(18-05-2010, 03:27 AM)KonduriRaghavakumar Wrote: I would like to clarify wrong side failure means that failing of the interlocking which results to unsafe, generally we employ only the fail safe sytems, hence there may be error in wiring or circuit.
Remember that systems that are designed to be failsafe do not always do so. This may be due to an error in design that got through check and test. It may be through a completely unanticipated (or perhaps incorrectly judged to be incredible) failure mode of a piece of equipment perhaps arising through age derioration, being subjected to greater than anticipated stress etc. There is aways the possibility of failure induced unintentionally by careless maintenance.
Possibly a failure may result from an error in circuit design or in data in particular through some start-up or other unusual transitory condition that hadn't been adequately considered. Can be problems at interfaces between different systems, particularly in unusual failure modes.
Never overlook the human; sod's law dictates that when something has been designed to be fool-proof that evolution eventually ensures that a more complete class of fool evolves to bypass any precaution. In the quoted example the train detection could be assumed to be via axle counter and this could have been inappropriately reset; hence the section, though occupied, would be showing clear. In the UK now we ensure that the protecting signal is still held at danger for the first train on the affected route(s) after any such reset; however this wasn't always the standard.......This is called "Aspect Restriction Following Restoration" and there seem to be some places where this is regarded as a nuisance and a means to "get rid of it quickly" by running a train around the layout to do the bare minimum to satisfy the locking (whilst not actually achieving all of the prime objective to physically prove the totality of routes are clear of foul vehicles) has become practice!!
PJW