Functional Safety Requirements
Artifact
Topic overview
Quick facts
- Definition of Functional Safety Requirements in functional safety
- Functional Safety Requirements in depth
- The case for Functional Safety Requirements
- Functional Safety Requirements in practice
- Where teams get Functional Safety Requirements wrong
- Functional Safety Requirements and its standards
- Industries where Functional Safety Requirements applies
- Ask our team about Functional Safety Requirements
- Terms related to Functional Safety Requirements
Safety requirements form a hierarchy that mirrors the design decomposition. At the top sit the safety goals or overall safety requirements produced by the hazard analysis, one per hazardous event, stating the behavior that must be prevented or achieved, each carrying its integrity level.
These decompose into functional safety requirements describing what the system must do (detect the condition, reach the safe state, within the time), then into technical safety requirements that commit to how the architecture achieves it, and finally into hardware and software safety requirements allocated to specific elements.
Each level inherits the integrity level of its parent unless a justified decomposition across independent elements reduces it, and each allocation to an architectural element is an explicit, recorded decision.
What makes a safety requirement adequate is stricter than ordinary requirements hygiene.
Every one must be verifiable, written so a test or analysis can pass or fail it, and must carry the safety-specific attributes the standards expect: the triggering condition, the safe state, the timing budget (the fault-tolerant time within which the reaction must complete), the operating modes in which it applies, and behavior on detected faults.
Vague aspirations like the system shall be safe are unusable; the working currency is statements like loss of encoder signal shall be detected within 50 milliseconds and shall trigger transition to safe torque off.
Traceability is the other load-bearing property: every requirement traces upward to the hazard that motivates it and downward to the design elements and verification evidence that implement and prove it.
This bidirectional trace is what an assessor samples first, and it is also the program's own impact-analysis tool, when a hazard rating changes or a component is swapped, the trace shows exactly what is affected.
The case for Functional Safety Requirements
Requirements defects are the dominant root cause of both safety incidents and late-program rework: a wrong or missing safety requirement is implemented faithfully, verified successfully against itself, and fails only in the field or in assessment, the most expensive places to fail.
For product leaders, the state of the safety requirements set is the truest single indicator of program health, an unstable or untraceable requirements tree means the verification budget is being spent proving the wrong things, and no downstream heroics recover that.
Functional Safety Requirements in practice
Safety lifecycle
Safety and systems engineering write each level of the hierarchy at its lifecycle phase: safety goals from the hazard analysis at concept, functional and technical safety requirements alongside architecture, hardware and software requirements at detailed design, all managed in a requirements tool with attributes and trace links rather than in prose documents.
Reviews at each level check verifiability, completeness against the parent level, and consistency of timing budgets across the chain, and every requirement is placed in the V&V plan with a method and coverage criterion.
Assessors walk the tree in both directions: pick a hazard and confirm requirements cover it through to test evidence, and pick a low-level requirement and confirm it traces to a hazard rather than existing as unjustified folklore.
Where teams get Functional Safety Requirements wrong
Functional Safety Requirements and its standards
In programs run to IEC 61508, Functional Safety Requirements is one of the things the assessor checks first - the umbrella functional safety standard defining the safety lifecycle, sil levels, and requirements for electrical, electronic, and programmable systems.
Functional Safety Requirements is defined and governed by ISO 26262 - automotive functional safety standard defining asil levels and the automotive safety lifecycle.
SRS ownership and review
| Aspect | Detail |
|---|---|
| Produced by | Safety/systems engineer |
| Inputs | Hazard analysis, safety goals, standards, interfaces |
| Outputs | Functional + integrity requirements per safety function |
| Reviewed by | Independent reviewer; assessor samples traceability |
| Common finding | Untestable or compound requirements |
Industries where Functional Safety Requirements applies
In autonomous systems, Functional Safety Requirements comes up in nearly every certification conversation: autonomous mobile equipment, off-highway autonomy, and driverless platforms.
In industrial machinery, Functional Safety Requirements comes up in nearly every certification conversation: fixed and mobile machinery, packaging lines, and manufacturing equipment.
Ask our team about Functional Safety Requirements
Ben TwomblyFounder & CEOLeads CSA's functional-safety, certification, and engineering programs for safety-critical products and organizations. His assessor-side background informs practical programs that connect standards, engineering work, and credible evidence.
Tom CyrPrincipal Safety Engineer, IFSPSupports industrial and rail functional-safety programs with safety analysis, requirements, lifecycle planning, and certification-readiness guidance.Terms related to Functional Safety Requirements
How each connects to Functional Safety Requirements:
Programs that deal with Functional Safety Requirements almost always involve HARA in the same phase.
Programs that deal with Functional Safety Requirements almost always involve Verification and Validation in the same phase.
Functional Safety Testing appears in the same lifecycle work as Functional Safety Requirements and is usually produced or reviewed alongside it.
IEC 61508 connects the two: Functional Safety Requirements and Functional Safety are assessed against the same lifecycle.
SIL is governed by IEC 61508 as well, which is why the two show up together in audits.
Both Functional Safety Requirements and ASIL sit under ISO 26262, so evidence for one usually references the other.
Both Functional Safety Requirements and Safety Function sit under IEC 61508, so evidence for one usually references the other.
Safe Torque Off is governed by IEC 61508 as well, which is why the two show up together in audits.
IEC 61508 connects the two: Functional Safety Requirements and PFH are assessed against the same lifecycle.
FMEA is governed by IEC 61508 as well, which is why the two show up together in audits.
Both Functional Safety Requirements and FMEDA sit under IEC 61508, so evidence for one usually references the other.
Both Functional Safety Requirements and Fault Tree Analysis sit under IEC 61508, so evidence for one usually references the other.