is proved)
b) the (W) NLR/RLR is cross-proved against the (M)NLR/RLR and vice versa
c) various controls which make the (W) step-up to a (M) when the forward route is set, the full overlap aspect conditions are satisfied and the signal itself is not approach locked. Actually this condition is only strictly needed for RRI [to prevent the aspect reverting briefly- since it takes a finite time for the route relays to change state and the cross-proving prevents the (M)NLR being unlatched until the (W)NLR has been latched up again]. In SSI the class of route can be exchanged within the execution of a single instruction which is executed between the output telegram being calculated for the next cycle of the interlocking- the signal remains off as the TFM only gets periodic updating on each major cycle of the SSI and remains displaying an aspect as previously calculated for a duration not exceeding 1 second and hence the reversion prblem would not occur.

I am not sure that providing circuits would actually be very useful to aid understanding- they are obviously site specific and I hope that my explanation above assists. If however you find a diagram more useful than a wordy explanation let me know and I'll try to look out / construct a typical when I have more time

Thanks PJW.