What is SIL (Safety Integrity Level)?

Indicator of Reliability of Mechanisms to Prevent Accidents

Various safety devices are used in factory machinery, railways, power plants, and other facilities to prevent accidents. For example, there are sensors that stop machinery when a person enters a hazardous area, and systems that shut down equipment when a hazardous condition is detected.

But what happens if that safety device malfunctions? If a device that is supposed to protect us fails to work, it could lead to a serious accident.

The concept used in this context is SIL (Safety Integrity Level).
SIL is an index that indicates how reliably a safety device or system will function (how trustworthy it is). In other words, it is a level representation of “how well that safety function will prevent accidents.”

SIL is divided into 4 levels

SIL has the following four levels:

SIL1：Basic safety level
SIL2：Higher safety level
SIL3：Very high safety level
SIL4：Extremely high safety level

The higher the number, the lower the probability of safety function failure. In other words, SIL4 represents the most stringent safety level.

However, not all equipment requires SIL4. The required level varies according to the severity of the hazards that would result from an accident.

How is “required SIL” determined?

Required SIL is not decided simply by a feeling of “Let’s make it high because it looks dangerous.” Generally, it is determined by evaluating the risk based on the following factors:

Significance of damage when an accident occurs
How frequently hazardous situations occur
Likelihood that people can avoid the hazard
How long people are exposed to hazardous conditions

These factors are the basis of analyzing the risks. They determine “how reliable the safety function needs to be” and thus decide “the required SIL.”

For example, SIL3 and SIL4, which are the high safety integrity levels, are required for equipment where an accident can affect many lives. On the other hand, SIL1 or SIL2 can be sufficient when damages are relatively minor.

In this way, SIL is a system for determining the required safety level according to the magnitude of the hazard.

PL as Another Indicator for Machinery Field

When we consider machine safety, there is an indicator PL (Performance Level) which is often compared with SIL.
PL is a concept, which is mainly used in machine safety standards, where the reliability of safety function is represented on a five-point scale from a to e.

PL a: Low safety level
PL b
PL c
PL d
PL e: Very high safety level

PL is, like SIL, based on the concept of the possibility of a safety function failing. Namely, both are indicators for evaluating the “reliability of safety functions.”

However, they differ in the fields in which they are used.

SIL: Plants, process industries, railways, etc.
PL: Industrial machinery (safety of machinery and equipment)

Therefore, PL is often used by machine manufacturers and is frequently found in the catalog of safety components.

Why SIL and PL are used in different fields

PL is mainly used in machine safety standards (ISO 13849) and is suitable for ensuring safety within a single machine. For example, it is an idea of establishing safety by combining components such as safety switches, relays, and sensors.

In contrast, SIL is an indicator which was initially created to consider the safety of a large system as a whole, such as plants. For example, entire factory production lines, chemical plants, and power plants, which are large scale systems where multiple devices are connected. SIL put its importance on “how well the entire system can prevent accidents.”

Era of Thinking Safety by “Numbers”

In the past, safety design relied heavily on experience and intuition. However, today, it is widely accepted to analyze risks and numerically assess safety to prevent accidents.
SIL and PL are the important indicators for this approach.

These concepts have made it possible to objectively decide how reliable a safety device is. To ensure safety in factories and infrastructures, SIL and PL play a crucial role.