Block diagrams are used to model systems which incorporate varying configurations outside of the basic series (in-line) configuration, and are especially useful in analyzing configurations that include redundant components.

When system components are connected in series, it indicates that all components must be operational for the system to be successful. Once a single component fails in the chain, the entire system is non-operational. But if the system utilizes backups for components, then one failure does not cause the entire system to fail. Instead, a backup unit can take over to keep the system up and running. Therefore, designing redundancy into a system can often lead to increased reliability and availability.

Different types of redundancy, such as parallel operating and standby non-operating, exist. When items are in a parallel operating configuration, all are not necessarily needed to be operational for the system to operate. For example, in a simple system, two batteries might be set up in redundant configuration so that if one battery fails, the backup battery can take over.

In this simple scenario, only one of the two components needs to be operational for system success. In block diagram terms, this is usually described as a 1::2 parallel operating configuration. In the case of a standby non-operating configuration, a system component is in standby mode awaiting failure of the first unit. In block diagram terms, this is referred to as a 1::2 standby configuration.

Models become even more complex when you consider factors such as the switch delay encountered in standby situations when the backup unit must be switched on. You might also need to account for the fact that a parallel operating unit has a higher failure rate than a standby unit because it is always powered. You can envision how analyzing the reliability and availability of these systems can become difficult without the help of software analysis packages.

Redundancy in system design is essential in cases where repair is either impossible, such as in a launched space probe, or extremely costly, such as in a ship at sea. In other systems, redundancy can play a vital role in ensuring system availability is kept to a maximum.

Block diagrams can be used to model series systems, parallel systems, and combinations of series and parallel systems. Parallel systems can be operating or in standby.

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While a standard block diagram process lets you compute reliability and availability, optimization and simulation tools take this a step further by allowing you to incorporate information about maintenance activities, spare parts, and repair resources.

Windchill RBD provides optimization and simulation tools so you can effectively address multiple facets of component maintenance, including the definition and allocation of repair teams, the stocking and shipping to and from offsite spares pools, and more. You can define maintenance plans that specify the time interval, cost, and the type of maintenance being performed. You can also specify labor costs and designate if repair is partial or perfect.

If you are unfamiliar with optimization and simulation techniques and would like to study this topic in more detail, the following is an excellent resource: