System failure rates

USC may be modeled as a power-series expansion of non-CCF component failure nates. No a priori physical information is introduced, so the methods are ultimately dependent on the accuracy of data to support such an expansion. A fundamental problem with this method is that if the system failure rate were known such as is required for the fitting process then it would not be neces.sary to construct a model. In practice information on common cause coupling in systems cannot be determined directly. NUREG/CR-2300 calls this "Type 3" CCF. 

The overall system failure rate including common modes was estimated to be the geometric mean of these extremes (equation... 

Reliability is a function the system failure rate or its reciprocal, mean time between failures (MTBF). The system failure rate in non-redundant systems is numerically equal to the sum of component failure rates. 

The system failure rate is less than 10% the operation shutdown rate is less than... 

Useful life period. After the weaker units die off in the infant-mortality period, the failure rate becomes nearly constant and the assemblies have entered the normal or useful life period. This period is characterized as a relatively constant failure rate and is also referred to as the system life of a product. Mean time between failures is calculated in this section of the curve. Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a system during operation. MTBF can be calculated as the arithmetic mean (average) time between failures of a system. Failure rates calculated from MIL-HDBK-217 and Telcordia-332 apply only to this period. 

Probability terms are often combined with equipment failure rates to come up with a system failure rate. P-IOIA has a failure rate of 0.5 year the probability that P-lOlB will not start on demand at the time P-101 A fails is 0.1 therefore, the overall failure rate for the pump system becomes (0.5 0.1) year or once in 20 years. 

It would generally be agreed that a significant environmental incident such as this tank overflow occurring once every three is not acceptable. Management may decide that they want the system failure rate reduced to 1 in 10 years, or better. How this can be done is discussed below. 

Determine the overall system failure rate using the predicted frequency/probability values for each event. 

Once more, the overall system failure rate appears not to have changed. Figure 15.25 does not present an accurate picture, however, because the Electrical Power Failure occurs twice, once to do with P-IOIA and once to do with P-IOIB. Hence it is a common cause effect Failure of Electricity leads to an immediate failure of the pumping system, so the fault tree should have the... 

It should be pointed out that the approximation techniques are only valid for very small system failure rates. 

Additive models do not really consider the structure of the software but instead attempt to predict the time-dependent failure rate forthe compoimd software from the components failure data. In e.g. (Xie and Wohlin 1995) it is assumed that any component failure causes system failure (i.e. aU components are assmned to be in series with each other), and the system failure rate Xs(t) is obtained from the components failure rates through... 

Kuball, S., J. May, and G. Hughes (1999). Building a system failure rate estimator by identifying component failure rates. Proceedings of the 10th International Symposium on Software Reliability Engineering (ISSRE 99), 32-41. 

The overall system failure rate is significantly lower because the failure rate of each aged component which has increased over time now is reduced. 

NOTE 2 A major distinguishing feature between random hardware faiiures and systematic failures (see 3.2.85) is that system faiiure rates (or other appropriate measures), arising from random hardware failures, can be predicted but systematic faiiures, by their very nature, cannot be predicted. That is, system failure rates arising from random hardware faiiures can be quantified but those arising from systematic failures cannot be statistically quantified because the events ieading to them cannot easily be predicted. 

This equation will formally have the form of a life function with constant failure rate. A5 can be viewed as the system failure rate in the case of a preset inspection interval. Such a view will of course make sense only when inspection intervals are small in comparison to the entire operation period to be considered. For the OR linkage, the following emerges from Equation (4-56) ... 

In this system failure rate can be reduced after thorough analysis and remedial measures. 

The economic impact of a spurious or nuisance trip of an ESD system can be disastrous. An ESD system is an important layer of protection to prevent and prevent hazardous situations from occurring. So, it is needless to mention that the ESD system must be extremely reliable and function on demand. During an emergency, it must put the process in a safe state in orderly fashion. Also ESD systems design shall be based on a fail safe independent system, that is, ESD systems are such that during a failure of a component the process reverts to a condition considered safe and not a vulnerable serious hazardous event. Reliability and availability are major parameters for ESD system performance. Reliability is a function of system failure rate (its reciprocal) and mean time between failures. Spurious trip conditions may initiate a so-called fail safe incident that may result in accidental shutdown of equipment or processes. However, undetected process design errors or operations may initiate dangerous incidents that may disable the safety interlock and may even cause accidental process... 

Conventional reliability assessments of random failure rates for hard-wired systems are based on measured failure rates for all the system s components and an assumption of perfect routine testing (i.e., the routine testing detects all latent faults). This assessment approach can often yield umealistically low predictions for actual systems failure rates. The system failure rates will in practice be dominated by common-mode failures and not by random failures. 

Fault trees, failure modes and effects analysis (FMEA), failure modes effects and criticality analysis (FMECA) and event trees use logic, reliability data (component failure rates), and assessed system failure rates, combined with human error failure rates (using methodologies such as HEART or THERP) and other methodologies such as software reliability assessment, to develop estimates of system failure frequencies, and hence plant accident frequencies. 

FMEA/FMecA Sometimes the number of components is just too large to make fault tree analysis practicable. In FMEA, a team identifies potential failure modes based on past experience with similar products or processes. For the assessment of system failure rate, the dangerous failure modes are listed and then probabilities are assigned and summed - this figure is then taken to be the probability of the unwanted system failure ("top event O. 

Tables 5.1 and 5.2 provide the failure rate and unavailability equations for simplex and parallel (redundant) identical subsystems for revealed failures having a mean down time of MDT. However, it is worth mentioning that, as with all redundant systems, the total system failure rate (or PFD) will be dominated by the effect of common cause failure dealt with later in this chapter.

However if this assumption is not valid then the following applies, where is the system failure rate of the system i.e., X has the following equations for the following configurations ... 

Each single component failure which causes system failure is described as a SERIES ELEMENT. This is represented, in fault tree notation, as an OR gate whereby any failure causes the top event. The system failure rate contribution from this source is obtained from the sum of the individual failure rates. 

The DTG-R signalling system failure rates as defined in the supplier s model (WRSL Signalhng and Train Control System Prehminary Fault Tree Hazard Analysis), have been used in the ISPv21 model ... 

It is to be noted that the right-hand side of Equation 3.25 is independent of time t. Thus, the left-hand side of this equation is simply the series system failure rate. It means that whenever we add up failure rates of units/items. 

The transportation system failure rate with respect to tires... 

Mikulski, J. Malfunctions of Railway Traffic Control Systems Failure Rate Analysis. Proceedings of the 3rd International Conference on Computer Simulation in Risk Analysis and Hazard Mitigation, 2002, p. 141-147. 

One alternative to empirical test evidence is the claim that compliance to established standards for safety-critical software will result in software with a tolerable dangerous failure rate. For example, compliance to lEC 61508 Level 4 is linked with dangerous system failure rates as low as 10 /hr [9]. Unfortunately there is very little empirical evidence to support such a claim. 

Handling these three types of failures under the specified operating conditions of the components is what we call maintenance. Preventive maintenance aims to eliminate potential failures before they occur, e.g. by measuring wear, or by replacement or retraining prior to rapid increases in the failure rate corrective maintenance rectifies failures on occurrence. As components fail and are repaired or replaced, the performance of the system will fluctuate, and at some point in time it is possible that the performance becomes so poor that we say the system has failed. This leads to the definition of system failure rate, X, as the inverse of the mean time between such failures and, given the specified operating conditions. 

No definition of reliability is complete without an understanding of the bathtub curve. It is generally accepted that a component s (or part or sub-system) failure rate... 

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