SIS Design and Engineering
We handle architecture, FMEDA, probability of failure calculation and cybersecurity to IEC 61511 clauses 11 and 12
What this element is and why it matters
Phase 4 takes the safety requirements specification and produces the as designed safety instrumented system. Our team handles architecture selection across one out of one, one out of two and two out of three voting, FMEDA grade component selection, probability of failure verification to IEC 61508 6, cybersecurity zone and conduit design to IEC 62443, and the full design documentation to IEC 61511 clauses 11 and 12.
Individual significance for organisations
Design quality determines how the safety instrumented system behaves across its life, including its actual integrity, its spurious trip frequency, its proof test cost and its ageing pathway. Poor design choices cost your facility across the 20 to 25 year life of the asset, so we make sure those choices are sound.
Contribution to Functional Safety Lifecycle Implementation
Phase 4 produces the as designed safety instrumented system that Phase 5 commissions and Phase 6 operates. The safety manual it produces to clause 16 is the master document for the entire lifecycle of the system.
What compliant execution looks like
How we implement this element
A focused six step methodology calibrated to deliver sis design and engineering as a working capability rather than a documented compliance artefact.
We select the architecture across one out of one, one out of one with diagnostics, one out of two and two out of three based on the safety integrity level target, the mean time to fail spurious budget, common cause failure mitigation and any prior use evidence.
We choose sensors, logic solvers and final elements certified by TUV, exida or SIRA, and we verify the FMEDA data sheets covering the dangerous detected and dangerous undetected failure rates, the safe failure fraction and the diagnostic coverage.
We calculate to IEC 61508 6 Annex B using fault tree or Markov methods, we include the beta factor common cause contribution to Annex D, and we verify that the safety integrity level target is met.
To clause 11 we check Route 1H using hardware fault tolerance and safe failure fraction against Route 2H using reliability data, and we document compliance with the constraints.
We apply the IEC 62443 zone and conduit model, segregate the system from the basic process control system and the corporate network, set access control, and align with NIST SP 800 82.
We issue the design report, the hardware and software specifications, the cause and effect matrix and the safety manual to clause 16.
Element implementation flow chart
A decision gated workflow that shows the actual sequence of activities from initiation through steady state operation, with key decision points highlighted.
What we produce
- A safety instrumented system design report with the architecture and the probability of failure calculation
- An FMEDA graded component selection register
- A safety manual to IEC 61511 clause 16
- An architectural constraint compliance record covering Route 1H or 2H, hardware fault tolerance and safe failure fraction
- A systematic capability and proven in use evidence file for each element
- A cause and effect matrix and a zone and conduit security diagram
Where execution fails
- A prior use claim made without statistical evidence
- Beta factor scoring that is optimistic against your site context
- Cybersecurity bolted on after the design is frozen
- An average probability of failure that passes while the hardware fault tolerance and safe failure fraction constraints are not met
Codes this element is built on
Explore related elements in this framework
Functional Safety Lifecycle Implementation full element index
Talk to us about implementing SIS Design and Engineering
We can scope this element implementation against your facility, regulatory context, and existing management system maturity, then integrate it with the other Functional Safety Lifecycle Implementation elements you already operate.