[SystemSafety] Preliminary Hazard Analysis - Approaches

[nfr]

Consider the title - "Preliminary".
At the preliminary (very early) stage, there is not yet a design - and, very likely, not even a definitive specification - so there is not much on which to base a bottom-up analysis.
Doing a top-down analysis at this early stage, based on definable hazardous events, does not preclude one or more bottom-up analyses at a later stage, when there's more on which to base them. And nor does it suggest that other hazardous events will later be identified.


On 24 Apr 2013, at 11:51, M Mencke wrote:


Dear all,



I have seen different approaches to Preliminary Hazard Analysis in the railway sector.



I have come across a top-down method, which (roughly) involves the following steps:



Use of an “Example Railway Hazard List” based on the hazardous events modeled by RSSB’s Safety Risk Model. The hazardous events are classified according to the type of event. For example, Collision, Fire, etc.



The events on this list are linked to the functions of the system being analysed. For example, avoiding a collision between two passenger trains can be considered a function of the signaling system.



The functions identified are broken down into components, modules, etc., therefore linking them to the top hazard.



This sounds similar to a Fault Tree Analysis; my question is, is it useful to apply this approach during initial design stages?



>From my point of view, if you select the top hazards which you consider to be mitigated by your system from a list at the beginning of your analysis, you could overlook some significant hazards which may be generated by a (sub)system/function failure that you did not initially identify.



Personally, I prefer a bottom-up approach. It seems to me that by analyzing failures of system functions through the application of keywords, etc., effects on the subsystem and the system can be easily identified, whereas the other way round, you have to be very familiar with the effects in order to be able to work downwards.



Any opinions would be appreciated.



Kind regards,



Myriam.


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