This trail of breadcrumbs has led us back to ISO 13849-1:2006, Safety of Machinery – Safety-Related Parts of Control Systems. This new standard is the basis for the PL and B10d ratings you see on many safety devices today. The ratings are ranked “a” through “e” in increasing risk to the operator, with “e” being the greatest risk. Within this standard, the EN-954 categories for circuit types survive, but are only part of the implementation. More common-sense approaches are allowed, taking into account variables such as mean time to dangerous failure (MTTFd) for devices, monitoring devices for failure, circuit types (cat.1-4), and even those hazards which cannot be guarded without impeding the work to be done (such as PPE, signage, training, etc.). All of these are on the table if the situation supports them. ISO 13849-1:2006 was developed with the support of ANSI, as this organization supplied representative engineers to help with development of this standard. ANSI is a contributing member of ISO standards development and adoption boards.