Effects of a Periodic Maintenance on the Safety Integrity Level of a Control System - Forms/Format 2010 - page 82

Tabl e 1 . Description of the states of the diagram in Fig. 3

State Description

1 Initial, failure-free state in the time t =0 (safe state)

2 Random undetectable failure is present in either the A or B

unit (safe state)

Random detectable failure is present in either the A or B unit
(safe state)

3

4 A failure is present in both units (hazardous state)

5 The state after a failure of any of the units has been detected

and negated (safe state)

Tabl e 2 . Description of the transitions of the diagram in Fig. 3

Transition Description

1 → 2 Occurrence of a random undetectable failure of either A or B

unit

1 → 3 Occurrence of a random detectable failure of either A or B

unit

2 → 3 Occurrence of a random detectable failure of the unit that

already has an undetectable failure

2 → 4 Transition will occur when the second (operational) unit fails

after any failure of the first unit

3 → 4 Transition will occur when the second unit (operational) fails

after any failure of the first unit

3 → 5 Transition will occur when the detectable failure of either A

or B unit has been detected and negated