So you’re faced with the question of whether that trusty old transmitter you’ve been using as part of your safety shutdown system can meet the “prior use” requirements described in Section 11.5.3 of ANSI/ISA-84.00.01-2004 (IEC 61511-1 Mod). 

Prior use, or Proven-in-use, is defined as a documented assessment showing there is appropriate evidence, based on the previous use of the component, that the component is suitable for use in a safety instrumented system.

Let’s focus on the question “how much operating experience is required to make sure a product has enough safety integrity?”

Lucky for us, IEC 61508 provides specific details on this.

For a given component’s version level IEC 61508 suggests 100,000 unit hours for components targeted for SIL 1 applications and 10 million unit hours for components targeted for SIL 3 applications.  Along with the operating hours, one must also provide documented proof test results for all detected dangerous failures.

Of course there’s a lot more discussion on ISA’s web site in an article written by Bill Goble.  www.isa.org