(13-09-2016, 08:32 PM)ledwardz Wrote: [quote pid='6406' dateline='1473743304']
[quote pid='6405' dateline='1473716267']
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 out 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?
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1. Yes it proves that the circuit to the PPI actually goes via the relay (and in more complicated circuits involving parallel paths and combinations of many relays that the overall logic of ANDs and ORs is correct to the design.
2. In any testing one should always attempt to "prove free, prove locked, prove free again"- in that way one can be more certain that cause leads to effect rather than being a random coincidence. Also that the end function is driven directly from the item being changed, rather than item A affecting item B that then affects item C which actually then produces the output. You wouldn't necessarily realise that the function is driven off a pure repeat relay (but if it were then it really wouldn't matter too much); it is more to prove that the relay contact is being continuously proved rather than one that is proved "initial only" and that once it has been strapped that some other element remembers and therefore the feed to the output is maintained for longer than the time for which the contact were strapped (either a timer or until some other condition occurs to unset the memory)
3. Yes this would be the feed to the relay coil. A false feed to the relay coil only actually proves the relay is capable of operating when presented with voltage of correct magnitude (and polarity when critical), the contact is a "front" (or where applicable a 2back" and that contact and spade connections are not high resistance. It is however far better to "exercise" the system by manipulating things in that relay's coil circuit so that it gets a "legitimate" feed- in that way the whole system functionality is confirmed little by little and design errors can be found at this stage so that by the time the Principles tester arrives then it all "seems to work ok" and the Principles testing isn't continually interrupted by needing to finding and resolve faults. The Principles test needs to concentrate on finding the "conceptual errors" that means that the functionality isn't 100% ok- it should not get distracted into putting effort into getting the system to work at all.
4. As 2.
PJW