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OT AssociatesPURE PLAY OT SECURITY
← The OT Field Guide
02

Safety holds the final word

No security change goes live until safety and operations have confirmed it cannot harm the process.

4 min readInteractive figureISA/IEC 62443-2-1IEC 61511
Interactive figure
A security change, through the plant's gates
FW-118: block Modbus writes from site operations (L3) to controllers (L1)
  1. GATE 1
    Process impact assessed
    OT security + control engineer
  2. GATE 2
    Tested on a representative system
    Test bench, never the live process
  3. GATE 3
    Process safety review
    Process safety engineer
  4. GATE 4
    Operations approval
    Asset owner · may veto or defer
  5. GATE 5
    Window and rollback
    Planned window, tested way back
  6. GATE 6
    Live and verified
    Verified after the change
Advance the change one gate at a time. Try Change B, or let operations defer Change A at gate 4.

Send a security change through the gates and see who can stop it.

Security serves the process

A firewall rule, a hardening baseline or a new monitoring sensor is a change to a live industrial system. In OT, a well-intended change that interrupts a control loop or blocks an alarm is itself an incident.

We therefore treat every security change as an engineering change: assessed for process impact, tested on representative systems, scheduled into the right window and always reversible.

Who decides

Security proposes; operations and process safety decide. The asset owner can stop or defer any change. A security team that cannot accept that will not be trusted in the control room, and a security programme without that trust will not survive its first trip.

Our implementation work is built this way: every segmentation or hardening step is validated, tested and reversible, with operations holding the final word.

Takeaways
  1. 01Treat security changes as engineering changes.
  2. 02Test on representative systems, never first on the live process.
  3. 03Operations and process safety can veto; plan for it.