ISO 13849 is the international standard for the design of safety-related parts of control systems (SRP/CS) for machinery. It defines the Performance Level (PL a-e) framework, an alternative to the SIL approach of IEC 62061 for the machinery sector.
ISO 13849-1 Defines the PL determination method (risk graph), Category 1-4 architectures, MTTFD and DCavg quantification, common cause failure CCF score. Annex A is the famous risk graph. Annex K (informative) maps PL ↔ SIL.
ISO 13849-2 Validation process : test plans, fault lists per technology (electrical, electronic, programmable, fluid power, mechanical), verification of failure exclusions.
(SRP/CS) (PL) (Cat) (PLr) (MTTFD) (DCavg) (CCF) If IEC 61508 is the grandfather and IEC 61511 is the process-industry adaptation, ISO 13849 is the machinery cousin — same family, different philosophy.
In machinery (welding robots, packaging lines, machine tools, presses), safety functions typically:
ISO 13849 chose to express integrity as Performance Level (PL a-e) rather than reusing IEC 61508’s SIL 1-4. The metric is PFHD (Probability of dangerous Failure per Hour) only — there’s no low-demand PFDavg equivalent. This is because machinery safety functions are typically demanded continuously or high-rate.
Severity
S1 (slight) ─── PL a
S2 (serious)
│
┌────────┴───────┐
F1 (rare) F2 (frequent)
│ │
P1: avoidable P1: avoidable
↓ ↓
PL b PL c
P2: hardly P2: hardly
↓ ↓
PL c PL d
F2 + P2 + irreversible
↓
PL e
This is a simpler decision tree than the LOPA used in IEC 61511. It works because machine safety risks are usually more standardized (a sharp moving part, a press, an electric shock zone) than process plant risks.
The PL achieved is determined by a 3D matrix (clause 4.5.4 of Part 1) :
| MTTFD per channel | Cat B / 1, DC=none | Cat 2, DC=low | Cat 2, DC=medium | Cat 3, DC=low | Cat 3, DC=medium | Cat 4, DC=high |
|---|---|---|---|---|---|---|
| Low (3-10y) | PL a | PL b | PL c | PL c | PL d | – |
| Medium (10-30y) | PL b | PL c | PL d | PL d | PL d | – |
| High (30-100y) | PL c | PL d | PL d | PL d | PL e | PL e |
In practice, a typical Cat 3 safety door (force-guided contacts + redundant inputs + dual-channel safety relay) easily reaches PL d with off-the-shelf certified components. Reaching PL e requires Cat 4 architecture (full diversity in some implementations) and consistent high DC > 99%.
ISO 13849 handles CCF differently from IEC 61508 :
The checklist scores items like : physical separation, diverse technology, EMC qualification, identical maintenance procedures, etc. Most well-engineered Cat 3 / Cat 4 systems pass easily.
The IFA (DGUV, Germany) publishes SISTEMA, a free desktop tool that implements the full ISO 13849-1 calculation flow. It’s the de facto industry tool, supported by every major safety vendor (Pilz, Sick, Schmersal, Siemens) who provide component data libraries directly importable into SISTEMA.
Our PFD ↔ SIL calculator covers the IEC 61511 side. For pure machinery design in PL terms, the SISTEMA workflow is hard to beat — and we will not duplicate it ; we’ll likely link to it and complement with conceptual education.
Historical accident. Both are harmonized under the Machinery Directive 2006/42/EC :
Both are valid. In practice, ISO 13849 dominates Europe for typical machinery (its tools and component certifications are widespread). IEC 62061 is more often seen in complex machinery with significant programmable electronic content (multi-axis CNC, large packaging lines).
There is ongoing work to converge them in a future joint revision — IEC/ISO 17305 was the proposed merged document, but it has been on hold. For now, designers pick one and stick with it.