Parallel Groups [Image: parallelGroup.png]

A parallel group represents a collection of items in which some items must be

functioning in in order for the group to function. The reliability of a parallel group

is based on the premise that all paths in the group must fail in order for the group to

fail.

[Image: parallel.png]

The reliability of the parallel group is the product of the reliabilities of each

individual item, with overall reliability increasing with increasing number of items in

the group.

Parallel Group Mathematical Expression

Expressed mathematically for the Exponential Distribution Type:

[Image: parallelexpeq.png]

Where n is the total number of items in a group, i is the item

counter, λ is the failure rate and t is time.

Expressed mathematically for the Weibull Distribution Type:

[Image: parallelweibeq.png]

Where β is the Weibull slope and η is the Weibull

characteristic life.

Parallel Grouping Equation Example

An example for the reliability of a 3 item parallel group is as follows:

[Image: parallel_group_eqn.png]

[Image: parallel_group_eqn2.png]

Creating a Parallel Group

To create a Reliability or Availability Grouping:

1. Right-click an the item in the Project explorer or System Model
2. Select 'RBD Model' Note: The RBD editor will show all items under the currently

selected item.

3. In the RBD editor, select items (hold SHIFT and left-click) to include in the Parallel group
4. Right-clicking the RBD diagram and selecting "Group → Series Group" from the right-click menu Note: An alternative method is to select the series icon

[Image: parallelGroup.png]

from the icon menu to create

the parallel group.

5. Verify that all selected items have been included in the parallel group

[Image: createparallel.gif]

The items in a parallel group can be rearranged (as well as included or omitted from

the group)in any order by left-clicking and dragging each individual item.

Note: Rearranging the items in the parallel group does not affect the group

reliability.

[Image: parallelmove.gif]