# How to calculate safety Integrity level (SIL)? **URL:** \ **Category:** Field Instrumentation\ **Created:** [October 14, 2019, 7:29pm UTC](https://forumautomation.com/t/how-to-calculate-safety-integrity-level-sil/7284 "2019-10-14T19:29:29Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![sivaranjith](https://forumautomation.com/letter_avatar_proxy/v4/letter/s/ba9def/32.png) [@sivaranjith](https://forumautomation.com/u/sivaranjith)\ **Post date:** [October 14, 2019, 7:29pm UTC](https://forumautomation.com/t/how-to-calculate-safety-integrity-level-sil/7284/1 "2019-10-14T19:29:30Z")
![SIL](https://cdn.forumautomation.com/original/2X/0/066536267837a3a1b2d1b92e8ce1e7f75343c20a.png) [color=#ff0000][size=5]Calculating SIL suitability levels:[/size][/color] [color=#1C4587][size=3]To General Monitors, it is a combined effort between corporate reliability, technology, and a third party to verify estimates to determine the [**SIL**](https://forumautomation.com/t/what-is-sil-in-process-industry-how-to-calculate-sil-level/4682) suitability standard to individual products. The steps in suitability include:[/size][/color] - failure rate prediction - FMEDA - Failure Path Investigation > [@SIS controllers(logic solvers) - Safety Instrumented Functions and Systems](https://forumautomation.com/t/sis-controllers-logic-solvers-safety-instrumented-functions-and-systems/3324): > > What is SIS? A Safety Instrumented Function, or SIF, is one or more components designed to execute a specific safety-related task in the event of a specific dangerous condition. A Safety Instrumented System, or SIS, is a collection of SIFs designed to bring an industrial process to a safe condition in the event of any dangerous detected conditions. Also known as Emergency Shutdown (ESD) or Protective Instrument Systems (PIS). SIS controllers: Control hardware for safety instrumented functio… [color=#ff0000][size=5]Step 1 Failure Rate Prediction:[/size][/color] [color=#1C4587][size=3]This initial step is the basis for all measurements of the material SIL. All failure rates of individual components contribute to the overall failure rate of the product within the system.[/size][/color] - [color=#1C4587][size=3]BOM component failure rates are calculated based on component type and in-circuit bias[/size][/color] - [color=#1C4587][size=3]The rate of product failure is the sum of all the rate of component failure.[/size][/color] [color=#ff0000][size=5]Step 2 Failure Mode Effects and Diagnostic Analysis (FMEDA):[/size][/color] [color=#1C4587][size=3]Effects of Failure Mode and Diagnostic Analysis (FMEDA) is a methodology that measures and quantifies the self-diagnostic potential of the material. The failure mode of each component and its failure rate is listed.[/size][/color] [color=#1C4587][size=3]Failure modes are determined as safe (fire / gas detectable product) or dangerous (fire / gas detectable product). Any safe and dangerous mode of failure is determined by the diagnosis of the product as Detectable or Undetectable.[/size][/color] [color=#ff0000][size=5]Step 3 Failure Path Investigation:[/size][/color] [color=#1C4587][size=3]Using the actual product, known failure modes not identified during the FMEDA phase are investigated. This requires leads for components to be shortened and opened and classification of failure identified. SD, DD, SU, DU.[/size][/color] [color=#ff0000][size=5]Step 4 Calculate SFF, SIL and PFD:[/size][/color] [color=#1C4587][size=3]The product’s rate of SIL is finally determined by the Secure Failure Fraction (SFF) and the probability of Demand Failure (PFD). The following equations are being used.[/size][/color] SFF = (lSD + lSU + lDD) / (lSD + lSU + lDD + lDU) PFD = (lDU)(Proof Test Interval)/2 + (lDD)(Down Time or Repair Time) [color=#1C4587][size=3]SIL level is then determined from the Safe Failure Fraction for type B equipment per the IEC 61508.[/size][/color] [color=#ff0000][size=5]Step 5 Third Party Review:[/size][/color] [color=#1C4587][size=3]For analysis and confirmation, all findings from the above measures are forwarded to a third party.[/size][/color] > [@SIS sensors, What is SIS system?](https://forumautomation.com/t/sis-sensors-what-is-sis-system/5655): > > SIS system: The [Safety Instrumented System (SIS)](https://forumautomation.com/t/sis-controllers-logic-solvers-safety-instrumented-functions-and-systems/3324) plays a vital role in providing a protective layer around industrial process systems. Whether called an SIS, emergency or safety shutdown system, or a safety interlock. The purpose of SIS is to take process to a “safe state” when pre-determined set points have been exceeded or when safe operating conditions have been transgressed. An SIS is comprised of safety functions (see SIF below) with sensors, logic solvers and actuators: [sis%20sensor%201…