External and Internal Requirements

There are two types of operating requirements: external and internal. External requirements refer to the status of other elements in the model that are not sub-components of the given element.

For example, assume that we define a computer as a system, and place inside the system several sub-components (e.g., motherboard, CPU, power supply, DVD drive, monitor). Specifying that a motherboard can operate only if the power supply is operating is an external requirement with respect to the motherboard (since the power supply is external to the motherboard).

Internal requirements reference things internal to the element. There are two types of things that are considered to be internal to a Reliability element:

1. Failure modes for the element. By default, any failure modes for the element are automatically added as internal requirements. In particular, when you add a failure mode to an element, GoldSim automatically inserts a “Not” condition in the internal requirements portion of the requirements tree: Not ~Failed[n], where n is the failure mode.

In the example below, there are two failure modes for the element, and these are added automatically to the requirements tree:

Note: Adding a failure mode as an internal requirement implies that it is assumed to be fatal to the component (i.e., if the failure mode occurs, the component itself fails and is no longer operative). However, if desired, you can specify that a failure mode is non-fatal (by manually removing it from the requirements tree) in order to model more complex failure mode behavior.

2. Child elements of a component that is being modeled as a system. You can manually refer to the operating status of child elements. For example, specifying that a computer can operate only if the power supply is operating is an internal requirement with respect to the computer (since the power supply is internal to the computer):