【正文】 erm Fault Occurrence refers to the fact that an undesired event has taken place and may or may not still exist. Fault Existence, however, implies that the fault has occurred and continues to exist. Therefore, the fault can be described as being either transient or permanent. During the construction of the fault tree, all systems analysts should use Fault Occurrence, rather than Fault Existence, as the focus of interest. 4. Comparison of Failure Causes and Fault Effects A failure is considered to be an inability to perform a normal function. Example: Valve does not open. A fault is a higher level Occurrence which is usually preceded by a lowerlevel failure, such as a casing cracking due to overheating because of a lack of coolant induction due to an inoperable valve (lower level of failure). However, a fault may also occur when no failure is present. Example: Coolant valve operates properly, but the signal to operate it encounters a delay. A fault has occurred, but there is no valve failure. Because of this, it can be stated that any failure causes a fault, but not every fault is caused by a failure. Failure Categories: a. Component, b. Environment, c. Human, d. Software. Fault Tree Construction Steps Summary ? Determine the level to which the examination should be constructed ? Begin with the systemlevel fault ? Fully describe all events which immediately cause this event ? With each lowerlevel fault, continue describing its immediate causes until a ponent level failure or human error can be attributed to the fault Fault Tree Construction Steps Summary (continued) ? Fully define each branch of the tree before beginning another branch ? During the construction of the tree, it is advisable to use a block diagram of the system to simplify determining the main branches ? If the results of the FMECA on the system are available at the time of the FTA it is advisable to use the results in defining the top event(s) Analyzing the Fault Tree 1. Determine the minimal cutsets to simplify the tree (qualitative analysis). 2. Determine the probability of each input event 3. Combine the probability inputs to logic gates as follows: a. AND Gate The probability of output is the product of the probabilities of the inputs (P0=Pi1? Pi2...?Pin) b. OR Gate The probability of output is the sum of the probabilities of the inputs (P0=Pi1+ Pi2...?Pin) 4. Combine the gate input probabilities until the probability of the top event is determined. ? Identify the system or equipment level fault state(s) [undesired event(s)] ? Construct the fault tree ? Perform the analysis to the ponent level FaultTree Analysis Procedures Criteria for Identifying the Undesired Event ? The top event must be measurable and definable ? The top event must be inclusive of the lower events ? The top event is the result of the lower events Clues about Causes ? Can any equipment failures contribute to this effect? ? Material faults? ? Human errors? ? Methods and Procedures? ? Software performance? ? Maintenance errors or the absence of maintenance? ? Inaccuracies or malfunction of measurement device(s)? ? Environments such as chemicals, dust, vibration, shock and/or temperature? Errors 1 Almost all errors are caused by human error. ? Fetfulness Sometimes we fet things when we are not concentrating. Example: A person fets to set his/her alarm clock at night. Safeguard: Establish a routine which includes checking before going to bed. ? Errors due to misunderstanding Sometimes we make mistakes when we jump to the wrong conclusion before we’ re familiar with the situation. Example: A person used to a stick shift pushes the brake petal in an automatic thinking it is the clutch. Safeguards: Training, checking in advance, standardizing work procedures. ? Errors in identification Sometimes we misjudge a situation because we view it too quickly or are too far away to se it clearly. For example, a $1 bill is mistaken for a $10 bill. Safeguards: Training, attentiveness, vigilance. Errors 2 ? Errors made by amateurs Sometimes we make mistakes through lack of experience. Example: A new worker does not know the operation or is just barely familiar with it. Safeguards: Training, skill building, work standardization. ? Willful errors Sometimes errors occur when we decide that we can ignore the rules under certain circumstances. Example: Crossing a street against a red light because we see no cars. Safeguards: Basic education, experience. ? Inadvertent errors Sometimes w