How do you design SIS for continuous or high-demand mode applications?

High-demand mode SIS design (demand rate > 1/year): use PFH (probability of failure per hour) instead of PFDavg, design for failure tolerance (system must continue to function with one failure), implement continuous diagnostics with high coverage, provide online repair capability without removing protection, and use redundant architectures with automatic switchover. Typical applications: rotating machinery protection, continuous process analyzers for safety. Calculate PFH from dangerous failure rates accounting for repair time. Architectural constraints are more stringent (SIL 2 requires hardware fault tolerance of 1). Consider voting during repair (1oo2 degraded to 1oo1). Some standards (IEC 61511) limit continuous mode to SIL 2 maximum.