A SIL 3 loop (one failure in 10,000 years) is mathematically robust against random hardware failures—but completely blind to a single malicious write command over Modbus TCP. TR84.00.09 introduced the concept of for security, arguing that a safety function can only claim its SIL if the supporting cybersecurity controls maintain the integrity of the logic, data, and timing.
ISA-TR84.00.09 didn’t just predict the collision of safety and security. It gave us the tools to survive it. The only question is whether we’ll use them before the next TRITON finds its target. Next time you see a SIL-rated safety controller, don’t ask, “Is it fail-safe?” Ask, “Is it cyber-safe?” And when you get a blank stare, hand them a copy of ISA-TR84.00.09. It’s short, it’s free for ISA members, and it might just save their plant. isa-tr84.00.09
In the world of industrial control systems (ICS), two documents get all the glory. There’s ISA-62443 (IEC 62443) , the sprawling, multi-part behemoth that serves as the constitution for industrial cybersecurity. And then there’s ISA-84 (IEC 61511) , the bible of functional safety (SIS/SIL). They sit on opposite ends of the engineering bookshelf, rarely speaking to one another. A SIL 3 loop (one failure in 10,000
Published in 2008 (and reaffirmed since), this document—formally titled “Security Countermeasures Related to Safety Instrumented Systems (SIS)” —asked a heretical question at the time: What happens when a cyber attack targets a safety system? It gave us the tools to survive it