IEC 61508 Safety Standards: Complete Implementation Guide

shieldIEC 61508 Overview

IEC 61508, titled “Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems,” is the foundational international standard for functional safety. Published in seven parts, it provides a framework for ensuring safety-critical systems operate reliably to prevent harm to people, property, and the environment.

For PLC programmers, IEC 61508 defines stringent requirements for safety system design, implementation, validation, and ongoing maintenance. Compliance is mandatory for industries including oil and gas, chemical processing, pharmaceuticals, nuclear power, and any application where system failure could cause serious injury or death.

assessmentUnderstanding SIL Levels

Safety Integrity Level (SIL) is a measure of risk reduction provided by a safety function. Higher SIL levels require more stringent design, validation, and testing to achieve lower probability of dangerous failure.

analyticsRisk Assessment and SIL Determination

Before implementing safety functions, you must conduct a thorough risk assessment to determine the required SIL level for each safety function.

verified_userSafety-Rated PLC Platforms

Safety applications require certified safety PLCs with hardware and software validated to IEC 61508. Here are the major safety PLC platforms:

codeSafety PLC Programming

Safety programming follows strict rules and uses certified function blocks to ensure reliable operation.

// Safety Program - Emergency Stop
// SIL 3 / PLe certified function

FUNCTION_BLOCK FB_EmergencyStop
VAR_INPUT
    EStop_Ch1 : BOOL;  // E-Stop input channel 1
    EStop_Ch2 : BOOL;  // E-Stop input channel 2 (redundant)
    Reset : BOOL;      // Manual reset button
END_VAR
VAR_OUTPUT
    SafeOutput : BOOL; // Safe motor enable
    Error : BOOL;      // Discrepancy error
END_VAR
VAR
    F_FDBACK : F_FDBACK;  // Certified feedback monitoring
END_VAR

// Emergency stop with redundant channels
IF (EStop_Ch1 AND EStop_Ch2) THEN
    SafeOutput := TRUE;  // Both channels OK
ELSE
    SafeOutput := FALSE; // At least one E-Stop active
END_IF;

// Channel discrepancy monitoring
IF (EStop_Ch1 XOR EStop_Ch2) THEN
    Error := TRUE;  // Channels disagree - fault condition
    SafeOutput := FALSE;
END_IF;

// Reset only allowed when both channels released
IF Reset AND EStop_Ch1 AND EStop_Ch2 THEN
    Error := FALSE;
END_IF;

END_FUNCTION_BLOCK

check_circleValidation and Testing

IEC 61508 requires rigorous testing to verify safety functions operate correctly under all conditions, including fault scenarios.

descriptionSafety Documentation Requirements

Comprehensive documentation is mandatory for IEC 61508 compliance and certification.

functionsCommon Safety Functions

IEC 61508 defines standard safety functions commonly implemented across industries. Here are the most critical ones:

buildProof Testing and Maintenance

Safety systems degrade over time. IEC 61508 requires periodic proof testing to detect dangerous undetected failures and maintain the required SIL rating.

verifiedAchieving IEC 61508 Compliance

Full IEC 61508 compliance requires a systematic approach throughout the entire safety lifecycle.