Proof Test of Safety Instrumented Systems (SIS) according to IEC61511

Based on the presentation on Proof Test of Safety Instrumented Systems (SIS) according to IEC 61511 by Functional Safety Expert Mr. Chen Zhenkang of TUV Rheinland Singapore,  at the ASEAN HSSE conference dated  18th Sep 2019.

  1. Introduction: Process Industry & IEC61511The process industry sector, such as Refineries, Petrochemical, Pharmaceutical, Specialty Chemical, Semiconductor, Tank  Storage, Power Plants, Pulp and paper, Water Treatment, FPSO and other oil and gas platforms, is becoming increasingly reliant upon complex safety systems for safety management.IEC 61511 is a technical standard which sets out requirements for the specification, design, installation, operation and maintenance of Safety Instrumented Systems (SIS), so that it can be confidently entrusted to achieve or maintain a safe state of the process. Its latest revision is edition 2 ( 2016) with amendments.

    3 important S-words in IEC 61511 are SIS, SIF and SIL.

    SIS: Safety instrumented System

    is a system of hardware and software used to implement one or more SIFs to reduce risk.

    SIF: Safety Instrumented Function

            is designed to achieve a required SIL and typically consists of sensors, logic solver and final elements.

    SIL: Safety Integrity Level

            is a discrete level (1,2,3 or 4), allocated to each SIF for specifying the safety integrity requirements to be achieved by SIS; is a property of a SIF, NOT a property of a system, sub-system, element or component.

    The co-relationship between these 3 terminologies (SIS, SIF, SIL) are shown in Figure 1.

  2. Proof Test: a SHALL requirement in IEC 61511“IEC 61511-1: 2016, 16.3.1” clearly states Proof Test is a SHALL requirement for the entire SIS including sensor(s), the logic solver and the final element (s).
  3. Why Proof Test?                                                                                                                                                               Safety functions, like all other functions,  are associated with failures, thus we need to understand and manage their failures.Analysis λDu (Dangerous/unsafe undetected failure) really bothers us: it will  make that the safety system cannot perform the desired action.Since λDu cannot be detected by diagnostics, then how to control it?

    Proof Test shall be performed to reveal the λDu .  It is just like the mock test in school, which helps the students to reveal their knowledge gap which cannot be identified by themselves (undetected) while will fail them in the coming major examinations (dangerous).


  4. What is Proof Test ?In practice 100% of “as new” condition is not achievable for other than low-complexity safety-related systems. But this should be the target.As a minimum , all the safety function which are executed are checked according to the safety requirements specification.
  5. When and How to Proof Test ?Phase 3: SIS Safety Requirement Specification (SRS)

    Requirements relating to Proof Test Interval, Proof Test Implementation and functions enabling Proof Test…

    Phase 4: SIS Design and Engineering

    Design for appropriate proof testing and if necessary online proof testing…

    Phase 5: SIS Safety Validation

    Safety Validation shall include proof test…

    Phase 6: SIS Operation and Maintenance

    Planning and procedure of Proof Test shall be developed for each SIF…

  6. TÜV  Rheinland & Functional Safety
    • TÜV Rheinland is a well-recognised certification body for the Functional Safety Products, Functional Safety Applications and Functional Safety Management System.
    • TÜV Rheinland Functional Safety Training Program is the only worldwide extended vocational training program in the area of Functional Safety where knowledge and competencies are approved of by a third neutral party and where certificates are issued.
    • As for the process industry sector, our functional safety service covers all safety life-cycle phases based on IEC 61511:2016 and IEC 61508:2010.
    • Local (Singapore) contact:; +65-67990339