Industrial Safety

Demystifying Functional Safety: SIS, SIL, and MooN Explained

Uncover the basics of SIS, SIL, and MooN.

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

First of all, here are a few confusing acronyms for you… SIS, SIL, and MooN…Stick around because we will attempt to address each term and discuss how they are related, or interact with each other.

What is 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.

Safety Instrumented System (SIS)

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

Ok… so let's be clear… the SIS is one Protection Layer in a multi-layered safety approach, as no single safety measure alone can eliminate risk.The regulatory process control system and the SIS are separate.The process control system operates the equipment or process and maintains it within safe parameters and is considered the first layer of protection.The SIS provides another layer of protection by monitoring the equipment or process. If an unacceptable condition or risk of an unsafe condition occurs, it reacts by shutting down the equipment or process.The Safety Instrumented System consists of a distinct set of devices operating independently from the process control system.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.