3 thoughts on “Difference between SIS, PLC and BPCS Systems”

  1. The key differences between Control System (DCS/BPCS) and the Safety system (SIS/ESD) are below.

    Control Systems

    * Control system works 24 / 7 (hours per day/days per week)
    * No guarantee on the state of outputs during failure of control system, most likely outputs on hold
    * High flexibility needed to develop and maintain (complex) control and automation applications
    * Improvements or changes in the configuration but also repairs are mainly implemented on-line
    * Accepted risk of plant disturbance in order to avoid a maintenance shut-down of the plant
    * No need to test control system regularly except for some back-up/redundant parts

    Safety Instrumented Systems

    * Safety system always works on hot standby “Sleeping” mode. (On-demand mode)
    * Predictable state of output on any revealed failure in the system “Failsafe” design
    * Fixed functionality, carefully minimized during design
    * No modification of safeguarding functionality in a running plant
    * Stringent procedures to make any change
    * Limited possibilities to repair the hardware while the plant is running
    * Explicit procedure and strategy to test for unrevealed failures of instrumented protective functions
    * Automatic tests (that are intended to reduce risk of unrevealed failures): line monitoring, partial stroke testing


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