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Situation
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Description
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Maintenance occurring during phase change
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The maintenance continues to occur in the new phase until it is finished, even if the maintenance properties in the new phase conflict with the maintenance that is underway. The maintenance continues even if the component is not in the new phase. Theoretically, this makes it possible for a maintenance activity to span many phases.
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Component is present in some phases but not in all phases
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During Monte Carlo simulation, a component is ignored in phases where it is not present. During such phases, the component will not fail or have any maintenance start. However, if maintenance for the component is occurring at the start of a phase in which the component is not present, the maintenance continues.
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Component is not connected to start and end nodes
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In a block diagram, it is acceptable for components to be present but not connected to the start and end nodes. These disconnected components are running and can fail and undergo maintenance, but such failures do not directly affect the availability of the overall system. A few ways in which a disconnected component can affect a system follow.
• The disconnected components can consume repair resources or spares from a common spares pool, thereby denying their use by connected components.
• Costs are still accrued for disconnected components.
• A connected component could have a trigger that is dependent on a disconnected component.
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Component fails in a phase where corrective maintenance is not enabled
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When a component fails in a phase where corrective maintenance is not enabled, it remains failed until a phase is reached where the component does have corrective maintenance enabled. At this point, the corrective maintenance is performed.
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Maintenance to occur in a maintenance only phase takes longer than the duration specified for this phase
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The duration specified for a maintenance-only phase is the minimum time that the phase lasts. The maintenance-only phase does not end until all of the maintenance begun in the maintenance-only phase has been completed. Maintenance that can never be completed during a maintenance-only phase is not started. For example, if the maintenance requires a spare and no spares are available, the maintenance is not started.
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Maintenance based on calendar time versus operating time
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Upon entering a new phase, a maintenance task this phase that is based on calendar time happens according to the interval going back to time 0. For example, assume that Phase 3 begins at 1000 hours and that Block 1 has a maintenance task that is to occur every 300 calendar hours. The maintenance task occurs at 1200 hours (1200 mod 300 = 0).
Maintenance tasks that are based on operating time use the operating time of the component. As indicated earlier, you use component identifiers to define the same components operating in different phases. For more information, see Component Identifiers.
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Standby component with switch delay in process of switching when phase changes
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If a standby component with a switch delay is in the process of switching when a phase changes, it continues switching if the redundancy is still present in the new phase. Otherwise, the switch is cancelled and effectively completed at the start of the phase.
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Component has different spare properties in different phases
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If a component has different spares properties in different phases, this case is treated as one spares pool that has different properties in each phase. If for any reason a case cannot be handled, an error message is shown.
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Component must have the same quantity in all phases where the component is present
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A component must have the same quantity in all phases where it is present. This restriction is due to the complications that arise when trying to handle the extra components present in phases where they are not needed. For example, if a component in Phase 1 has a quantity of 3, and in Phase 2 this same component has a quantity of 2, what happens to the third unit in Phase 2? Is it in standby, inactive (zero failures), or fully active? Is it a spare? Should maintenance be done on it? Is so, what type of maintenance?
When a component is not present in a phase, it is assumed to be inactive, which means it has zero failures and no maintenance performed on it. However, making these assumptions for extra components in a phase would not be correct in all cases. If you require different quantities of a component in different phases, to model this situation, use multiple blocks with junctions to specify what happens to the extra components.
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