Functional safety is the part of a system's safety that depends on automatic protection working correctly in response to its inputs, or failing in a predictable way. The aim is freedom from unacceptable risk of harm to people, property or the environment. Standards such as IEC 61508 define how to engineer it.
The idea in one example
A guard door on a machine has a switch. When the door opens, a controller must stop the motor. Functional safety is about whether that chain of switch, logic and motor control reliably does its job, and what happens if one part breaks.
It covers the whole chain, including non electronic parts such as valves and actuators that the electronics monitor or control.
Fail-safe and predictable failure
Fail-safe means a system fails in a predictable manner, ideally toward a state that does no harm. Protection is meant to cope with likely systematic errors, hardware failures and stress from operation or environment.
Systematic errors are design or software mistakes that happen every time the conditions repeat. Random hardware failures happen by chance at some rate. Functional safety handles the first with disciplined engineering and the second with probability.
The usual process
- Identify hazards and the safety functions that must exist, using methods such as HAZID and HAZOP, and review of past accidents.
- Decide how much risk reduction each function needs. The result is expressed as a Safety Integrity Level (SIL), Performance Level (PL) or Automotive Safety Integrity Level (ASIL), depending on the sector.
- Find how devices fail, commonly with failure mode and effects analysis, and estimate failure probabilities.
- Design, verify and validate, then keep proving it through operation.
Which standard
IEC 61508 is the core standard. Simpler machinery systems may use ISO 13849. Sector standards include ISO 26262 for road vehicles, DO-178C for avionics software and IEC 61511 for the process industries.
We cover the levels in more detail in our posts on safety integrity levels and on SIL, PL and ASIL compared.
How it differs from security
Safety asks whether the system is dangerous when something goes wrong by accident. Security asks whether someone can make it go wrong on purpose. They overlap: an attacker who can change a safety function turns a security failure into a safety one. Plants and vendors increasingly assess both together, which is why guidance on industrial security and on safety systems now refers to each other. Check the current editions of the standards you work under for what they require.
What this means for developers
If you write software that sits in a safety function, expect traceable requirements, independent review, stricter testing and controlled change. Ordinary agile habits can fit, but evidence and approval gates matter more. If you only write the surrounding software, know which parts are safety related and treat changes to those parts carefully.
Quick check
Is functional safety the same as workplace safety?
No. Workplace safety covers broad practice such as guarding and training. Functional safety is specifically about automatic protective functions working correctly.
Does a standard certify a product as safe?
A standard sets a process and targets. Whether a product meets them is shown by assessment against that standard for a defined use.
Sources
These are secondary summaries; consult the standards themselves for requirements.
Last reviewed: 19 October 2026.
