Method for Evaluating the Dependability of Factory Automation Systems in View of the Capability of Safety Devices
The important factors for dependability are safety and reliability. When dependability is evaluated, in view of safety along with the probability of an accident occurring, both the degree of accident damage and the capability of installed-safety devices should be included as items to be evaluated. To establish a barometer to quantitatively determine the dependability of a system, a study to standardize and rationalize the safety index is currently under way. However, to standardize the Tree Analysis for Safety (TAS) which is used to determine the safety index of a system, TAS has been unable to deal with the degree of damage occurring in accidents and has not been able to cope with a wide range of accidents in a consistent manner. For a TAS standard that can be applied to accidents, we propose that a mechanism, to select safety devices applicable in accordance with protection capabilities, be built into TAS by identifying the upper limit of the protection capability of safety devices by considering the potential amounts of hazard potential factors which greatly affect the degree of accident damage. We have applied this system to accidents during normal hours and to maintenance in Factory Automation systems, and have confirmed that this system could be used as an evaluation method.