Application of Safety Relay in Safety Instrumented Systems

Safety relay is a kind of interface component that connects controllers with actuators, primarily serving the functions of electrical isolation and power driving. The internal circuit of the module is designed to be fault-safe, and the output circuit employs multiple redundancies, ensuring reliable disconnection of the output circuit even when external inputs are cut off or internal components fail, thereby guaranteeing the disconnection of the power source for the actuators.

The Necessity of Safety Relay

According to IEC 61511, it is not specified that all components in a SIS system must be safety products or have a Safety Integrity Level (hereinafter referred to as SIL, which ranges from 1 to 4). However, the chosen components must demonstrate proven usage, meaning that based on usage records in previous operational environments and applications, it is evaluated that the device has a sufficiently low hazardous failure rate, making it suitable for use in safety instrumented systems.

At the same time, proven usage requires appropriate evidence to support that these components and subsystems are suitable for SIS systems. The applicability of the evidence should include:

• The manufacturer has a comprehensive quality control and management system;

• The component or subsystem has proper identification and technical documentation;

• Good performance in similar process operation states and independent environments;

• This information needs to be organized and summarized into documentation for storage.

In principle, proven usage evaluation is not applicable to SIL3 systems. However, based on current situation surveys, ordinary relays have not undergone systematic safety evaluation and cannot provide λ-type and various related parameters needed for Safety Integrity Level calculations; ordinary relay manufacturers do not track the performance of their products under specific process operating conditions and independent environments, and cannot provide evidence of good product performance.

However,  SIL certified safety relay is entirely different. The product:

• Is designed from the outset to be fault-safe;

• Adopts higher-than-normal product testing standards;

• Provides corresponding documentation that clearly specifies complete installation, operation, and maintenance instructions and requirements;

• Is subject to regular quality control and management system audits by the manufacturer;

• Document management covers the entire product lifecycle.

Therefore, against the backdrop where chemical accidents continue to occur and strict inspections are conducted, using SIL certified safety relay is more compliant with regulatory requirements and reduces usage risks.

Selection Considerations for Safety Relay

Safety relay's failure rates need to be involved in the overall system's SIL verification calculation and serve as an important basis during selection. Additionally, as an intermediate connecting component, the electrical compatibility and matching between the upper-level control devices and lower-level actuators must also be considered. The table below lists some electrical performance parameters of a manufacturer's SIS controller DO modules.