SIL, Performance Level (PL) and ASIL are three ratings for how dependable a safety function must be. SIL comes from IEC 61508 and process industries, PL from ISO 13849 for machinery, and ASIL from ISO 26262 for road vehicles. They share ancestry but use different scales and methods.
The three at a glance
- SIL: IEC 61508 and IEC 61511, process industry and general use, scale 1 to 4
- PL: ISO 13849, machinery, scale a to e
- ASIL: ISO 26262, road vehicles, scale A to D plus QM
ISO 26262 adapts IEC 61508 to automotive use. ISO 13849 is described as a tailored, simplified version for the machinery sector.
How each is derived
SIL comes from a risk analysis, often LOPA in process plants, that sets the risk reduction a safety function must provide.
PL uses a required PL (PLr) from the manufacturer's risk assessment. The achieved PL depends on the architecture category (B, 1, 2, 3 or 4), the quality of components, diagnostics and the probability of dangerous failure per hour. PL e is the most demanding and sits at or below about one dangerous failure in 100 million hours.
ASIL comes from rating each hazardous event on severity, exposure and controllability. ASIL D needs the highest rating in all three. Each step down in one factor lowers the level by one, and QM means standard quality management is enough.
Why they are not interchangeable
Although people publish informal tables lining the scales up, the methods differ. SIL for a low demand function uses probability of failure on demand. PL and the high demand SIL use failure per hour. ASIL includes controllability, which asks whether a driver could prevent harm, a factor with no direct equivalent in a plant. Treat any mapping as a rough guide, and ask the relevant assessor before relying on one in a safety case.
When you meet more than one
A vehicle factory may hold a car (ASIL) built by robots (PL) fed by a process line (SIL). Components such as PLCs and drives are sold with ratings for one scheme, and integrators need to read the stated assumptions. A drive rated for a performance level in a machine is not automatically suitable for a process shutdown function.
What software teams should take away
Ask which standard governs your product, then follow its software requirements. Our overview of what functional safety is is a good starting point, and the post on IEC 61508 for software teams covers the lifecycle.
Quick check
Is PL e the same as SIL 3?
Informal comparisons exist, but the standards do not make them identical. Confirm with a qualified assessor for your application.
Which one applies to a delivery robot?
It depends on the product and regulation. Machinery, road vehicle and other rules differ, so identify the governing standard first.
Sources
Secondary summaries; verify against the standards.
Last reviewed: 19 October 2026.
