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Back Contacts in timer relay
#1
Hello, please the Signalling professors of the forum for some explanations.

I need a clarification about the use of back contacts either for energizing or releasing timer relay mainly used in overlap.

In RRIs, is it safe to use back contacts for energizing or releasing timer relay? Is there any specification that prohibits the use of back contacs for energizing timer relay? What is the major problem for using back contacts?

I will be very happy if you answer me with as many explanations you could.

Thanks
Konstantinos
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#2
(12-06-2019, 01:39 PM)kochronaki Wrote: Hello, please the Signalling professors of the forum for some explanations.

I need a clarification about the use of back contacts either for energizing or releasing timer relay mainly used in overlap.

In RRIs, is it safe to use back contacts for energizing or releasing timer relay? Is there any specification that prohibits the use of back contacs for energizing timer relay? What is the major problem for using back contacts?

I will be very happy if you answer me with as many explanations you could.

Thanks
Konstantinos

The problem about using back contacts is that everything is designed so that failures overwhelmingly will cause the relay to adopt the de-energised state and thus “fail-safe”.  By using a back contact to give a release, then you defeat that. 
In reality there are 3 states-  clear, occupied and “don’t know, but must assume to be occupied”.  By driving the timer from the unknown state then you will cause the release to be given even if it is actually clear, so there may not actually be a train where you think it is.  This would be a WRONGSIDE FAILURE.

However you could mitigate the risk by designing the overall system to make it to be a PROTECTED WRONGSIDE FAILURE. Provided  that you proved that the release had not been given prematurely given, by proving the timer not timing in the controls of the signal reading up to the one for which the approach release is being used, then the overall scenario could be made acceptably safe.  It is a matter of understanding the hazards and the associated risks of the design , attempting to mitigate and ensuring that the residual risk is acceptably low.

If the train detection is a track circuit, this is designed to be a vacancy detector which is what is appropriate for its primary role.  It really does not matter if you use a front contact made to prevent the timer from timing, or a back contact made to start the timer timing- anything that caused the track to drop will start the timer.  Hence use of a back contact of a TPR has to be acceptable because there is nothing better you can do without providing a different form of presence detector .Conversely if you use a back contact of a repeat relay to operate the timer, then you have increased the number of failure modes that could lead to a false release being given.  Using a front contact of the TPR to permit a signal to clear whilst using a back contact of a repeat relay to operate the timer to implement approach release could mean that one blown fuse might give an unrestrictive aspect sequence on the approach to a junction signal that has been falsely cleared too early and the train driver finding out too late that the train has been routes over a slow speed set of points, being unable to brake in time and then derailing.

It is all about understanding the CONTEXT of the situation, rather than having a very black and white rule; however you should always be cautious when using back contacts of relays to be clear why you are needing to do so and think it through enough to know that it is ok.
PJW
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#3
Hello, i'm coming back!!!

If a signal has a facing point which is near of it and is detect and locked for overlap purpose and the timer relay is failed to start, what has to be the reaction of the system?
For example until changing the timer the signal cannot take a procced aspect?
Is the any typical circuit?
How is the general idea for Qrj1 timer relay when is faulty? Is there any way to understand the station master than the reason you cannot set the route is from overlap purpose?

Thanks a lot!
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#4
kochronaki Wrote:Hello, i'm coming back!!!

If a signal has a facing point which is near of it and is detect and locked for overlap purpose and the timer relay is failed to start, what has to be the reaction of the system?
For example until changing the timer the signal cannot take a procced aspect?
Is the any typical circuit?
How is the general idea for Qrj1 timer relay when is faulty? Is there any way to understand the station master than the reason you cannot set the route is from overlap purpose?

Thanks a lot!
The treatment of facing points in the overlap varies between different railways and over vintages of standards. 
I am not sure if your question is really asking
a) about the detection of the points in the aspect of the signal reading up to the exit signal beyond which they are situated, or
b) but the fact that the points are subject to Time of Operation Locking. 

For b) when the berth track of the exit signal becomes occupied they become locked in their position until that track has been occupied so long that the train must be assumed to have stopped and therefore won't SPAD. 
In that case if the timer doesn't start then the points will remain locked indefinitely until the berth track is cleared.

For a) you'd be talking about a different timer designed to prevent the signal reverting to danger whilst the points are moving from one lie to the other; in that case if the timer didn't work then as soon as the initial detection of the points were lost then the signal would go back to danger.

Please express your question again and I'll try to understand what you really want to know
PJW
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#5
Let me try again.

Assume we have a timer (QRJ1 relay) that is used to measure the time before one can cancel a set route. This timer begins counting as soon as the train passes approach lock.

Question 1. How should the system react if the timer malfunctions when the train passes the approach lock section?
Question 2: Should the system be designed to specifically account for this situation?
Question 3: Are there any specifications about these types of timers with respect to which contacts to use (back or front)?
Question 4: Do the same apply for timers used in overlap (to release facing points)?

Thank you for your patience with my question.
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#6
Please anybody for help ?
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