When Is Safety Performance Level Calculation Required?

Understanding Performance Level in Modern Machine Safety Systems

Machine safety has changed dramatically over the last several decades. The days of relying solely on hard guarding and administrative controls are over. Modern manufacturing increasingly depends on safety-related control systems to reduce risk while maintaining productivity. Light curtains, interlocked guards, safety PLCs, safe torque off circuits, pressure-sensitive mats, laser scanners, and two-hand controls are now common in industrial environments.

But as these systems become more advanced, one critical question emerges:

How do you prove the safety system is reliable enough to protect workers?

That is where Safety Performance Level calculations come into play.

What Is Safety Performance Level?

Performance Level (PL) is a measurement of the reliability and effectiveness of a safety-related control system. It is defined in the international standard:

• International Organization for Standardization ISO 13849-1 — Safety of machinery — Safety-related parts of control systems

The standard evaluates how likely a safety system is to successfully perform its intended safety function when needed.

Examples of safety functions include:

• Preventing machine startup while a guard door is open

• Stopping hazardous motion when a light curtain is interrupted

• Removing energy during emergency stop activation

• Preventing unexpected restart after power loss

• Ensuring a robot slows or stops when personnel enter a collaborative workspace

Performance Levels range from:

• PL a → Lowest reliability

• PL e → Highest reliability

Higher risk situations generally require higher Performance Levels.

Why Performance Level Exists

Traditional machine safeguarding often focused on whether a safeguard existed, not whether it would reliably function during real-world operation.

For example:

• A guard interlock may fail internally

• A relay contact may weld closed

• A wire may short to another conductor

• A sensor may lose detection capability

• A safety PLC input may fail unnoticed

If the failure defeats the safety function and no one realizes it, employees may unknowingly be exposed to hazardous energy or motion.

Performance Level calculations attempt to quantify:

• The likelihood of dangerous failures

• The system’s ability to detect failures

• The architecture redundancy of the system

• The expected reliability over time

The goal is not perfection. The goal is reducing the probability of dangerous failure to an acceptably low level based on risk.

When Is Performance Level Calculation Required?

This is one of the most misunderstood topics in machine safety.

There is no OSHA regulation that explicitly states:

“You must calculate Performance Level.”

However, OSHA absolutely requires employers to provide effective safeguarding.

Under the Occupational Safety and Health Act’s General Duty Clause and numerous machine-specific regulations, employers must protect workers from recognized hazards. If a safety control system is used to reduce risk, the employer must be able to demonstrate that the system is sufficiently reliable for the application.

This becomes especially important when relying on:

• Interlocked guards

• Presence sensing devices

• Safety PLC systems

• Electronic safeguarding

• Safety-rated sensors

• Control-reliable systems

• Functional safety systems

In practice, Performance Level calculations become necessary whenever a safety-related control system is being used as a risk reduction measure.

Situations Where PL Calculations Are Commonly Needed

1. New Machine Design

Machine manufacturers routinely perform PL calculations during machine design to comply with international standards and customer requirements.

This is especially common for:

• Robotic systems

• Automated assembly equipment

• CNC machinery

• Packaging systems

• Conveyors

• Presses

• Automated storage systems

Many global manufacturers now require ISO 13849 validation as part of machine acceptance.

2. Machine Modifications

One of the biggest mistakes companies make is modifying a machine without reevaluating the safeguarding system.

Examples include:

• Adding a new guard door

• Changing safety relays

• Integrating robots

• Adding automatic modes

• Altering machine speed

• Integrating multiple machines into a line

• Changing safety logic

Even relatively small modifications can invalidate the original safety design assumptions.

3. Using the Minor Servicing Exception

This is one of the most important applications for Performance Level calculations.

Under OSHA’s Lockout/Tagout standard, certain tasks may qualify for the minor servicing exception if alternative protective measures provide effective employee protection.

Many companies use:

• Interlocked guards

• Presence sensing devices

• Safe speed monitoring

• Safe torque off systems

• Safety-rated control systems

instead of full lockout during specific tasks.

However, this approach only works if the safeguarding system is highly reliable and appropriate for the risk.

A weak or poorly designed safety circuit may not provide equivalent protection.

This is where PL calculations become critical.

If the system is being relied upon in place of lockout during certain tasks, proving the reliability of the safety function becomes extremely important from both a regulatory and liability standpoint.

What Determines the Required Performance Level?

ISO 13849 uses a risk assessment approach.

The required PL depends on factors such as:

• Severity of injury

• Frequency of exposure

• Ability to avoid the hazard

For Example:

• Minor hazard with limited exposure - PL b or c is Required

• Serious crushing hazard - Pl d is required

• High-speed automated robotics - PL e is required

The higher the risk, the higher the required system reliability.

