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What are the two types of SIL?

SIL, or Safety Integrity Level, is a term used in the field of functional safety to measure the reliability of systems. It provides a quantitative measure for the probability of a system failure and the risks associated with it. There are two main types of SIL, namely SIL 1 and SIL 2. In this article, we will explore these two types in detail, their characteristics, and how they are applied in ensuring safety in various industries.

SIL 1: A Moderate Level of Safety Integrity

SIL 1 represents the lowest level of safety integrity. Systems that fall under this category are considered to have a moderate level of integrity and are designed to prevent significant hazards. The primary objective of SIL 1 systems is to reduce the risk to an acceptable level. Here are some key features of SIL 1:

Reliability Requirements

Systems classified as SIL 1 require a minimum average probability of failure on demand (PFDavg) ranging from 10^-2 to 10^-1. This means that the system should be able to perform its intended function without failure in at least 90% of demanded operations.

Risk Reduction Measures

To achieve the required level of safety, SIL 1 systems may incorporate various risk reduction measures such as redundant components, error-checking mechanisms, and safe-state defaults. These measures help enhance the reliability of the system and minimize the likelihood of failures.


SIL 1 systems find applications in industries where failures can lead to minor injuries or discomfort but not severe harm or loss of life. Examples include consumer electronics, appliances, and non-critical machinery.

SIL 2: A High Level of Safety Integrity

SIL 2 represents a higher level of safety integrity compared to SIL 1. Systems classified under this level have stricter requirements to ensure a more robust and reliable performance. Here are some important aspects of SIL 2:

Reliability Requirements

SIL 2 systems have a more stringent average probability of failure on demand (PFDavg) ranging from 10^-3 to 10^-2. This means that the system should have a failure rate of less than 1 in 10,000 to 1 in 100 operations.

Risk Reduction Measures

To achieve the required safety level, SIL 2 systems often incorporate additional measures beyond those found in SIL 1 systems. These may include more redundancy, sophisticated diagnostics, and fault-tolerant designs. Such measures help to minimize the chances of failures and reduce any potential risks significantly.


SIL 2 systems are commonly used in industries where failures can result in serious injuries or harm to the environment. Examples include chemical plants, power generation facilities, and transportation systems.

To summarize, SIL 1 and SIL 2 are two types of Safety Integrity Levels with varying requirements and objectives. SIL 1 provides a moderate level of safety integrity suitable for applications where failures pose minimal risks. On the other hand, SIL 2 offers a higher level of safety integrity and is applied in industries where failures could lead to substantial harm or damage. By categorizing systems into these levels, organizations can ensure the implementation of appropriate safety measures and maintain the overall reliability of their systems.


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