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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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