Background - Using Event-B for Critical Device Software Systems - page 16

safety [ 53 , 92 ]. This development cycle is accepted by all industry sectors in devel-
oping the advanced safe critical systems. The life-cycle phases are briefly described
as follows:

• The initial concept phase is used to identify the functional requirements of the

system, related environment where the system will be operated, and possible de-
sign approaches for developing the system.

• The second phase is used to set the goal for management and technical activities

to consider the safety implications of the developing system through assessing the
required safety level to ensure that the system achieves and maintains the required
level of functional safety. The goal should be produced at the beginning of system
life-cycle and it must be reviewed at regular interval.

• In Phase 3, hazard identification process is applied to identify the possible

hazards, which might arise during construction, installation, operation, mainte-
nance and disposal of the system. This hazard identification process is applicable
throughout the system life-cycle. The main formal techniques for hazard analysis
are FHA, FTA, FMEA and HAZOP.

• Risk assessment process is used to identify a set of possible risks through

analysing the identified hazards, and check against tolerability criteria. A set of
actions must be taken to reduce the overall risks. The action can be decided under
consideration of possible consequences of hazards to a tolerable level. The risk
assessment process helps to discover possible requirements for the safety integrity
level for the system.

• The safety requirements are separately assessed for different parts of the system

and the whole system is reviewed to ensure that the risk will be reduced to an
acceptable level and system is safe in use. Any critical system is too complex in
functionality. To implement the safety functions, a simple technology should be
used to avoid the overall complexity of the system.

• This phase of the safety life-cycle is related to system implementation, where

safety related parts or components are implemented to satisfy the safety require-
ments.

• Assessment of the specific components or parts of the system must comply with

the safety requirements to ensure that the component of the system meets the
given safety requirements. The assessment process is based on analysis and au-
diting techniques.

• Safety validation phase is used to verify the system against the claimed safety

properties. This process assures that the system have been achieved a set of goals
and system is safe to use in practice. Moreover, during the verification process
arising problems are also resolved.

• This phase of safety life-cycle related to the system operation and maintenance,

which ensures that the system will be safe during the maintenance process. Vari-
ous safety related system problems arise due to a poor maintenance process. Thus
the system must be designed for maintainability. The use of the system in differ-
ent environment should also be analysed to evaluate the system behaviour and
must ensure the safety of the system.