What Goes Into a Performance Level Calculation?

A proper PL calculation evaluates several technical factors.

Category Architecture

ISO 13849 defines system architectures called Categories.

Examples:

• Category B

• Category 1

• Category 2

• Category 3

• Category 4

Higher categories generally include more redundancy and fault detection.

MTTFd (Mean Time to Dangerous Failure)

This estimates how long components are expected to operate before a dangerous failure occurs.

Components with higher reliability improve overall system PL.

Diagnostic Coverage (DC)

Diagnostic coverage measures how effectively the system detects failures.

Examples:

• Cross-monitoring relays

• Pulse testing

• Input discrepancy monitoring

• Short circuit detection

Common Cause Failure (CCF)

This evaluates whether a single event could defeat redundant channels simultaneously.

Examples:

• Poor wiring separation

• Environmental contamination

• Shared power supplies

• Excessive vibration

What Are the Benefits of High-Performance Safety Systems?

Many companies view machine safety as a compliance burden. That mindset misses the operational advantages of modern functional safety systems.

Reduced Injury Risk

The most obvious benefit is preventing catastrophic injuries.

Modern safety systems can dramatically reduce exposure to:

• Crush hazards

• Amputations

• Unexpected startup

• Robotic motion

• Stored energy

• High-speed mechanical movement

Increased Productivity

This surprises many people.

Well-designed safety systems often increase productivity compared to overly restrictive lockout practices.

Examples include:

• Faster tool changes

• Safer jam clearing

• Reduced downtime

• Quicker recovery after interruptions

• Improved maintenance efficiency

This is one reason safety-rated monitored systems have become so popular in advanced manufacturing.

Reduced Need for Full Lockout in Certain Tasks

When properly designed, validated, and risk assessed, safety control systems can allow certain repetitive tasks to occur without complete energy isolation.

Examples may include:

• Minor adjustments

• Sensor cleaning

• Clearing small jams

• Product changeovers

• Visual inspections

This can significantly improve operational efficiency while maintaining worker protection.

Improved Diagnostics and Troubleshooting

Modern safety systems can identify faults before failures become dangerous.

Advanced systems may detect:

• Wiring faults

• Device failures

• Channel discrepancies

• Communication loss

• Unsafe operating states

This reduces unexpected downtime and improves maintenance response.

Better Legal Defensibility

If an incident occurs, companies are often asked:

“How did you determine the safeguarding system was adequate?”

A documented risk assessment and validated Performance Level calculation demonstrates a structured engineering approach rather than guesswork.

Common Misconceptions About Performance Level

“We Only Need PL for Robots”

False.

Any safety-related control system may require PL evaluation.

“A Safety Relay Automatically Makes the System Safe”

False.

The entire safety function must be evaluated, including:

• Inputs

• Logic

• Outputs

• Wiring

• Architecture

• Diagnostics

“OSHA Doesn’t Mention PL, So We Don’t Need It”

Dangerous assumption.

OSHA frequently references consensus standards and expects employers to provide effective protection. Modern machine safeguarding increasingly depends on recognized functional safety principles.

“Only Engineers Need to Understand This”

Safety professionals should absolutely understand the fundamentals of functional safety.

Many EHS professionals are now heavily involved in:

• Machine risk assessments

• Safeguarding validation

• LOTO alternative methods

• Capital project reviews

• Robot integration reviews

• Functional safety management

Understanding Performance Level is rapidly becoming a core competency in advanced manufacturing safety.

The Future of Machine Safety

Manufacturing is moving toward increasingly intelligent safety systems.

Examples include:

• Safety-rated robotics

• Collaborative robots

• Vision-based safety systems

• Zone-based safeguarding

• Safe speed monitoring

• Integrated safety networks

• IoT-connected safety devices

• Real-time safety diagnostics

As these technologies expand, functional safety concepts like Performance Level will become even more important.

The future of machine safety is not simply “putting guards on machines.”

It is designing systems that intelligently reduce risk while preserving operational efficiency.

Final Thoughts

Performance Level calculations are not just an engineering exercise. They are a structured method for determining whether a safety system can truly be trusted to protect workers.

If your facility relies on:

• Interlocked guarding

• Presence sensing devices

• Safety PLCs

• Alternative protective measures

• Robot safety systems

• Functional safety architectures

then understanding Performance Level is becoming increasingly important.

For many modern applications, especially those involving automated machinery and alternative protective measures under Lockout/Tagout, validating the reliability of the safety function is no longer optional from a practical standpoint.

The companies that understand functional safety today will be far better prepared for the future of industrial safety tomorrow.