Safety Category


Thread Starter



I would like to confirm my understanding of Safety category 2 and 3. I'm new to this and just researched on the internet. We are designing a Emergency Shutdown system on a Biogas Plant.

Safety category is based on the wiring strategy and not solely based on the safety rating of the devices (i.e. e-stop, safety relays, etc.). My understanding is category 2 has a single input and output. Category 3 has a redundant input and output.

Is my understanding correct?

Also, are there any standard regarding safety category based on the application? As stated, we are designing the ESD on a Biogas Plant.

Ivan Joseph
You need to understand process safety and functional safety. There are levels of safety:

-design of process
-pressure relief, piping design
-process control
-Safety Instrumented System

Every system or level is designed to keep the process safe.

A HAZOP will determine the risks in your plant and what systems will mitigate the risk. If some of the residual risks are significantly high in impact and probability they will need a very reliable interlock, trip or ESD system. The level of reliability required determines the SIL level of the system.

The reliability factor of every component in a SIS system determines it's SIL level.