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responsible for plant safety ought to become familiar with these figures. Te base rate for errors of omission is one chance in one hundred and the base rate for errors of commission is three in one thousand opportunities. However, many factors influence these base rates and in both directions.


Constructing a Fault Tree Fault trees are group efforts. Tey


must be facilitated by someone who is knowledgeable of both the method and the overall failure and general process language. Te facilitator must be sufficiently organized to deal with the potentially great many details and focus on one area and one “branch” of the fault tree logic at a time. Practically, the fault tree is sketched out beginning with a well defined “top event.” It is critical that every- one knows what is the exact defect or unwanted occurrence. Te more specifically the “top event” is described, the easier it will be to include potential causes and define their probability. As intermediate events are added, these are built down to the basic events in each branch, branch by branch. Once the structure is defined, the


Fig. 5. This FTA is used to illustrate contribution analysis. The “top event” is caused by intermediate events A, B and C, which in turn are caused by other events. The circles indi- cate basic events. Probabilities of occurrence are provided next to the event.


might take much longer than simply gathering a collection of parts and measuring them dimensionally, but the results may be just as important in the evaluation of the “top event.” Calculat- ing frequency of occurrence from a capability study for a given limit may be found by reference to a text on sta- tistical process control or one focused on capability evaluation. Human Error: A fair number


of basic events likely will be human errors, such as forgetting something, misadjustments or failure to follow procedure (assuming one exists). Hu- man error is distressingly common, and its probability has been studied extensively. It is unnecessary for the engineer to guess wildly at how often an individual on the line, for example, is going to forget a step in a proce-


dure. Such evaluations have already been made and are available in several reference materials. Quality engi- neers, industrial engineers and those


Table 1. Contribution of Causal Branches Top Event Probability:


Event A Event B Event C


Calculated Probability 0.000031


0.000003 0.000048


Table 2. Contribution Analysis of Event A Event A Probability:


Calculated Probability


Event D Event E


44 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2014


0.00001 0.000021


determination of the probability of the basic events must be assigned to group members to research. Te team should include representation from the production, maintenance and quality groups, at least. People familiar with the processes in detail, including what goes wrong and how often, provide a great reality check on the construction and fitness of an FTA.


0.000082


Percent Contribution to the Top Event


37% 3.7%


58.5%


0.000031


Percent Contribution to Event A


32.3% 67.7%


Percent Contribution to the Top Event


12.2% 25.6%


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