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Safety Instrumented Systems (SIS)


ANSI/ISA-S84.00.01, (IEC 61511 Mod), Application of Safety Instrumented Systems for the Process Industries, (called IEC 61511 / ISA 84 here) addresses the application of Safety Instrumented Systems (SIS) where needed.

SIS are intended to help take a process to a safe state when predetermined conditions are violated, such as set points for pressure, temperature, level, etc. Its objective is to define requirements for SISs. SISs are also called Emergency Shutdown Systems, Safety Shutdown Systems, and Safety Interlock Systems. SISs provide safety control functions and complement the Basic Process Control System (BPCS) which provides normal process control. The IEC 61511 / ISA 84 standard addresses the entire lifecycle for SISs. Compliance with the standard requires significant effort.


Services Offered:

  • Determination of required Safety Integrity Levels (SILs).
  • Layers of Protection Analysis (LOPA), Risk graphs, and Risk Matrices are used to help evaluate where an SIS may be desired. A report documenting the results is provided.
  • Preparation of Safety Requirements Specifications (SRSs).
  • Documentation of the requirements for SISs and their SIFs.
  • Failure Modes, Effects and Diagnostics Analysis (FMEDA).
  • We determine safe and dangerous, detected and undetected SIF failure rates using failure data on the SIF components. The results are provided in a report.
  • Safety Integrity Level (SIL) verification / validation.
  • We determine the SIL values provided by the SIFs in a SIS by calculation. This is a requirement of the IEC 61511 / ISA 84 standard. The results of the calculations are documented in a report.
  • SIS lifecycle activities.
  • We provide assistance with other SIS lifecycle activities such as Functional Safety Assessments, e.g. Pre-Startup Safety Reviews. The results are provided in a report.
  • Development of procedures for SIS installation, commissioning, testing, operation, maintenance and change reviews.
  • We develop and document procedures that meet prevailing good industry practices.
  • Modification of PSM Programs to Address SISs.
  • We help clients to modify their PSM programs to address SIS, such as additions to process safety information, Process Hazard Analysis (PHA), operating procedures, and mechanical integrity. We help update PSM program documentation.

Other information


OSHA has endorsed IEC 61511 / ISA 84 as a “national consensus standard” in a March 23, 2000, OSHA letter to ISA. This letter states that the standard is considered “a recognized and generally accepted good engineering practice” for SIS. Paragraph (d)(3)(ii) of the Process Safety Management (PSM) standard specifies, “The employer shall document that equipment complies with recognized and generally accepted good engineering practices”. The letter states that in evaluating whether an employer’s engineering practices with respect to SISs comply with PSM, OSHA would consider, among other factors, whether the employer meets the requirements of IEC 61511 / ISA 84. In the letter, OSHA states that it is also important to note that there is a large percentage of processes which are not covered by PSM which may include SISs covered by the standard. OSHA states that the employer may be in violation of the General Duty Clause, Section 5(a)(1) of the OSH Act if SISs are utilized which do not conform with IEC 61511 / ISA 84, and hazards exist related to the SISs which could seriously harm employees. Consequently, this means that companies must comply with IEC 61511 / ISA 84, not only for PSM-covered processes but also for other processes that use SISs where hazards to personnel may be present. Regardless of regulatory requirements, IEC 61511 / ISA 84 is an internationally recognized standard and represents good engineering practice.