When accidents occurring in a factory automation (FA) system are analyzed, it is found that many of them cannot be expressed in terms of probability. They include accidents occurring when an interlock did not function as designed due to the dead angle in the object detection sensor, and those caused by workers and maintenance crew who switched off the power supply of a safety device or entered the interior of the safety protection cage contrary to warnings. It is difficult to evaluate these accidents in terms of reliability alone. An evaluation method for the overall safety of a system is desired.
In this paper, a new evaluation method for the safety of the system for humans is described by means of a matrix consisting of situations and of the safety policy used to remove, nullify, or localize the hazard source. Each element of the matrix is systematically evaluated on a six-stage safety scale from 2 to +3, with 0 denoting a state realizable and attainable by effort and a state +1 a state that can be attained at present. This evaluation method is applied to accidents in FA systems, including nine usual situations, five maintenance situations, and two cases of third-party accidents. It is demonstrated that this method can be used for evaluation.