What are Safety Relays?

 

Safety relays are dedicated safety control devices used to monitor and manage machine safety functions such as emergency stops, safety gates, light curtains, two-hand controls and safety switches.

Unlike standard control relays or PLC logic, safety relays are designed with redundant monitoring and fault detection to help ensure machinery moves to a safe state when a hazardous condition occurs.

Align Automation integrates safety relay systems into industrial control architectures, working alongside OEM machine builders and electrical contractors to deliver practical, reliable and maintainable safety solutions.

 

Ensuring Operational Integrity: Safety Relays

 

In any automated system, safety is the highest priority. While a PLC or PAC manages the functionality and efficiency of your machine, Safety Relays act as the independent, reliable "watchdogs" that ensure the system halts immediately if a hazard is detected. They are the essential link between safety devices (like light curtains, emergency stops, and interlocks) and the machine’s power circuit.

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The Role of Safety Relays

 

A safety relay is a specialized device designed to monitor safety-critical circuits. Unlike a standard relay, a safety relay is built with redundancy and self-monitoring capabilities.

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If a fault occurs—such as a welded contact inside the relay or a broken wire in the safety circuit—the safety relay detects this internal failure and prevents the machine from restarting until the fault is cleared. This prevents a "fail-to-danger" scenario where a machine might continue running even if the emergency stop was pressed.

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Why Use Dedicated Safety Relays?

 

Even in systems controlled by advanced PACs, dedicated safety hardware is vital for several reasons:

• Failure Detection: They use redundant circuits to monitor themselves. If one component fails, the second provides a backup, and the system locks out.

• Simplicity and Determinism: Safety relays operate with hard-wired logic that is independent of the main machine software. This ensures that safety functions are executed instantly and cannot be "overridden" by a software bug or a frozen CPU scan.

• Certified safety relays or safety controllers are commonly used to help achieve compliance with machinery safety standards (such as ISO 13849-1 and IEC 62061). 

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Types of Safety Relays

 

Depending on the complexity of your machine, there are two main ways to approach safety control:

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1. Traditional/Modular Safety Relays

 

These are stand-alone devices, typically used for simple machines with a few safety points.

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• Best for: E-stop monitoring, safety gate monitoring, and light curtain integration.

• Key Benefit: Easy to install, simple to troubleshoot, and cost-effective for localized safety needs.

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2. Configurable/Programmable Safety Controllers

 

As machines—like 6DOF motion platforms—become more complex, you may have dozens of safety inputs. Configurable safety relays allow you to create "safety logic" using a graphical interface.

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• Best for: Systems with multiple safety zones, complex muting requirements, or high-density I/O.

• Key Benefit: Reduces wiring complexity and provides more granular diagnostic data back to the main PLC/PAC.

Integration with Motion Systems

 

For dynamic applications like 6DOF motion platforms, safety relays must work in tandem with the motion control hardware:

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• Safe Torque Off (STO): Modern servo drives (like the Lexium series) feature built-in STO functions. A safety relay can trigger this function, cutting the power to the motor stages instantly without needing to cut the main power to the entire machine. This allows for faster recovery times and less mechanical wear during frequent stops.

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• Monitoring Feedback: Most safety relays provide a status signal to the PLC or PAC. This allows your Human-Machine Interface (HMI) to display exactly which gate is open or which E-stop has been pressed, significantly reducing downtime during troubleshooting.

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Best Practices for Safety Design

1. Risk Assessment: Always perform a formal risk assessment for your machine to determine the required Performance Level (PL) or Safety Integrity Level (SIL) for your safety circuits.

2. Keep Safety Independent: Whenever possible, keep safety-rated sensors and actuators wired to a dedicated safety relay or safety PLC, rather than relying solely on the standard machine control logic.

3. Routine Testing: Periodically test your safety relay functionality to ensure that all redundant channels are active and responsive.

4. Machine safety requirements vary between applications. Final safety architecture should always be determined through a formal risk assessment and applicable standards review.