Safety, Cybersecurity, and Reliability
What this domain tests
Safety, Cybersecurity, and Reliability tests whether you can separate and integrate three risk domains: functional/machine safety, industrial cybersecurity, and equipment/system reliability. Expect risk assessment, safety lifecycle, SIL/PL, zones and conduits, defense in depth, redundancy, and failure analysis.
Core rules and concepts
- Safety and cybersecurity are different risk disciplines. A firewall does not make a safety function safe, and a SIL rating does not secure a network.
- Follow the safety lifecycle: hazard and risk assessment, safety requirements specification, design, verification, validation, operation, proof test, and MOC. For process safety, use IEC 61511 and SIS/SIF; for machinery, use ISO 13849 and performance level.
- SIL is a property of the safety function, not a device. Verify PFDavg or PFH, architectural constraints, proof-test interval, and common-cause credits.
- For cybersecurity, use IEC 62443 zones and conduits, asset inventory, least privilege, patching, monitoring, incident response, and recovery. Assume breach.
- For reliability, use FMEA/FMECA, MTBF/MTTR, redundancy, and single-point-of-failure analysis. Redundancy without diagnostics can create a hidden failure.
Worked example
A transmitter has dangerous undetected failure rate lambda-du = 2 x 10^-6 per hour. Proof-test interval TI = 1 year = 8,760 h. For a 1oo1 architecture, approximate PFDavg = lambda-du x TI / 2 = 2 x 10^-6 x 8,760 / 2 = 0.00876. That falls in the SIL 2 range (0.001 to 0.01), but it is close to the SIL 3 boundary and leaves little margin. If the required SIL is 2, the design may pass; if SIL 3 is required, it fails. Reasoning: always show the formula, units, and TI assumption. A 2oo3 architecture or shorter proof test would lower PFDavg, but common-cause and proof-test effectiveness still need review.
Traps that cost marks
- Treating a SIL-certified device as a SIL-rated function.
- Ignoring proof-test interval and common-cause failure.
- Using a firewall as the only cybersecurity control.
- Putting safety and control on the same network without justified zones and conduits.
- Assuming redundancy eliminates failure; hidden failures and common cause remain.
- Forgetting recovery and backup testing as part of reliability.