Functional safety is a critical consideration when designing modern industrial automation systems. Safety PLCs, safety I/O, emergency stops, interlocks and protective devices must work together to reduce risks to people, equipment and processes.
Two of the most commonly used concepts are SIL (Safety Integrity Level) and PL (Performance Level). Although they are related, they come from different standards and should not be treated as interchangeable ratings.

1- What Is Functional Safety?
Functional safety focuses on ensuring that safety-related control systems perform their required safety functions when needed.
Examples include:

– Emergency stop systems
– Safety gates and door switches
– Light curtains
– Two-hand controls
– Safe motor stopping
– Overspeed protection
– Burner management
– Pressure protection
– Safety interlocks
The objective is to reduce the probability that a dangerous failure of the safety system will prevent the required safety function from operating.

2- What Is SIL?
SIL — Safety Integrity Level is primarily defined by the IEC 61508 family of functional safety standards.
There are four SIL levels:
– SIL 1 -> Lowest
– SIL 2 -> Medium
– SIL 3 -> High
– SIL 4 -> Very high

In industrial automation, SIL 1, SIL 2 and SIL 3 are the levels most commonly encountered.
A higher SIL corresponds to a lower probability of dangerous failure for the safety function.
However, SIL should not simply be selected based on the type of PLC. The required SIL is determined through a safety analysis of the application and the specific safety function.

3- What Is PL?
PL — Performance Level is defined by ISO 13849-1, which is widely used for machinery safety.
The five Performance Levels are:
PL a → PL b → PL c → PL d → PL e
With PL e representing the highest level of performance.
The required PL is determined through a risk assessment of the machine.
The assessment considers factors such as:
– Severity of injury
– Frequency of exposure
– Possibility of avoiding the hazard
– The resulting risk determines the required performance level for the safety function.

4- SIL vs. PL
SIL and PL are based on different standards and calculation methods ( IEC 61508 family, ISO 13849-1) :
– Used broadly for functional safety systems
– Particularly common in machinery
– Based on safety integrity concepts
– Based on architecture, reliability and diagnostic capability

They can sometimes be compared using probability-of-failure metrics, but SIL 2 should not automatically be considered identical to PL d.
The applicable standard and safety architecture must always be considered.

5- How to Select a Safety PLC
Selecting a safety PLC should start with the safety function, not the PLC itself.
A typical process is:
Risk Assessment → Required Safety Level → Safety Function → Architecture → Safety PLC → Validation

For example, a machine may require a safety function such as:
– Emergency stop → Safe logic → Motor power removal
– The complete chain must be capable of achieving the required safety level.
– This means that the safety PLC alone does not determine the final SIL or PL.

6- Safety PLC Architecture
A typical safety control system may include:
Safety Sensors → Safety I/O → Safety PLC → Safety Outputs → Final Control Elements
Depending on the application, the architecture may include:
– Redundant inputs
– Dual-channel emergency stops
– Safety relays
– Safety PLC
– Redundant outputs
– Safety contactors
– Safe drives
– Safety communication networks
The complete safety-related control system must be analyzed and validated.

7- Safety Communication
Modern safety systems increasingly use industrial networks to exchange safety-related information.
Examples include:
– PROFIsafe
– CIP Safety
– Safety over EtherCAT
– Modbus Safety, depending on the application and implementation
These technologies can reduce wiring and simplify distributed safety architectures while maintaining the required safety functions.

8- Validation Is Essential
Programming a safety PLC is only one part of a functional safety project.
The safety system must be validated against the defined safety requirements.
Testing should verify:
– Emergency-stop functions
– Safety gates
– Sensors
– Safety outputs
– Safe stopping
– Fault detection
– Diagnostic functions
– Restart conditions
– Failure scenarios
The validation process should demonstrate that the implemented safety function achieves its required safety performance.

9- Machinery vs. Process Applications
The applicable safety standards can depend on the type of installation.
For machinery, ISO 13849-1 and IEC 62061 are commonly relevant.
For broader functional safety systems, particularly in process industries, the IEC 61508/IEC 61511 framework may be applicable.
The correct standard should therefore be established during the initial engineering and risk-assessment phase.

Conclusion
SIL and PL are not simply labels attached to a safety PLC. They represent the required safety performance of a complete safety-related function.
A robust approach is:
Risk Assessment → Safety Requirements → Safety Architecture → Component Selection → Programming → Verification → Validation
At ICS Automatix, we support industrial automation projects involving PLC and safety systems, control architectures, modernization and commissioning. Our approach focuses on selecting appropriate technologies and architectures according to the application’s technical and functional safety requirements.