Functional Safety in Instrumentation Systems (SIS, SIF & SIL)

Functional safety is the risk reduction provided by the functions implemented to ensure the safe operations of the process.

The IEC 61508 standard covers functional safety of electrical/electronic/programmable electronic safety-related systems. The purpose of IEC 61508 is to use safety instrumented systems that reduce risk to a tolerable level by following the overall hardware and software safety life cycle procedures and maintaining the associated documentation.

Compliance with IEC 61508 ensures the compliance with industry safety standards such as:

• IEC 61511 Process Industry
• IEC 61513 Nuclear Industry
• IEC 62061 Machinery Industry

All these standards address the entire safety life cycle and are based on performance analysis. Compliance to these standards reduces the risks.

IEC 61508 defines 3 successive levels of safety assessment:

1. Safety Instrumented Systems (SIS)
2. Safety Instrumented Functions (SIF)
3. Safety Integrity Level (SIL)

1. Safety Instrumented System (SIS)

Safety instrumented system is defined as instruments system used to implement one or more safety instrumented functions. It is made up of any combination of items like sensors, logic solvers, final control elements, etc. In other words, SIS is an instrumentation and control system that detects ‘’out of control’’ conditions and automatically returns the process to a safe state.

Related: Methods of Protecting Instruments from Explosive Atmospheres

2. Safety Instrumented Function (SIF)

Safety instrumented function is defined as a function to be implemented by a SIS which is intended to automatically achieve or maintain a safe state for the process with respect to a specific hazardous event.

Safety instrumented function (SIF) is an independent safety loop or interlock that automatically brings the process to a safe state in response to specific initiating events.

3. Safety Integrity Level (SIL)

SIL is the safety integrity level of a particular safety instrumented function (SIF) which is being implemented by a safety instrumented system (SIS).

SIL is a measure of risk reduction provided by a specific SIF.

SIL is normally expressed as a number from 1-4:

• SIL 4 provides the greatest risk reduction.
• SIL 1 provides the lowest risk reduction.

You can also read: Hazardous Locations Classifications