Jul 09, 2026
If you work with industrial machinery, safety systems or machine risk assessments, you have likely come across ISO 13849. It is one of the most widely used machinery safety standards in the world and plays a critical role in ensuring safety-related control systems perform reliably when required.
Whether you're designing new machinery, modifying existing equipment or carrying out a machinery safety assessment, understanding ISO 13849 is essential. The standard provides a framework for designing and validating safety-related parts of control systems, helping to ensure that safety functions reduce risk to an acceptable level.
In simple terms, ISO 13849 helps answer an important question: Can the machine's safety system be relied upon when it is needed most?
ISO 13849 is an international machinery safety standard titled: ISO 13849-1: Safety of Machinery – Safety-Related Parts of Control Systems – Part 1: General Principles for Design.
The standard is used to assess and design safety-related control systems found on industrial machinery. These systems are responsible for carrying out safety functions such as stopping dangerous movements, monitoring safety devices and preventing access to hazardous areas.
Examples of safety functions covered by ISO 13849 include:
The purpose of the standard is to ensure these systems perform with a level of reliability appropriate to the risks they are controlling.
Many machinery hazards are controlled through safety systems rather than physical guards alone.
For example, when an operator opens an interlocked guard, the machine may automatically stop. If the safety system responsible for that stop function fails, the consequences could be severe.
Simply installing safety devices is not enough. Organisations must also consider the reliability of the systems controlling those devices.
ISO 13849 provides a structured methodology for evaluating whether a safety function is likely to perform correctly when required. This helps reduce the risk of dangerous failures and improves overall machinery safety.
For manufacturers, compliance with ISO 13849 is often an important part of demonstrating machinery safety during UKCA and CE marking projects.
A safety-related part of a control system is any component involved in carrying out a safety function.
This may include:
These components work together to detect hazardous situations and place machinery into a safe condition when required.
For example, when an interlocked access door is opened, several components may work together to stop machine movement. ISO 13849 helps determine whether the entire safety function achieves an appropriate level of reliability.
One of the key concepts within ISO 13849 is the Performance Level, commonly referred to as PL.
A Performance Level represents the ability of a safety function to perform reliably under foreseeable operating conditions. Performance Levels are classified as:
As risk increases, a higher Performance Level may be required.
For example, a safety function designed to prevent minor injuries may require a lower Performance Level than a function intended to prevent fatal injuries.
The required Performance Level is determined through a risk assessment process that considers:
A common source of confusion is the difference between Required Performance Level (PLr) and Achieved Performance Level (PL).
The Required Performance Level (PLr) is determined through risk assessment and represents the minimum reliability needed for a particular safety function.
The Achieved Performance Level (PL) is calculated based on the actual design of the safety system.
For compliance, the achieved Performance Level must meet or exceed the required Performance Level identified during risk assessment.
If it does not, additional design measures may be necessary to improve system reliability.
Another important concept within the standard is the use of system categories.
Categories describe the architecture of the safety-related control system and influence its ability to tolerate faults.
The standard defines the following categories:
As the category increases, so does the system's fault tolerance and diagnostic capability.
For example, Category 3 and Category 4 architectures are commonly used where higher levels of risk are present because they are designed to maintain safety even if certain faults occur.
However, category alone does not determine the final Performance Level. Other factors such as reliability data and diagnostic coverage must also be considered.
Determining the achieved Performance Level requires several factors to be evaluated.
These include:
Because of the complexity involved, many engineers use software tools such as SISTEMA to calculate Performance Levels and verify compliance with ISO 13849.
The objective is to demonstrate that the safety function achieves the required level of reliability throughout its expected operational life.
ISO 13849 does not replace a machine risk assessment.
Instead, it works alongside standards such as ISO 12100.
A typical process involves:
In this way, ISO 12100 helps identify the risks, while ISO 13849 helps ensure the safety systems used to control those risks are sufficiently reliable.
ISO 13849 is relevant whenever machinery relies on control systems to achieve safety.
Common applications include:
It is particularly important where safety functions depend on electrical, electronic, pneumatic or programmable control systems.
Despite being widely used, ISO 13849 is often misunderstood.
Some of the most common mistakes include:
These issues can result in safety systems that appear compliant on paper but fail to achieve the required level of risk reduction in practice.
Designing a safety system is only part of the process.
ISO 13849 also requires validation to confirm that safety functions perform as intended. Validation activities may include testing, inspection and review of calculations to ensure the system meets the required Performance Level.
Without validation, there is no evidence that the safety system will reliably perform its intended function when needed.
ISO 13849 is one of the most important machinery safety standards used today. It provides a framework for designing, assessing and validating safety-related control systems to ensure they achieve an appropriate level of reliability.
By introducing concepts such as Performance Levels, system categories and safety function validation, the standard helps organisations ensure that machinery safety systems perform correctly when required.
When used alongside machine risk assessments and standards such as ISO 12100, ISO 13849 plays a crucial role in reducing machinery risks and supporting safe machine design, modification and operation.
This Article was reviewed and approved by Warren Spiers, owner of Spiers Engineering Safety. Warren has 20+ years of expertise in PUWER and machinery safety compliance. Connect with him on LinkedIn