Strap and Function Testing

[ledwardz]

Hi There,

I am relatively new to the rail industry working primarily on Manchester Metrolink and do not have a background in testing. I am wondering if someone can explain strap and function testing to me. Internet sources regarding the subject seem convoluted.

as an example;

If i place a voltage on a set of motorised points detection relay (so in effect mimiking that the points are set in reverse) and then monitor the PPI position is this strap and function testing?

Would this type of test be done by a functional tester or a principle tester?

I imagine its a functional tester as it only tests one small part of the system and verifys that the circuit is built to design rather than validating that the design is fit for purpose and safe to signalling standards.

If this is a functional tester then what is the principle tester responsible for? does he check any part of the electrics or is he more concerned with the 'function'. e.g If my interlocking is set to a particular route do the protecting signals reflect this? Would he test axle counter points blocks to ensure the points remain locked until the block is traversed and that they are positioned in order that the points have enough time to throw? (again this seems like functional test)

Basically, if someone could give me a principle testing example explained at a basic level i'd appreciate it.

Thanks,

Lee.

Assuming that a PPI is a Points Position Indicator driven from a point detection relay, the strap and function would be:

1. with point detection relay down, bridge our with a strap (i.e. one of a known number of clearly identified items which is electrically conductive to short out the contact at its terminals) the front contact of the relay, to check get correct voltage feed to the PPI and it responds accordingly, 
2 remove and check returns to original state,
3. now false feed relay to get the feed again,
4. remove and check back to original.

Obviously where there are more contacts in circuit or the circuit is double cut or polar, then a bit more complicated!  Item 1 is the "strap" and item 3 is the "function".
This is undertaken by a functional tester.



A Principles tester in essence checks the completed system operates as intended and in most cases would not concern themselves with the operation of the PPI.  So you are broadly correct; they would check that a signal only show a proceed aspect when all the necessary points set in the correct positions and that the points were held in those positons until the train had gone sufficiently far that the locking is no longer needed and that they are then free to move again.  It is a "functional test of the whole system"- the difference is that it relies on knowledge and experience to know which points are to be locked by which routes in what circumstances and the application of that generic knowledge to the particular site arrangements rather than checking the implementation directly to the design or any standard check list. 

Certainly in very simple situations (such as an automatic block signal that demands one block section clear and the signal ahead alight in a very standard manner), there is a case for saying that there is no need for a Principles tester to perform the activity which could be done perfectly well by a functional tester- the only caveat is that the decision that it truly is a "bog standard" case with nothing special is itself a Principles decision, so it does need a prior review but the Principles tester has done their bit by making that decision and delegating the defined task to a suitably qualified functional tester. Perhaps on Manchester Metrolink there is much in this category without the complexity that there is at many NR sites and this may be one reason why the distinction isn't obvious to you.

Does that help?

Hi PJW.

Thanks for taking the time to respond. That has clarified my understanding of a principles testing engineer. still not really sure if i have understood the strap and function testing though. I have wrote below why i think each part of the test is done.

1. with point detection relay down, bridge our with a strap (i.e. one of a known number of clearly identified items which is electrically conductive to short out the contact at its terminals) the front contact of the relay, to check get correct voltage feed to the PPI and it responds accordingly,
This just checks the PPI circuit is as per design and that that contact of the relay is doing the expected function?

2 remove and check returns to original state,
This ensures there is no other feed to the PPI that shouldnt exist?

3. now false feed relay to get the feed again, (you mean the actual coil and not the contact this time?)
This ensures that the relay is functioning correctly?

4. remove and check back to original.

This ensures that there is no other feed to the relay that shouldnt exist?