What is ISO 13849-1?
ISO 13849-1 is an international standard that sets out requirements for the functional safety of machine control systems. The standard focuses specifically on safety-related parts of control systems (SRP/CS: Safety-Related Parts of Control Systems), such as emergency stops, light curtains, safety relays and Safety PLCs.
In OT environments, ISO 13849-1 helps mitigate risks on moving, cutting or driving machinery using demonstrable safety measures in both hardware and software.
🧠 Purpose of ISO 13849-1
- Risk management – Safe operation of machines under both normal and faulty conditions
- Structure and design – Requirements for architecture, reliability, fault detection
- Determination of Performance Level (PL) – A measure of risk control
- Applicable at electromechanical and programmable level
- Alternative or complement to IEC 62061 (SIL standard)
✅ What are Performance Levels (PL)?
| PL | Description | Application |
|---|---|---|
| PL a | Low risk | Visual signalling without an active function |
| PL b | Limited risk | Mechanical guard without locking |
| PL c | Moderate risk | Motor activation after gate detection |
| PL d | High risk | Light curtain with feedback and two-hand control |
| PL e | Very high risk | Emergency stop with redundancy and fault detection |
The PL depends on three factors: Severity, Frequency, Possibility of avoidance (S, F, P).
🔧 Important concepts in ISO 13849-1
| Term | Explanation |
|---|---|
| SRP/CS | Safety-Related Parts of Control System |
| MTTFd | Mean Time To Dangerous Failure (component reliability) |
| DC (Diagnostic Coverage) | The degree to which the system detects internal faults |
| CCF (Common Cause Failure) | Measure against simultaneous failure of redundant systems |
| Redundancy | Use of duplicated circuits/components at PL d and e |
📦 Practical applications
| Machine function | Examples of safety components |
|---|---|
| Emergency stop | Red push button + safety relay with self-monitoring |
| Access door | Locking system with position switch and feedback |
| Two-hand control | Control panel requiring both buttons simultaneously |
| Motion limitation | Encoders with limiting via Safety PLC |
| Light curtain protection | Zone monitoring that stops the motor on interruption |
🔁 ISO 13849-1 and other standards
| Standard | Relation to ISO 13849-1 |
|---|---|
| IEC 62061 | Alternative standard for functional safety, based on SIL |
| IEC 60204-1 | General electrical equipment of machines |
| Machinery Directive | Directly references ISO 13849-1 as a harmonised standard |
| ISO 12100 | Risk assessment methodology preceding PL determination |
| ISO 14119 | Protection via interlocks, locking of guards/doors |
⚠️ Common practical mistakes
- No proper PL determination before design
- Mismatch between component PL and required PL
- Insufficient fault detection (low DC) at PL d/e
- Failing to account for CCF factors (such as a shared power supply)
- Use of standard PLCs for safety functions without validation
📌 In summary
ISO 13849-1 provides a structured framework for designing safe machines. The Performance Level model makes it possible to objectively demonstrate risk mitigation through calculation, component selection and fault detection.