Reliability HotWire: eMagazine for the Reliability Professional
Reliability HotWire

Issue 54, August 2005

Reliability Basics

Treating Common Cause Failures in Fault Trees

Common cause failure analysis is important in reliability and safety studies, as common cause failures often dominate random hardware failures. Systems affected by common cause failures are systems in which two or more events have the potential of occurring due to the same cause. Some typical common causes include impact, vibration, pressure, grit, stress and temperature. ReliaSoft provides an innovative approach to handle common cause failures. Failures that cause different events to happen can be modeled with mirror blocks in BlockSim.

Mirror Blocks

Mirror blocks allow you to place the exact same block in multiple locations within a Reliability Block Diagram (RBD) or a Fault Tree. This can be useful for many purposes, such as modeling bi-directional paths within a diagram and common cause failures. To use mirror blocks, select the block with the properties that are to be mirrored and select Mirror Block under the Block menu. The mouse pointer will turn into a "mirror." Next, click the block to which the properties are to be mirrored. The properties of the source block will be mirrored in the destination block. A grey box will appear in the upper left corner of the block to indicate that the block is a mirrored block. In mirrored blocks, the duplicate block behaves in the exact way that the original block does. The failure times and all maintenance events are the same for each duplicate block as for the original block.

Mirror blocks guarantee that multiple blocks will exhibit the same behavior (e.g. failures...) and experience the same action (e.g. corrective maintenance, inspections...) simultaneously. Entering the same properties for different blocks in a diagram does not ensure that these blocks will act like mirrored blocks. For example, because of the randomness, blocks that have the same failure distribution and the same parameters could still fail at different times when performing a simulation. 

Note: Mirror blocks are not available when your copy of BlockSim is set to the Simple setting. Mirror blocks are available in BlockSim for both RBDs and Fault Trees.

Using Mirror Blocks for Common Cause Failure Analysis

Common cause failures have traditionally been handled using the Beta, MGL, Alpha and BFR models. ReliaSoft's BlockSim has a simpler and more effective approach to handling common cause failure that relies on the use of the mirror blocks feature. Therefore, the traditional common cause failure analysis methods will not be discussed in this article. The following example illustrates the BlockSim approach.

Consider the following example in which Event A could cause both an X Failure (if it happens along with a B event) and a Y Failure (if it happens along with a C event).

The above example describes a simple common cause failure problem. In this example, the A event is the common cause. A failure distribution needs to be specified for the A, B and C events. The events' failure distributions are listed next.

You can use the mirror block feature to indicate that the two A events are actually the same event and to specify that if event A occurs then Failures X and Y could occur.

The probability of a system level failure occurrence can be found using the Quick Calculation Pad as follows.

If the two events A in this fault tree example were not mirrored, the results would have differed, as the following figure shows.

The difference becomes more significant when the model complexity increases.

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