What Is SIL? Safety Integrity Levels Explained for Industrial Machinery.

What Is Safety Integrity Level?

Safety Integrity Level, or SIL, is a classification used in functional safety to describe the integrity required of a safety-related function.

Under IEC 61508, Safety Integrity Levels range from SIL 1 through SIL 4.

Higher levels correspond to more demanding safety-integrity requirements.

In industrial robotics, SIL concepts are relevant to functions designed to reduce risks associated with machinery and control-system failures.

For example, an industrial drive's protective function may be evaluated according to applicable functional-safety standards.

Key Takeaways

• SIL describes the safety integrity of a defined safety-related function.
• IEC 61508 defines four Safety Integrity Levels, from SIL 1 through SIL 4.
• SIL is not a general rating of how safe an entire robot or factory is.

How Does Safety Integrity Level Work?

Imagine an industrial machine with a hazardous moving mechanism.

Engineers determine that a particular protective function is necessary.

For example, the function may need to stop hazardous movement when a specified condition occurs.

A risk assessment helps determine the level of risk reduction required.

The safety function is then designed and evaluated according to the relevant standards.

The evaluation considers the probability of dangerous failure and other requirements involving architecture, systematic capability, and development practices.

The achieved level must satisfy the applicable requirements for the function.

What Are SIL 1 Through SIL 4?

IEC 61508 defines four levels.

SIL 1: Lowest safety-integrity level within the scale.

SIL 2: More demanding integrity requirements.

SIL 3: Higher integrity requirements.

SIL 4: Highest integrity level within IEC 61508.

These levels are associated with ranges of dangerous failure probability.

For example, under IEC 61508's high-demand or continuous-operation framework, SIL 2 corresponds to a dangerous failure rate of at least 10⁻⁷ and less than 10⁻⁶ per hour, within the relevant standard's conditions.

However, a SIL assessment involves more than selecting a failure-rate range.

Systematic capability and architectural requirements also matter.

SIL vs. Performance Level

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

SIL: Associated with standards such as IEC 61508 and IEC 62061.

PL: Associated with ISO 13849.

The methods have different classifications and requirements.

For example, ISO 13849 uses Performance Levels from PL a through PL e.

Although relationships between some levels can be evaluated for particular safety functions, they should not be treated as automatically interchangeable.

The applicable standard and complete design matter.

What Is SIL 3 Certification?

A product may be assessed for suitability in safety functions up to a particular SIL under defined conditions.

However, a component carrying a SIL-related certification does not automatically make every system containing it compliant with that level.

For example, a safety-rated drive may be one component of a protective stopping function.

Sensors, control logic, wiring, configuration, and the complete machine response still need appropriate evaluation.

The function's achieved integrity depends on the full design.

Does Higher SIL Mean Better Safety?

A higher SIL represents more demanding safety-integrity requirements for a particular function.

However, a machine with a SIL 3 function is not automatically safer in every respect than a machine with a SIL 2 function.

The machines may have different hazards, protective measures, and operating conditions.

The required level should be based on the actual risk rather than choosing the highest label available.

Frequently Asked Questions

Does every robot safety function require SIL 3?

No. Requirements depend on the risk assessment and applicable standards.

Is SIL 4 commonly required in industrial robot cells?

Not generally. SIL 4 is associated with especially demanding safety-integrity requirements and is uncommon in ordinary machine-safety applications.

The Bottom Line

Safety Integrity Level provides a framework for evaluating the dependability of defined safety-related functions. Its importance lies in matching engineering evidence and performance to the risk reduction required by the application.