Intro4u2u

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 below apply to typical applications.
DIN V 19250

In Germany, the methodology of defining the risk to individuals is established in DIN V 19250, “Control Technology; Fundamental Safety Aspects To Be Considered for Measurement and Control Equipment.” DIN V 19250 establishes the concept that safety systems should be designed to meet designated classes, Class 1 (AK1) through Class 8 (AK8). The choice of the class is dependent on the level of risk posed by the process. DIN V 19250 attempts to force users to consider the hazards involved in their processes and to determine the integrity of the required safety-related system.
DIN V VDE 0801

As the use of programmable electronic systems in safety system designs has become prevalent, it is necessary to determine whether the design of a PES is sufficiently rigorous for the application and for the DIN V 19250 class. DIN V VDE 0801, “Principles for Computers in Safety-Related Systems,” sets forth the following specific measures to be used in evaluating a PES:

* Design
* Coding (system level)
* Implementation and integration
* Validation

Each measure is divided into specific techniques that can be thoroughly tested and documented by independent persons. Thus, DIN V VDE 0801 provides a means of determining if a PES meets certain DIN V 19250 classes.
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 the flowchart on page 10) 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 4 SILs that are indexed to a specific probability-to-fail-ondemand (PFD) (see figure 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 safetyinstrumented 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.
Draft 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 61508 is intended primarily for manufacturers and suppliers of devices. IEC 61511 is intended for SIS designers, integrators, and users in the process-control industry.