2001 Q1 FAILSAFE & OTHER TECHNIQUES

[PJW]

Hey all,

I've had a go at Q1 from 2001.

please have a look and give me any comments.

i did it in something approximating exam conditions (by hand) and then typed it up (without correcting any grammar!) although I probably spend 40mins or so on it.

I was hoping that others may comment, but since they haven't yet....

Not bad but probably a little too brief.

I am glad that having given the example of a track circuit as example of fail safe design that you later went on to explain that depending on the usage this may not be the case- certainly NOT failsafe for trying to detect the PRESENCE of a train. You probably should have explained how the desgn of the signalling system as a whole attempts to mitigate such risk (e.g. by explaining the rationale behind Raynes Park control).

Should also have mentioned that there are certainly some non failsafe failure modes of the TC.

When discussng advantages and limitations, your answer seemed to be comparing to achieving via a means of less safety integrity. However I think the question really intended you to compare with other means of achieving an equally high / higher safety integrity by using other techniques (duplication, redundancy, failure detection and negation of output etc).

Similarly in the section about overcoming the limitations I think you should have considered the practicability of achieving certain functionality in a failsafe manner. Your point re providing a degraded mode to "work around" the failure is certainly valuable but wasn't the whole answer and should also have been explained more in risk terms since this was a module 1 question (i.e. a system that can only achieve safety by failing to a restrictive state at the first hint of any abnormality within it, may in fact be increasing overall system risk because of the necessity of keeping trains moving and thus places great dependence upon the human which is often not the most reliable of components within the system and one that cannot be relied upon to operate in a failsafe manner)