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Safety circuits are essential in machinery to prevent accidents and ensure operator protection. The ISO 13849-1 standard provides guidelines for designing and verifying these safety functions through systematic calculations. This article outlines the step-by-step process for designing and verifying safety circuits according to ISO 13849-1.
Understanding ISO 13849-1 Requirements
ISO 13849-1 specifies the safety performance levels for control systems. It emphasizes risk assessment, safety function design, and verification. The standard categorizes safety functions into Performance Levels (PL), ranging from PL a (lowest) to PL e (highest). Achieving the appropriate PL depends on the risk severity and the safety function’s reliability.
Step 1: Risk Assessment and Safety Requirements
The process begins with identifying potential hazards and assessing their severity. Based on this, safety requirements are defined, including the necessary Performance Level (PL). This step determines the target reliability for the safety circuit.
Step 2: Safety Function Design
The safety function is designed to reduce risk to an acceptable level. It involves selecting appropriate sensors, logic units, and actuators. The design must ensure the safety function can reliably detect faults and initiate safe shutdowns or stops.
Step 3: Calculating the Diagnostic Coverage (DC)
Diagnostic coverage measures the ability of the safety system to detect faults. It is expressed as a percentage and influences the Safety Integrity Level (SIL) or Performance Level (PL). Higher DC improves the safety function’s reliability.
Step 4: Determining the Failure Rates
Failure rates of components are obtained from manufacturer data or standards. These rates are used to calculate the probability of dangerous failures. The goal is to ensure the failure probability aligns with the target PL.
Step 5: Calculating the Overall Reliability
The overall reliability of the safety circuit is calculated by combining individual failure rates and diagnostic coverage. The formula considers the probability of dangerous failures per hour, ensuring it meets the requirements for the desired PL.
Step 6: Verification and Validation
The final step involves verifying that the safety circuit meets the calculated reliability targets. Testing, fault injection, and documentation are used to validate the safety function’s performance. Regular maintenance and periodic testing are recommended to maintain safety levels.