Understanding Performance Level (PL) and Its Importance in Safety Systems
In the context of industrial automation, safety is paramount, particularly in environments that involve robots, automated machines, and other high-risk machinery. As automation continues to advance, the risks associated with these technologies are becoming more complex, and so are the safety standards designed to mitigate them. One of the most critical safety concepts is Performance Level (PL), a crucial element in determining the effectiveness of safety devices and systems. This article provides an in-depth understanding of what Performance Level (PL) is, why it matters, and how it is evaluated and applied in industrial safety systems, particularly in robot cells.
What is Performance Level (PL)?
Performance Level (PL) is a standardized measurement used to define the safety performance of safety-related control systems, including safety devices such as sensors, emergency stop buttons, and safety interlocks. It is part of the broader framework established by the international standard ISO 13849-1, which governs the design and operation of safety-related parts of control systems. PL quantifies the level of risk reduction that a safety system can achieve in the event of a failure.
In simpler terms, PL tells us how reliable and effective a safety system is at preventing harm in the event of an emergency. It is based on a combination of factors including the reliability of the safety components, redundancy, and the probability of failure.
How is PL Evaluated?
PL is evaluated by taking into account the risk assessment of a particular system, the category of the safety device, and the performance requirements. It is determined by analyzing several key factors:
1. Risk Assessment
According to ISO 12100 (the standard for risk assessment in machinery), before a Performance Level can be assigned, a risk assessment of the machinery or system must be conducted. This assessment considers the severity of potential harm, the likelihood of harm occurring, and how effectively the safety system mitigates the risk.
Severity of harm: What is the potential severity of injury or damage if the system fails?
Probability of occurrence: How likely is the failure to happen under normal operation?
Frequency of exposure: How often are operators exposed to the risk?
2. Categories of Safety Devices (ISO 13849-1)
The performance of a safety system is heavily influenced by the category or type of safety device used. ISO 13849-1 defines several categories, with each category offering a different level of reliability:
Category B: Safety functions are implemented with a single component. The system has a relatively low level of reliability.
Category 1: A system that includes more reliable components but is still susceptible to failure under certain conditions.
Category 2: A system with components designed to increase reliability, but the system might still rely on the manual intervention of an operator.
Category 3: A higher level of reliability where components are designed to fail safely, reducing the risk of a hazard during operation.
Category 4: The highest level of reliability, where the failure of any single component will not result in a failure of the safety function.
The higher the category, the more stringent the requirements for redundancy, diagnostics, and monitoring of safety components.
3. Safety Functions and Architecture
Each safety function must be designed with a specific PL in mind. Safety functions in industrial machinery typically include emergency stop systems, safety interlocks, safety sensors, and safety mats. The architecture of the safety system, which may include redundant components (e.g., dual-channel systems), plays a vital role in determining the level of performance.
A system that incorporates redundancy—such as two separate safety circuits that can take over in case one fails—tends to achieve a higher Performance Level.
4. Performance Level Calculation (ISO 13849-1 and IEC 62061)
According to ISO 13849-1, the calculation of PL is influenced by the Safety Integrity Level (SIL) (as per IEC 61508 and IEC 62061) and the mean time to failure (MTTF) of the components. To determine PL, manufacturers assess the following criteria:
Mean time to dangerous failure (MTTFd): The average time it takes for a safety component to fail in a dangerous state.
Diagnostic Coverage (DC): The effectiveness of the system in detecting faults.
Common Cause Failure (CCF): The risk that a failure in one component could cause the entire system to fail.
The formula then uses these inputs to generate a Performance Level (PL) from PL a (the lowest level) to PL e (the highest level). Each PL corresponds to a specific probability of failure:
PL a: 10^-5 to 10^-4 failures per hour
PL b: 10^-4 to 10^-3 failures per hour
PL c: 10^-3 to 10^-2 failures per hour
PL d: 10^-2 to 10^-1 failures per hour
PL e: 10^-1 or fewer failures per hour
The higher the PL, the more reliable and safer the system, with fewer failures per hour of operation.
Why PL Matters: The Impact on Safety and Compliance
The Performance Level of safety devices directly impacts the safety of workers and the compliance of a facility with international standards. Below are a few key reasons why PL is so important in safety systems, particularly for robot cells:
1. Minimizing Risks to Human Life
Robot cells and automated machinery can present significant dangers to workers, especially when safety devices fail. The primary role of safety devices—whether sensors, light curtains, or emergency stops—is to prevent accidents and ensure that the system stops or mitigates danger when a fault occurs. Assigning the proper Performance Level to these devices ensures that the system meets the necessary reliability to keep workers safe from injury.
2. Meeting Regulatory Standards
International standards such as ISO 13849-1 and ANSI/RIA R15.06 (which addresses safety requirements for industrial robots) provide the framework for assessing the safety performance of robot cells and their safety devices. By complying with these standards, companies can ensure that their systems are safe and legal, reducing the likelihood of OSHA violations or other regulatory issues.
3. Improving Operational Efficiency
When safety systems are well-designed, with a properly calculated PL, robot cells are less likely to experience unexpected shutdowns, which can cause costly production delays. Additionally, well-designed safety systems help to minimize downtime by reducing the risk of major safety incidents, ensuring a smoother workflow.
Conclusion: The Vital Role of Performance Level in Safety Systems
Performance Level (PL) is a critical metric in the design, selection, and implementation of safety devices for robot cells and other industrial machinery. By understanding and properly calculating PL, manufacturers can ensure that their safety systems are effective, reliable, and compliant with industry standards.
As automation becomes more pervasive, the need for sophisticated safety systems becomes even greater. By prioritizing high-performance safety devices, companies can minimize risks to workers, improve productivity, and ensure compliance with evolving global safety standards.
Citations:
ISO 13849-1: Safety of machinery—Safety-related parts of control systems—Part 1: General principles for design.
IEC 61508: Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems.
ANSI/RIA R15.06: Industrial Robots and Robot Systems – Safety Requirements.
