Safety Integrity Level (SIL) is a discrete measure of the reliability required for a safety function in a robotic or automated system, defined by IEC 61508 on a four-level scale (SIL 1–4). A higher SIL demands greater hardware fault tolerance, stricter software development processes, and more comprehensive validation to ensure the safety function reduces risk to a tolerable level. SIL is assigned during risk assessment and drives the entire safety lifecycle of a system.
SIL allocation results from a quantitative or qualitative risk assessment that considers the severity of the hazard, the frequency of exposure, and the likelihood of avoiding harm. A SIL 2 safety function, for example, requires a probability of dangerous failure on demand between 10⁻³ and 10⁻². Achieving the required SIL involves a combination of hardware fault tolerance, diagnostic coverage, and systematic process rigour across the entire safety lifecycle.