Guidelines for handling secondary risks This case study includes examination of ways to defend against three types of hazards:
The guide proposes adding resilience features for indicating hardware fault, Apparently, such indicators were not added to all the valves. In particular, it was not added to the valve that controlled the entry of water to the instrument air line.
Guidelines for handling secondary risks
The additional components required to
alert about
component failure (sensors, algorithms, displays, sound alarms) are not only
costly, but also risky, because they are liable to fail, providing opportunities
for new kinds of incidences (as was the case with the PORV failure in the TMI
accident).
Guidelines about reducing the system complexity
Detecting faults in the fault indicators
Fault indicators are liable to fail, and it is
important to notify the operators when this happens. It is important to
distinguish between the case of
component failure and that of indicator failure.
The guide recommends adding
secondary indication of failure for each of the
primary indicators, and provides tips for adding the
secondary indicators
without adding to the system complexity, by coding. Typically, when the
indicator fails, it does not send signals to the control unit. Therefore, a most
effective way to detect faults in the indicators is by designing the indicators
such that they always send signals about the component state, whether it
functions properly or not.
Guidelines about detecting faults in the fault indicators
The guide proposes that the
mode violation is detected in the
scenario center, by checking the
scenario rules, and providing immediate
feedback to the operatos.
The guide suggests that
mistakes are due to insufficient specification of the possible situations
The guide proposes that the possible
situations should be analyzed and transformed into rules, stored in the
Situation analyzer.
Improper operator's actions can be detected by the activity center, as deviations from the rules specifying proper action.
Guidelines for hazard visibility assurance
Updated on 25 Jun 2016.