Background - Using Event-B for Critical Device Software Systems - page 11

stated conditions for a specified period of time ”[ 54 ]. Reliability can be used for
prediction, analysing, preventing and mitigating failure over time of a complex crit-
ical system. In the context of safety, there are several elements of reliability. These
elements are operational reliability and performance reliability. Operational reliabil-
ity can estimate the probability of failure of a system, while performance reliability
measures the adequacy of features to successfully perform under the specific condi-
tions. Reliability analysis aims to protect a system from failures of its components,
software and hardware [ 67 ].
A fundamental challenge in reliability analysis is the uncertainty for failure
occurrences and consequences. To protect a system, a quantitative approach has
been pushed forward for the design, regulation and management of the safety
of hazardous systems. The reliability assurance is a process that is considered
by manufacturers during product development according to the regulating stan-
dards [ 18 , 22 , 33 , 54 , 58 ]. The reliability is quantified in terms of probability. Re-
liability has a time oriented characteristic that can be expressed as the Mean Time
Between Failures (MTBF) [ 95 ]. When we use probability or characteristics of the
underlying life distribution to measure reliability, it must be emphasised that re-
liability is a relative measure of the performance of a system. It is relative to the
user requirements, system failures, expected lifetime of the device, operating envi-
ronment conditions, system functionality and behaviour of the system changes with
time.
Reliability engineering is a function to calculate the expected reliability of a sys-
tem, process and behaviour in advance. The main objective of reliability engineering
is to deliver reliable product in order to satisfy behaviour requirements, safe oper-
ation, lower cost, and to maintain company reputation [ 95 ]. Nowadays, reliability
engineering is a well established discipline that can provide an integration of for-
mal methods to investigate the system requirements, correctness of the system by
addressing the following questions: (1) why a system fails? (2) how to develop a
reliable system? (3) how to measure the reliability of design, process and operation
of a system? and (4) how to maintain system reliability during system operation
through fault diagnosis and prediction [ 17 , 116 ].
2.2.2 Safety
Safety can be defined as “ freedom from those conditions that can cause death, in-
jury, occupational illness, or damage to or loss of equipment or property, or damage
to the environment ”[ 83 ]. Safety can provide some standards to ensure quality and
functionality of a system. The safety standards eliminate all potential risks that can
cause loss of life, injuries or property damage. Critical systems that meet certifica-
tion standards, are safe to use in practice. It provides confidence to the user to use
for their purpose in daily life.
Safety is like reliability that concentrate on the designing phase of a system.
A system must be designed for safety. System safety is an engineering and manage-
ment discipline that encapsulates human, machine, environment, designing, testing,