In engineering, fault tolerance means the property of a technical system to maintain its functionality even when failures and error conditions occur. Fault tolerance increases the availability of a system. This paper describes a way to implement fault-tolerant safety functions that allow a machine or system to continue to operate under certain fault scenarios without neglecting the requirements for personal protection.
Part I of the white paper describes the theoretical basis for operation in a degraded state. Part II (this document) describes the necessary requirements for operation in a degraded state. A prerequisite for the application of Part II is the consideration of Part I.
Requirements are defined for the subsystems suitable for degraded operation. A procedure is described how the integrator of a safety function can design the de- graded operation and implement it in the machine using subsystems suitable for it. In addition, it provides the manufacturer of safety components with guidance on this.