Demystifying Functional Safety: SIS, SIL, and MooN Explained

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⌚Timestamps:

00:00 - Intro
00:24 - What is Functional Safety?
01:27 - Safety Instrumented System (SIS)
02:51 - Safety Integrity Level (SIL)
05:47 - MooN system
07:01 - Summary

In this video, we will explore the concepts commonly included within the domain of Functional Safety.

The International Electrotechnical Commission (IEC), defines functional safety as identifying potential dangers and activating protective measures to prevent or reduce the impact of hazardous events.

It seems logical then that the goals of functional safety are to ensure a system or equipment is operating appropriately and actively prevent the failure of a system from causing harm to people and property.

All regulatory process control systems are designed and installed with safety in mind.

Even so, the risk of injury, fire, explosion, and other catastrophes is not at a tolerable level.

Deploying a functional safety system allows for an automated, safe shutdown of processing units in instances of unusually risky or potentially catastrophic situations that exceed the capabilities of the automatic process control system or trained operators to rectify.

Adding another layer of protection called the Safety Instrumented System, or SIS, moves us closer to achieving the desired functional safety condition.

The regulatory process control system and the SIS are separate.

A Safety Instrumented System consists of sensors, logic solvers, and final control elements with the sole aim of ensuring the process enters a safe state upon encountering specific conditions.

The logic solver is often a Safety PLC manufactured by vendors such as Allen Bradley, Omron, or Schneider.

In functional safety, SIL is a measure of the probability of failure on demand (PFD) for a Safety Instrumented System. PFD is the probability that a device or logic solver will fail causing the SIS not to respond when demanded.

There are four discrete SIL integrity levels. The higher the SIL level, the lower the PFD for the safety system.

A SIL integrity level applies to an entire system. Individual devices or components do not have SIL ratings.

To be clear, there is no such thing as a SIL-rated device. You can't buy SIL-rated temperature transmitters. You can buy a transmitter suitable for use in a SIL environment.

The International Electrotechnical Commission IEC 61508:2010 standard classifies the safety functions into four discrete SIL integrity levels.

Alright, so how are the SIL environment requirements determined? In a nutshell, each hazard is assigned to a target SIL. The determination process involves Hazard Analysis and Risk Assessment to bring the overall risk to an acceptable level.

Not every company uses the same determination process. There's the Layer of Protection Analysis (LOPA) method whereby all known process hazards and layers of protection are carefully examined.

Some companies use an Assignment Matrix to assign a SIL based on a qualitative ranking of the event likelihood, event consequences, and the availability of regulatory process control safeguards already in place.

Let's go to the MooN system concept… Not the actual MooN in the sky, but the "M out of N" system.

The MooN system introduces a collective decision-making framework. It is defined as a system with N units (components, channels, etc.), in which M out of N units are sufficient to initiate an action.

There are a required minimum of M units to vote in agreement to command the execution of the safety action.

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