IEC 61508 is the parent functional safety standard from which many of the domain-specific standards descend, and understanding it explains the shared logic running underneath all of them. Engineers meet ISO 26262 in automotive, EN 50128 in rail, IEC 61511 in process industries, and adaptations of the same thinking in medical and other sectors. What is easy to miss is that these are not independent inventions. They are sector-specific tailorings of one general framework, and that framework is IEC 61508.
Functional safety is a specific idea worth stating precisely. It is the part of overall safety that depends on a system or equipment operating correctly in response to its inputs. A safety function is something the system does to keep risk tolerable: a sensor detects an overpressure and a controller opens a relief valve. Functional safety asks whether that function will be there, correct, and timely when it is needed. It is not about whether a component is well built in general; it is about whether the safety-related function performs on demand.
The organizing concept is the Safety Integrity Level, or SIL. IEC 61508 defines four, from SIL 1 up to SIL 4, and a SIL is a target for how much risk reduction a safety function must deliver and, correspondingly, how unlikely it must be to fail. SIL 4 demands the greatest integrity and the lowest tolerable failure probability. The level is not chosen by taste; it is derived from a hazard and risk analysis that weighs how severe an outcome is, how often the hazardous situation arises, and what other protections exist. The SIL then drives the rigor of everything downstream, exactly as the Design Assurance Level does in avionics.
IEC 61508 also insists on the safety lifecycle. Rather than bolting safety analysis onto a finished design, the standard lays out phases from concept and hazard analysis, through the allocation of safety requirements to safety functions, into realization, operation, and eventual decommissioning. Safety requirements are derived, allocated, and then verified and validated against, and the standard expects the whole thing to be managed, documented, and traceable. This lifecycle framing is why the derived standards feel so similar: they all inherit the same spine.
A distinction the standard makes carefully is between systematic and random failures. Random hardware failures happen at quantifiable rates and can be addressed with redundancy, diagnostics, and probabilistic targets such as the average probability of failure on demand. Systematic failures, including virtually all software faults, come from errors in requirements, design, or implementation, and no amount of redundancy removes them. They are managed instead through process rigor, review, and verification that scales with the SIL. Getting the requirements right and traceable is the primary defense against the systematic failures that redundancy cannot catch.
For a systems engineering team, the practical takeaway is that a SIL is only meaningful if the safety requirements it drives are captured, allocated to the right functions, and verified with evidence. That is a requirements and traceability problem at its core. Hitt Hosting SE treats safety requirements as first-class linked objects, maintains allocation and verification coverage as live data, and flags the downstream impact when a safety requirement changes. Whether a program calls its target a SIL, an ASIL, or a Design Assurance Level, the underlying discipline the tool supports is the same one IEC 61508 codified.