(18-08-2010, 11:28 AM)jenni.joseph9 Wrote: Hi,
I tried to have a go at the Track Circuit FTA as per ur comments.
Attaching those sheets and I couldn't find a scenario where I can apply AND gate.
Can you please have a look and let me know the comments.
Thanks in advance.
Regards
You can introduce an AND gate when you think of something that will create a failure, but not all by itself.
For example:
a) the track insulations may be quite bad but the track circuit will still work fine UNTIL it rains and the ballast gets wet
b) a double tail track circuit in a non-electrified area has a pair of IRJs approximately opposite each other separating it from the adjacent track. Should one IRJ fail the two tracks will continue to operate properly but just share a common rail (as they would anyway in an electrified area where there is a traction return rail); however should the second of the pair of IRJ fail then at least one (and probably both) of the tracks will fail
c) where the track leads are duplicated, if one gets severed then the track continues to operate almost normally; if however the second one also fails before the first is replaced, then the track fails
d) if there are rusty rails then a lightweight vehicle might be undetected and the track circuit Show Clear When Occupied- a wrong side failure. However a longer heavier train is most unlikely to be lost; indeed the probablility of a lightweight train being detected is significantly increased when it is stationary compared to when it is moving
The things that I liked about your Fault Trees
1. Complete separation of the rightside and wrongside failure scenarios
2. The initial breaking down of the "Track circuit drops" into constituent high level causes: No Power at Feed / Insufficient current for relay to pick / open circuit between feed and relay.
The things that were not so good:
1. Whereas considered what might cause a 110V fuse to blow in a lot of detail, rather glossed over the feedset and relay themselves
2.Not sure what relay contact you are envisaging could have become high resistance that would cause track relay to drop. I guess that you are not assuming cascade track circuits (and if so you should have explained) so the only contact would be of the TR itself that would then cause the TPR to be down- this is a reasonable fault mode to consider but would have to be incorporated higher up your fault tree as it is an alternative fault than the track relay itself being down (yet the interlocking and signaller would see it ias a track circuit failure)
3. It is generally best not to combine too many legs into the same "gate" but to split into two or three at each stage, gradually getting into more detail as progress down the page.
The idea of the Fault Tree is to show the relationship between various faults and the failure which can result and indeed be able to plug in numeric valuesat the foot of the tree to calculate the probability of the failure at the top.
4. I didn't understand why "tail cable severed" fed directly into the top event rather than being counted as a "bad connection"
.
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By coincidence Andy Witton produced a Fault tree for use at Derby last weekend for the rightside failure scenario; I'll add here for comparison
PJW