Understanding ISO 13849: Ensuring Safety in Machine Control Systems

Machines play a vital role in modern industry, but they also pose risks to operators and maintenance personnel. Safety in machine control systems is not just a legal requirement; it protects lives and reduces costly downtime. ISO 13849 is an international standard that guides the design and implementation of safety-related parts of control systems. This post explains how to use ISO 13849 to build safer machines and meet regulatory demands.

Performance Level (PL)	Average Probability of Dangerous Failure per Hour (PFH_d)
a	≥ 10⁻⁵ to < 10⁻⁴
b	≥ 3 x 10⁻⁶ to < 10⁻⁵
c	≥ 10⁻⁶ to < 3 x 10⁻⁶
d	≥ 10⁻⁷ to < 10⁻⁶
e	≥ 10⁻⁸ to < 10⁻⁷

What ISO 13849 Means for Machine Safety

ISO 13849, titled Safety of machinery – Safety-related parts of control systems, provides a framework to assess and design control systems that reduce risks. It focuses on the parts of the control system responsible for safety functions, such as emergency stops, guards, and interlocks.

The standard helps manufacturers and engineers:

• Identify safety functions needed for a machine

• Evaluate risks associated with those functions

• Design control systems to meet required safety levels

• Verify and validate safety performance

  
  

ISO 13849 applies to electrical, electronic, and programmable electronic control systems. It is widely accepted in industries like manufacturing, automotive, and packaging.

How to Assess Risk and Define Safety Functions

Before designing a control system, you must understand the hazards and risks involved. ISO 13849 guides you through a risk assessment process:

1. Identify hazards: Look at all machine movements, energy sources, and user interactions that could cause harm.

2. Estimate risk: Consider the severity of injury, frequency of exposure, and possibility of avoiding harm.

3. Determine risk reduction: Decide what safety functions are needed to reduce risk to an acceptable level.

   
   

For example, a robotic arm in a factory might pose crushing hazards. The safety function could be an emergency stop that immediately cuts power when triggered.

How to Design Safety-Related Control Systems

ISO 13849 introduces the concept of Performance Levels (PL), which measure how reliable a safety function is. PL ranges from a (lowest) to e (highest), with higher levels meaning lower probability of dangerous failure.

To design a control system that meets a required PL, follow these steps:

• Select components: Use safety-rated sensors, controllers, and actuators.

• Apply redundancy: Duplicate critical parts to avoid single points of failure.

• Use diagnostics: Implement self-checks to detect faults early.

• Consider architecture: Choose a structure that supports the required PL, such as dual-channel or monitored systems.

  
  

For instance, a safety light curtain protecting a conveyor belt might require a PL d system, meaning it must detect faults reliably and stop the machine quickly.

How to Calculate and Verify Performance Levels

ISO 13849 provides formulas and tables to calculate the achieved PL based on:

• Mean Time To Dangerous Failure (MTTFd) of components

• Diagnostic Coverage (DC), or how well faults are detected

• Common Cause Failures (CCF), or failures affecting multiple parts simultaneously

• System architecture

  
  

You can use software tools or manual calculations to determine if your design meets the required PL.

Verification involves:

• Testing components and subsystems

• Checking wiring and connections

• Validating software logic if programmable controllers are used

• Documenting results for compliance

  
  

How to Implement Safety Functions in Practice

Here are practical tips for applying ISO 13849 in machine control systems:

• Start early: Integrate safety design from the concept phase, not as an afterthought.

• Use certified components: Choose parts with known reliability data and safety certifications.

• Train your team: Ensure engineers and technicians understand safety principles and the standard.

• Document thoroughly: Keep records of risk assessments, design decisions, calculations, and tests.

• Plan maintenance: Include regular inspections and testing to maintain safety performance over time.

  
  

Example: Emergency Stop Circuit Design

An emergency stop (E-stop) is a common safety function. To meet ISO 13849 requirements:

• Use a dual-channel E-stop button wired to a safety relay.

• The relay monitors both channels and stops the machine if either channel fails.

• Include feedback loops to detect wiring faults.

• Calculate the PL based on component reliability and diagnostics.

• Test the circuit regularly to confirm proper operation.

  
  

How to Keep Safety Systems Effective Over Time

Safety is not a one-time effort. Machines and their environments change, so safety systems need ongoing attention:

• Regular inspections: Check for wear, damage, or modifications that affect safety.

• Functional tests: Verify safety functions operate correctly under real conditions.

• Update documentation: Reflect any changes or upgrades in risk assessments and designs.

• Train operators: Keep users aware of safety procedures and emergency responses.

  
  

Following ISO 13849 helps create a culture of safety that protects people and assets.