Safe and Non-Safe Coupling Relays – Differences, Functionality, and Areas of Application

In the process industry, it is essential to securely transmit safety-related signals, such as output signals from a safety controller, to the peripheral devices. This is a critical requirement, as a faulty signal flow can interrupt the safety chain, making it impossible to guarantee a safe shutdown in the event of danger.

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### Differences between Safe and Non-Safe Coupling Relays
| Criterion | Safe Coupling Relays | Non-Safe Coupling Relays |
|----------------------|--------------------------------------------------------|----------------------------------------------------------|
| Safety Standard | SIL 3 / PL e (standards-compliant) | No safety certification |
| Redundancy | Yes (multiple redundancy) | No (single architecture) |
| Self-Monitoring | Yes (real-time fault detection) | No (no self-monitoring) |
| Reliability | Highest operational safety | Low operational safety in safety-critical systems |
| Use in Safety Chains| Yes (safety-related signals) | No (not allowed) |
| Application | Emergency stop, protective guard monitoring, machine stop| Conventional control circuits without safety function |

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### How Safe Coupling Relays Work
Safe coupling relays are designed to maintain the safety function even in the event of internal errors or failures. The redundancy and self-monitoring of the relay outputs ensure that a malfunction is detected and reported immediately. This is achieved by cyclic testing of the internal contacts.

If a safety shutdown (e.g., emergency stop) is triggered, safe coupling relays reliably open the switch paths to the peripherals. Thanks to redundancy, it is guaranteed that even if one of the internal contacts fails, the function is not lost.

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### Typical Applications of Safe Coupling Relays
Safe coupling relays are used in various safety-critical areas of industry. Key areas of application include:

• Galvanic isolation of digital 24V to 230V signals
- Transmission of signals from 24V controllers to 230V switching elements
- Ensuring interference-free signal transmission
- Potential isolation to prevent overvoltage and electrical interference


• Safe coupling of sensor signals
- Connection of safety sensors (e.g., pressure monitors) to the control system
- Protection of the control system from malfunctions and tampering


• Reinforcement of contacts
- Amplification of control signals with limited output power
- Control of actuators, motors, and switching elements


• Filtering of test pulses
- Safety controllers send periodic test pulses to verify the functionality of the safety components
- Safe coupling relays filter out these test pulses, ensuring correct signal evaluation

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### Conclusion
Safe coupling relays guarantee standards-compliant signal transmission in safety-critical applications. They prevent the interruption of the safety chain and enable the safe operation of machinery and systems according to the highest safety standards SIL 3 / PL e.

From galvanic isolation to signal amplification – safe coupling relays are indispensable in modern safety systems. They help protect people and machinery and ensure reliable shutdowns in the event of danger.