wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

21-2 Industrial Communication Systems
contexts, such as electrical safety, personal safety, safety on the streets, system safety, or functional safety.
In this chapter, we are exclusively concerned with the latter.
We use the definitions according to [IEC61508]. Safety is defined as “freedom from unacceptable risk
of harm.” and functional safety is the part of the overall safety that relates to the correct functioning
of some system (e.g., a control system or an industrial communication system) to maintain safety.
The functions that need to be performed correctly in order to achieve safety can hence be classified as
safety related.
Generally, safety is not an absolute property. To state that the risk related to a specific system has been
reduced to zero is generally not a realistic statement. Risk—measured as a combination of probability
and severity of an undesired event—is something that we are confronted with continuously. The crucial
question is the risk acceptability. What level of risk are we willing to accept. This complex question will
not be delved into here, but it should be said that this depends on society, on time, and on the domain
in question. Luckily, for industrial communication and control systems, this question has already been
posed and answered many times, such that reference can be made to these results.
Safety should not be confused with security: Security is concerned with the protection against
intentional attacks, in order to safeguard integrity, confidentiality, and availability of services. Safety
is concerned with the avoidance of unintended accidents involving harm to people and the environment.
A common denominator is that both disciplines—safety and security—have the goal of avoiding
unwanted events. This also makes it hard to determine the success of safety (and security) measures,
since they were successful if something unwanted does not happen.
A basic concept in safety engineering is the concept of a hazard. A hazard is defined as a “potential
source of harm” [IEC61508]. If all hazards are identified and corrected and adequate measures are taken
to control and mitigate the hazards, safety is achieved. This is the reason why the identification of hazards
has to be done at the very beginning of system development. Typical hazards for industrial communication
systems would be the loss or corruption of a message.
The importance of hazards and their identification before they actually cause an accident also highlights
the proactive nature of safety. Safety engineering is concerned with predicting unwanted events
before they happen, not (only) to learn from failures.
21.3 Safety Standards
In order to guarantee the achievement of a well-defined goal, in our case sufficient safety, standardization
plays an important role. Standardization helps to make systems interoperable and comparable. The
goal of safety standards is to devise generally applicable rules and recommendations, in order to be able
to build, operate, and maintain a safety-related system.
21.3.1 Overview of Safety Standards
The general problem with standards is that they must achieve a compromise between general applicability
and practical usefulness for specific systems giving concrete guidance. A high-level standard
may be applicable to a wide range of systems, but in order to be applicable to such a large group
of systems, it must make several generalizations, which make the standard less understandable
for the engineers building a system in a specific domain. Standards that contain a lot of specific
information are on the other hand only applicable for a small number of systems and applications.
In order to handle this compromise, standards are often classified as generic, domain specific, and
application specific.
Standards are abundant, and there is no exception in the domain of safety-critical and safety-related
systems. A selection of some standards that are all concerned with safety-related systems can be found
in Figure 21.1. Bear in mind that this is surely not a comprehensive figure, but it illustrates the proliferation
of safety standards.
© 2011 by Taylor and Francis Group, LLC