Safety Considerations Guide for Trident v2 Systems - TUV ...

12 Chapter 1 Safety Concepts
Safety Standards
Over the past several years, there has been rapid movement in many countries to develop
standards and regulations to minimize the impact of industrial accidents on citizens. The
standards described in this section apply to typical applications.
General Safety Standards
IEC 61508, Parts 1–7
The IEC 61508 standard, “Functional Safety: Safety Related Systems,” is an international
standard designed to address a complete SIS for the process, transit, and medical industries. The
standard introduces the concept of a safety life cycle model (see Figure 5 on page 9) to illustrate
that the integrity of an SIS is not limited to device integrity, but is also a function of design,
operation, testing, and maintenance.
The standard includes four SILs that are indexed to a specific probability-to-fail-on-demand
(PFD) (see Figure 2 on page 5). A SIL assignment is based on the required risk reduction as
determined by a PHA.
ANSI/ISA S84.01
ANSI/ISA S84.01-1996 is the United States standard for safety systems in the process industry.
The SIL classes from IEC 61508 are used and the DIN V 19250 relationships are maintained.
ANSI/ISA S84.01-1996 does not include the highest SIL class, SIL 4. The S84 Committee
determined that SIL 4 is applicable for medical and transit systems in which the only layer of
protection is the safety-instrumented layer. In contrast, the process industry can integrate many
layers of protection in the process design. The overall risk reduction from these layers of
protection is equal to or greater than that of other industries.
IEC 61511, Parts 1–3
The IEC 61511 standard, “Functional Safety: Safety Instrumented Systems for the Process
Industry Sector,” is an international standard designed to be used as a companion to IEC 61508.
IEC 61511 is intended for SIS designers, integrators, and users in the process-control industry.
Application-Specific Standards
NFPA 85
NFPA 85, “Boiler and Combustion Systems Hazards Code,” outlines the United States
requirements for operations using single burner boilers and multiple burner boilers.
Safety Considerations Guide for Trident v2 Systems