TABLE OF CONTENTS When there are units in a system with multiple modes of function such as (1) normal, (2) failed open, and (3) failed short in the case of a resistor, or (1) normal, (2) failed open energized, (3) failed open de-energized, (4) failed shut energized, and (5) failed shut de-energized in the case of a valve, the following procedure is recommended to determine the system's reliability:
1. Define system success.
2. Determine the total number of permutations (P), with repetitions permitted, of all modes of function of all units in the system. This may be calculated from
where
| M n | = | number of modes of function of a group of units in the system, |
and
| N n | = | number of units in the system having M n modes of function. |
For example, if in a system of three units, two units ( N1) have three modes ( M 1) of function, and one unit ( N 2) has two modes ( M 2) of function, then
or
P = 18.
3. Prevpare a table having the following headings:
Permutation number.
Units in system identified by unit number and the mode of function of each unit.
System output.
Is system successful?
Probability of occurrence of the permutation resulting in system success.
4. Fill in the first column in the previous table with the numbers 1 through P.
5. Under "Units and modes of function," enter all possible permutations of modes of function of...
