Contained Load Sharing Blocks

Add a contained block to a container by selecting the container and then choosing Diagram > Blocks > Add Block.

The properties available for configuring a contained load sharing block will vary depending on whether it is in an analytical RBD or a simulation RBD. In addition to the common block properties, you will need to describe how the block behaves.

The first step is to assign a URD for the block. As explained in the topic on standard blocks, the URD uses a failure model to describe the block’s reliability characteristics; for blocks in simulation diagrams, the corrective task and/or scheduled tasks associated with the URD are also applied to the block.

What's Changed? The load on contained load sharing blocks is calculated differently in version 8. Version 7 required a life-stress relationship for such configurations and based the re-calculation of load after block failure on that relationship; version 8 calculates load using the weight proportionality factor as a multiplier. Because of this, if you convert a diagram that uses load sharing containers from version 7 to version 8, you will need to manually configure the contained load sharing blocks in version 8.

The following options may be available for contained load sharing blocks in both analytical and simulation diagrams:

Weight Proportionality Factor is compared against the factors in the other blocks in the load sharing container to determine how the load will be shared. For example, if the container holds two blocks and one has a factor of 3 while the other has a factor of 9, then the first block will receive 25% of the load (3/12) and the second block will receive 75% of the load (9/12). Alternatively, if the factor for both blocks is set to 9, then both blocks will receive 50% of the load (9/18).
Duty Cycle allows you to model situations where the actual usage of a block during system operation is not identical to the usage for which you have data (either from testing or from the field). This can include situations where the item:
- Does not operate continuously (e.g., a DVD drive that was tested in continuous operation, but in actual use within a computer accumulates only 18 minutes of usage for every hour the computer operates).
- Is subjected to loads that are greater than or less than the rated loads (e.g., a motor that is rated to operate at 1,000 rpm but is being used at 800 rpm).
- Is affected by changes in environmental stress (e.g., a laptop computer that is typically used indoors at room temperature, but is being used outdoors in tropical conditions).

In these cases, continuous operation at the rated load is considered to be a duty cycle of 1. Any other level of usage is expressed as a percentage of the rated load value or operating time. For example, consider the DVD drive mentioned above; its duty cycle value would be 18 min / 60 min = 0.3. A duty cycle value higher than 1 indicates a load in excess of the rated value.

The ReliaWiki resource portal has more information on duty cycles at: http://www.ReliaWiki.org/index.php/Time-Dependent_System_Reliability_(Analytical).

Supplemental Costs allows you to choose or create models to represent costs that are always associated with the block. Cost per Failure uses a cost model, and Downtime Rate uses a cost per unit time model. If no models are assigned, it is assumed that there are no additional costs.

When you are working with a simulation diagram, the following options are also available:

Operates even if system is down indicates that the block continues to operate even if the system is not operating. If it is not selected, the block does not continue to accumulate age when the system is down.
Belongs to a maintenance group if selected, you can specify the maintenance group that the block belongs to.
Enable State Change Triggers allows you to specify the starting state of the block (i.e., off or on) and its state upon repair, then specify events that will activate and/or deactivate the block during simulation. You can choose to activate or deactivate the block when items in specified maintenance groups go down or are restored. The current block does not need to be part of the specified maintenance group to use this functionality. This allows you to model a cold standby configuration (i.e., one where the component cannot fail when in standby) within the load sharing container.

To add a state change trigger, click the Add icon in the Add a State Change Trigger field. The State Change Trigger window will appear, allowing you to define the trigger. Each trigger that you add will then be displayed in the Block Properties window. To edit an existing state change trigger, click the Edit icon for the state change trigger to open the State Change Trigger window.

Once the block is configured, you can click the Optimum Replacement icon in the Properties area to open the Optimum Replacement window for the current block; this window allows you to determine the most cost-effective time to replace the component based on costs for planned (i.e., preventive) and unplanned (i.e., corrective) replacement.