How to Select, Configure, and Apply Safety Relays Based on Machine Risk Assessment and Performance Level (PL) Rating
In industrial automation, ensuring the safety of personnel and equipment is non-negotiable. After a machine risk assessment is completed, and a Performance Level (PL) rating is defined according to ISO 13849-1, the next critical step is selecting and configuring the right safety relay. This vital component ensures that safety functions are executed correctly in response to hazards identified during risk analysis.
This blog post explores how to select, configure, and apply safety relays based on PL ratings, with practical examples and industry best practices to meet functional safety compliance.
What Is a Safety Relay?
A safety relay is an electromechanical or electronic device designed to reliably monitor safety-related inputs and trigger predefined safety outputs. Its job is to shut down or isolate power when hazardous conditions are detected—providing a fail-safe mechanism for machine safety functions.
Safety Relay vs. Standard Relay:
| Feature | Safety Relay | Standard Relay |
|---|---|---|
| Redundancy | Dual or triple channel | Single channel |
| Diagnostic Coverage | Built-in fault detection | None |
| Fail-safe Operation | Yes | No |
| Standards Compliance | ISO 13849-1, IEC 62061 | General-purpose |
Step 1: Complete Machine Risk Assessment
Before selecting a safety relay, perform a formal risk assessment using standards like:
- ISO 12100 – Risk assessment and risk reduction
- ISO 13849-1 – Safety of machinery – PL-based safety control design
- IEC 62061 – SIL (Safety Integrity Level) for electrical control systems
Key Elements to Assess:
- Severity of possible injury (S)
- Frequency and duration of exposure (F)
- Possibility of avoiding the hazard (P)
The outcome defines the required PL rating (PLr) for each safety function:
| PL Rating | Risk Level |
| PLa | Low |
| PLb | Low to Medium |
| PLc | Medium |
| PLd | High |
| PLe | Very High |
Step 2: Identify Safety Functions to Be Controlled
Determine what machine operations must be controlled or stopped during unsafe conditions. Common safety functions include:
- Emergency stop (E-stop)
- Guard door monitoring
- Two-hand control
- Light curtain or laser scanner monitoring
- Safe speed monitoring
Each function may require a separate safety relay or a multi-channel relay depending on complexity.
Step 3: Select the Suitable Safety Relay
Use the PLr as your benchmark to select a relay capable of achieving or exceeding the required rating.
Selection Criteria:
| Factor | Description |
| PLr Compliance | Must match or exceed PLr determined from risk assessment |
| Input Type | E-stop, guard switch, light curtain, etc. |
| Number of Channels | Single-channel, dual-channel, or triple-channel redundancy |
| Reset Mode | Manual reset, automatic reset, monitored reset |
| Reaction Time | Must meet stopping time requirements of the hazard |
| Diagnostics | Fault detection and LED indicators |
| Expandability | Support for additional safety inputs or network integration |
Popular Industry-Approved Safety Relays:
| Manufacturer | Model | Typical Use |
| Pilz | PNOZ X, PNOZmulti | General safety and complex systems |
| Siemens | SIRIUS 3SK1/3SK2 | Modular relay logic and networking |
| ABB | Jokab Safety RT Series | E-stop, guard interlock, light curtain |
| Allen-Bradley | Guardmaster 440R | Compact E-stop and interlock monitoring |
| Schneider Electric | XPS Series | Standalone safety functions |
Step 4: Configuration and Wiring Guidelines
Typical Safety Relay Configuration Example: E-Stop Circuit (PLd)
- Connect Dual-Channel E-Stop Button to the relay’s inputs.
- Wire Relay Output Contacts to the power contactor or safety-rated actuator.
- Install Reset Button with monitoring if manual reset is required.
- Enable Cross-Short Detection to monitor line breaks.
General Wiring Best Practices:
- Use shielded cables for long-distance inputs
- Avoid mixing safety and non-safety circuits in the same cable run
- Ensure adequate spacing in control panels
- Label all terminals and include in electrical drawings
- Test outputs regularly during commissioning
Step 5: Validation and PL Verification
Once installed, verify the safety function meets the desired PL using ISO 13849-1 methods:
- Use software tools (e.g., SISTEMA from IFA) to calculate PL based on component B10d, DC (diagnostic coverage), MTTFd (mean time to dangerous failure), and structure category
- Conduct functional safety validation test procedures
- Keep documentation for auditing and maintenance
Real-World Application Example
Scenario: A robotic palletizing cell includes a gate switch and E-stop for operator access. A risk assessment determines PLd is required for gate access.
Implementation:
- Use a Pilz PNOZ X3 safety relay with dual-channel input from the gate switch and E-stop button
- Connect output to safety contactor that de-energizes robot servo motors
- Manual reset mounted outside the gate
- Achieves PLd compliance through redundant circuits and diagnostics
Troubleshooting Safety Relay Systems
| Symptom | Likely Cause | Solution |
| Relay won’t reset | Fault in input circuit or output load | Check wiring and reset logic |
| Intermittent shutdowns | Faulty sensor or noise | Use shielded cables and verify sensor health |
| LED flashing | Internal diagnostic error | Refer to manufacturer fault codes |
Best Practices for Safety Relay Use
- Perform regular testing and validation during maintenance shutdowns.
- Log safety trips and resets using HMI or SCADA to identify patterns.
- Avoid software overrides that bypass relay functions.
- Design for future scalability—choose relays that support modular expansion.
- Ensure team training on functional safety, wiring, and response protocols.
Conclusion: Safety Starts with the Right Relay
Selecting and configuring a safety relay is not just about meeting a technical requirement—it’s about protecting lives and ensuring compliance. By starting with a thorough risk assessment, identifying required PL levels, and choosing the right relay for each function, you build a safer and smarter machine.
From E-stop buttons to complex light curtain zones, safety relays form the backbone of risk mitigation in automation. Choose wisely, wire correctly, and always verify your system with international safety standards.