What Is Performance Level? PL Ratings in Machine and Robot Safety.

What Is Performance Level?

Performance Level, or PL, is a classification used by ISO 13849 to describe the capability of safety-related parts of a machine control system to perform a defined safety function under specified conditions.

Performance Levels range from PL a through PL e, with PL e representing the highest level of safety-related performance within the classification.

In robotics, Performance Levels help engineers evaluate functions such as protective stopping, guard monitoring, and other safety-related controls.

A Performance Level applies to a defined safety function or safety-related control arrangement, not automatically to the overall safety of an entire robot.

Key Takeaways

• Performance Levels range from PL a to PL e under ISO 13849.
• The required Performance Level, or PLr, is determined through risk assessment.
• The achieved Performance Level depends on system architecture, component reliability, diagnostic coverage, and other relevant factors.

How Does Performance Level Work?

Imagine a robotic production cell containing moving equipment.

A protective gate is installed to restrict access to hazardous movement.

The gate has an interlock connected to a safety-related control system.

When the gate opens, the system must initiate the required protective response.

Engineers evaluate how reliably that safety function needs to operate.

The required Performance Level is determined from the risks associated with the application.

The system is then designed and evaluated to determine whether the achieved Performance Level satisfies the required level.

What Are PL a Through PL e?

ISO 13849 defines five Performance Levels.

PL a: Lowest performance capability within the defined scale.

PL b: Higher performance than PL a.

PL c: Intermediate performance level.

PL d: Higher performance level commonly encountered in industrial safety-related systems.

PL e: Highest performance capability within the defined scale.

These levels are related to ranges of average probability of dangerous failure per hour.

However, selecting a level is not simply a matter of choosing the highest available rating.

The required performance must be determined through appropriate risk assessment.

What Is the Difference Between PLr and PL?

PLr is the required Performance Level for a defined safety function.

PL is the Performance Level achieved by the designed and evaluated safety-related control system.

For example, a risk assessment may determine that a protective function requires PL d.

Engineers must then design and validate a system that achieves at least the required performance under the applicable standard.

A component with a particular rating does not automatically establish that the complete function achieves that level.

What Determines the Achieved Performance Level?

Important factors include:

System architecture: How the safety-related components are arranged and how they respond to faults.

MTTFd: Mean time to dangerous failure, used in reliability assessment.

Diagnostic coverage: The effectiveness of detecting certain dangerous failures.

Common-cause failures: Failures that can affect multiple components through a shared cause.

Systematic measures: Design and validation practices relevant to dependable safety functions.

These factors must be evaluated together.

Performance Level vs. Safety Integrity Level

Performance Level and Safety Integrity Level, or SIL, are both used in functional safety.

However, they belong to different standards frameworks.

PL is associated with ISO 13849.

SIL is used in standards including IEC 61508 and IEC 62061.

Both address safety-related performance, but their assessment requirements and classifications differ.

They should not be treated as automatically interchangeable.

Does PL e Mean a Robot Is Completely Safe?

No.

A high Performance Level concerns the reliability of a specified safety function.

A robot can have appropriately rated safety functions while still presenting hazards from sharp tools, falling objects, stored energy, or other physical conditions.

Overall safety depends on the complete application and its risk-reduction measures.

Frequently Asked Questions

Does every safety function need PL e?

No. The required level is determined through risk assessment.

Can a complete safety function achieve a lower PL than one of its components?

Yes. The performance of the complete system depends on the architecture and all relevant components, not just one device's rating.

The Bottom Line

Performance Level helps engineers evaluate how dependably machine safety-related control functions operate. It is an important part of functional safety, but must always be considered in the context of the specific function and its required risk reduction.