What is an SRS (Safety Requirement Specification)? Why is it important?

What is a Safety Requirement Specification? (SRS for short)?

A Safety Requirement Specification is one of the most important documents in any Safety Instrumented Systems project. This document forms the design basis of the Safety Instrumented System and has the detailed specifications of every Safety Instrumented Function (SIF) which comprises the SIS.

When is the document made?

If you refer to the Safety Lifecycle according to IEC 61511 (ISA S84 in the US), you will see that there is a separate phase in the lifecycle only focused on the SRS. This is after the Hazard and Risk analysis is done and the different protection layers have been allotted the different safety functions. Now, it is time to exactly specify the details of every SIF. The output of this phase is the SRS.

Safety Life Cycle

Why is the SRS important?

If the SRS is incomplete or poorly made, the design engineer either will not have clear direction on how to build the SIF and consequently make a poor job out of it, or assume something that he/she thinks is correct (since it was not clearly specified) and ultimately result in a SIF that will not meet the original intent. Worse, by the time the asset owner comes to know of this, it will be pretty late in the lifecycle and can mean either lost time or lost money (mostly both) because everything has to be reworked again.

What does an SRS contain?

A Safety Requirement Specification document, will have many specifications and will answer many questions such as

  • What is the Safety Integrity Level of each SIF?
  • What is the SIF architecture?
  • Will there be any redundancy and/or voting logic?
  • What will be the accuracy of the sensors & transmitters?
  • How will a SIF be bypassed?

There are many more questions and specifications than given above. If you have application software, which is part of the SIS functionality, then there should be a software SRS too. The IEC 61511 standard for Safety Instrumented Systems  in the Process industry (ISA S84 in the US), has details of what an SRS should contain.

SRS Video

To understand more about SRS please see the video below.

Where can I get an SRS template or more SRS information?

You can take the Abhisam Safety Instrumented Systems certification course today. Learn everything about Functional Safety, Safety Instrumented Systems and SIL including what needs to be done during every phase of the Safety Lifecycle.

Safety Instrumented Systems Training

Take this free Functional Safety Mock Test to know how well you are in the subject.

Is there any Safety Instrumented Systems book that explains this?

Sure, you can download the free Safety Instrumented Systems book from here.

The Abhisam Quick Guide to Basic Functional Safety & SIL