Intermittent fault

In the case of an intermittent fault, the dynamic behaviour of some system component is abnormal for short time intervals. The occurrence and the length of these time intervals are unpredictable which makes it difficult to react appropriately. After detection of faulty values, fault diagnosis takes some time, but then the intermittent fault may have gone for an unpredictable period of time. 

Faults are classified according to their root cause. On the other hand, the fault occurrence classification is based on temporal evolution of the system variables. An abrupt fault means that all of a sudden a normally operating system starts behaving abnormally a progressive fault refers to the gradual drift in the behavior of a system away from its normal behavior an intermittent fault refers to abnormal behavior for a small duration of time which is not uniformly repeated and an incipient fault refers to the state of a system where its behavior is in the borderline between normal and abnormal behavior. Note that it is not always possible to eategorize these faults in a real system because the controller action may hide the faults or change the temporal evolution of measured variables. 

On the other hand, when more than one fault can influence the system at the same time, advanced diagnostic methods are used. These methods are based on parameter estimation. Sensitivity bond graph formulation [12] allows real-time parameter estimation and thus it is possible not only to isolate multiple faults but also to quantify the fault severities. Parameter estimation in single fault [2] or multiple fault scenarios [12] are essential steps to be performed before fault accommodation. The parameter estimation scheme also gives the temporal evolution of system parameters. Thus, it is possible to identify and quantify different kinds of fault occurrences. A progressive fault shows gradual drift in estimated parameter values and intermittent fault shows spikes in the estimated parameter values. The advances made in the field of control theory have made it possible to develop state and parameter estimators for various classes of nonlinear systems. Analytical redundancy relations may also be used in optimization loop for parameter estimation because it avoids the need for state estimation. Interested readers may see Ref. [3] for further details and some solved examples. 

Intermittent fault The fault which occurs for some time then vanishes and again reappears. These faults are mainly due to hardware. Many times the system does not function well when a part of it is hot, but if it is allowed to cool down it again starts functioning well, or system malfunctions due to one loose connection—all these are examples of intermittent fault. 

The most important characteristic of fault is duration. So, fault can he classified utilizing this characteristic feature. Based on duration, the fault can he classified as a permanent/solid/hard fault and a transient/intermittent/soft fault. In most systems the majority of faults are transient faults (above 80%). Transient faults, or intermittent faults, can be defined as random failures that prevent the proper operation of a unit for only a short period of time—not long enough to be tested and diagnosed as a permanent failure. 

Transient/intermittent faults Transient faults occur once and then disappear. The majority of DCS or computer system faults (80—90% [4]) are the transient type, for example, a message is sent but does not reach the recipient, but when it is resent, it reaches the recipient. There is a peculiar characteristic of transient faults. This is termed intermittent fault, which occurs and vanishes in a cyclic manner. A simple loose connection in a component can cause such a situation. Transient and intermittent faults are mainly caused by random failure of components and the faults stay for short while and are then diagnosed or tested. 

Such a simulator allows 30 or 40 faults to be simulated every day. The great number of manipulations is a cause of errors. The intermittent faults are not simulated. 

Multiple faults and intermittent faults are not simulated. The duration of commutation exceeds 1 ms. 

NOTE 2 An intermittent fault occurs time and time again—and disappears. These faults can happen when a component (1.15) is on the verge of breaking down or, for example, due to a glitch in a switch. Some systematic faults (1.131) (e.g. timing marginalities) could lead to intermittent faults. 

The aforementioned differences obviously establish different contexts of use for UEP and UEC codes. On one hand, UEP codes are used when some information in part of the word is more important than in other parts (e.g. control information in communication messages or headers in multimedia transmission). On the other hand, UEC codes are appropriate in scenarios where constant bit error rates cannot be assumed (e.g. VHM or VLSI circuits affected by intermittent faults). 

This challenge is of great interest when the bit error rate (BER) is variable along the same codeword. In addition to VHM, FUEC codes may also result of interest in automotive, aerospace or avionics industries, where different sources of interference, noise or process variations may result in areas with variable BER (vBER). In these applications, intermittent faults in VLSI circuits increase the fault rate in the affected bits. In addition, the occurrence of multiple faults makes necessary to consider diverse error patterns in data storage and transmission. 

A final important remark relates to the type of (spatial and/or temporal) variability exhibited by the BER in different scenarios. For instance, VHM presents only spatial variability, since the BER depends on the error position in the storage media. On the other hand, intermittent faults show both spatial and temporal BER variability, since errors may vary not only their location but also their fi equency or duration. In this latter case FUEC codes are also necessary, but they require adaptive detection and tolerance mechanisms. This is indeed the next big challenge to cope with our research. 

Anon., CISHC Chem. Safety Summ., 1984, 55(218), 34-35 Diethylamine fumes from a reactor were usually absorbed in a glass scrubber through which sulfuric acid was circulated, but an unresolved fault in the level sensor caused the acid circulation pump to operate intermittently. While the pump was not running, amine fumes condensed in the dip pipe, forming a sohd crust (of the sulfate) which allowed a quantity of condensed amine to accumulate out of contact with the acid. When the pump was restarted, the neutralisation exotherm was sufficient to shatter the scrubber and distort the mesh guard around it. 

Whereas diffusion of gas at depth is widespread, mass flow is often localised (near the ground surface, in faults) or intermittent (volcanic emptions). Mass flow is an important consideration in the application of gas geochemistry to mineral because of its significant role in the interchange of atmospheric air and soil air, and therefore its influence on gas composition in the shallow subsurface from where most samples and measurements are taken. Lovell (1979) reviewed soil aeration in this context. 

An apical anomaly comprises either a continuous zone of elevated gas concentrations or an area with erratic elevated gas concentrations directly over the oil or gas field. An annular or halo anomaly has the form of a doughnut of continuous or discontinuous high gas concentrations surrounding a central zone of lower or background values, the latter overlying the surface projection of the oil or gas field. In a linear or belt anomaly, high gas concentrations are found continuously or intermittently along a line or confined in a belt, usually associated with faults, fracture zones, or matured source beds. 

Intermittent inspection for detailed faults grading apples... 

Complexity introduces additional challenges when it comes to evaluating faults and live service incidents. A key element in undertaking a root cause analysis is to be able to faithfully reproduce deviant behaviour. Faults in complex systems are more likely to be intermittent, unpredictable, non-deterministic and seemingly random. In particular it can be challenging to predict combinations of failures which might impact the system as a whole. Without an accurate set of pre-conditions on which to base the analysis any attempts to fix the issue will be severely hampered. 

This plant was operated with three electrical generators TG-2, TG-3 y TG-4. The power electrical system was synchronized to the public network through a "link" principal transformer. The electric network of this plant was ungrounded during a fault or there was not ground reference in system by a previous fault. Subsequently, in another site a fault occurs in a switchboard called "extension bus bar of TG-2" at voltage level of a 13.8 kV. The fault consists of an arc flash between bus bar and switchboard walls when the network was steady. Transient oscillations of the frequencies derived of the arc were timed to the circuit, and it was in resonance. The evidence shows that intermittent arc flash occurs, pa-omoted electro-erosion as is shown in Fig. 3.5.2.1. 

Fig. 3.5.2.I. Electroerosion damages by repetitive or intermittent ground fault arc flash.

Malfunction Is an intermittent, i.e. a temporary irregularity in the fulfillment of a system s desired function. A malfunction is due to one or more faults [3]. 

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