The most recognized standard detailing machine safety system design integrity levels is ISO 13849-1 (EN 954-1) Safety Related Parts of Control Systems, which establishes five levels of safety circuit integrity, the lowest being Category B and the highest, Category 4.

Because Category 3 and 4 applications require the use of both redundancy and self-checking, they are often equated with the US standard of control reliability. However, in a Category 4 application, the requirement is to detect a fault at or before the next demand on the safety system.