Fault tolerance characteristics

System Interconnect Reliability From the standpoint of reliability, the shared memory system in the global bus both have problems in the area of single-point failures If a failure of the bus or the central memory occurs, the entire system is incapacitated A ring system, when bypass hardware is employed, demonstrates very good fault tolerant characteristics. 

To overcome the one-sidedness of safety evaluation, based on the characteristics of parallel structure and parallel processing, safety status of oil depot can be evaluated comprehensively by choosing suitable evaluation index Using the fault tolerant characteristics of neural network, non numerical index can be calculated by choosing suitable function and data structure to realize fuzzy safety evaluation of oil depot. 

Table 1/6.1.1 1 Fault Tolerance Characteristics Name Explanation...

The next revision of the standard is likely to define a Type 3 device in which the fault tolerance characteristics fall between those of Type 2 and Type 4 devices. 

Further investigations by Lam et al. showed that PANI nanofibers films provided good actuation characteristics, provided fault tolerance, and had a negligible influence on the mechanical properties of the film, but lost conductivity over time... 

B.2.3.5 SIS component specification all system components have proven characteristics (for example, PFD, SFF, fault tolerance, systematic requirements for the specified SIL) as mandated in lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 flEC 61511-1 Mod). 

Following the state-of-the-art review, the next step is to implement fault tolerance techniques. We will start by explaining in detail and implementing two known software-based techniques, called Variables and Inverted Branches (AZAMBUJA 2010b), which will later be used as a complement to hybrid fault tolerance techniques. These techniques have been proposed in the past years and achieved high fault detection rates at low performance degradation levels and therefore are useful not only as an introduction to software-based fault tolerance techniques, but also to be combined with hardware-based and hybrid techniques. Then, three novel hybrid techniques will be proposed and implemented, based on both software and hardware replication characteristics. The three hybrid techniques will be divided into their software and hardware sides and described in detail, concerning both operation description and implementation. 

An accurate evaluation of dependability parameters such as reliability value or failure rate is not needed. Consequently, the concept of fault tolerant level may be used to specify dependability of each mission of the system. It consists in fixing a minimal required mnn-ber of failures that can induce the unavaUabiLity of the mission. From a practical point of view, this method is attractive. Indeed, it enables a system to be evaluated without having lot of information about the reliability characteristics of all usable devices. 

The steps executed after fault detection are termed alarm interpretation which classify the actual fault, its characteristics (occurrence time, fault size, consequences, etc.), and the root cause. Fault characterization and quantification is required to determine the immediate process state and to determine whether the fault can be safely accommodated at that process state. Based on this input, fault accommodation may be performed through reconfiguration when standby devices in healthy condition are available or through fault-tolerant control (FTC) where the... 

Each of these factors provides information regarding the process risk of continued operation, which is the primary concern when the intent is to continue normal operation in the presence of a known SIF fault. Fault tolerance is a design characteristic of the SIF and is discussed in ISA-TR84.00.04-1, Annex K. 

No claims shall be made in the safety manual, in respect of the hardware fault tolerance or the safe failure fraction or any other functional safety characteristic that is dependent on knowledge of safe and dangerous failure modes, unless the underlying assumptions, as to what constitute safe and dangerous failure modes, are clearly specified. ... 

Here, general characteristics and features of fault tolerant designs are discussed. 

The following are general characteristics found in fault tolerant systems ... 

A Center for Chemical Process Safety (CCPS) publication gives the following definition An IPL is a device, system or action which is capable of preventing a scenario from proceeding to its undesired consequence independent of the initiating event or the action of any other layer of protection associated with the scenario. The effectiveness and independence of an IPL must be auditable [12]. Discussions on layer of protection analysis (LOPA) were covered in Chapter V, so they are not repeated here. However, a few characteristic features of protection layers are presented. Similar to fault tolerance and security, this is also important so that the control system is always safe. As per lEC 61511 standard the core idea for integrated safety and security is defense-in-depth with independent layers of protection to reduce process risk. The strategy behind this is that the BPCS, critical... 

The results of simulation of the system with likely faults are presented in Figure 3. There is no significant difference in the shape and properties of the obtained acceptance ratio characteristic. It should be noted, though, that the slopes of the surface are shifted towards greater numbers of drivers and vehicles. This indicates that the required level of redundancy ensuring fault tolerance is greater that in case of the functional redundancy. 

NOTE 2 Wear does not occur in software. Limitations in reliability are due to faults in requirements, design and implementation, or due to contextual changes. NOTE 3 Dependability characteristics include availability and its inherent or external influencing factors, such as availability, reliability (including fault tolerance and recoverability), security (including confidentiality and integrity), maintainability, durability, and maintenance support. 

In addition to that there is also PV (product verification, product validation), which should confirm that the lifespan requirements are met also for the tolerances of the production of supplied components. Design-FMEA formally questioned which error sequences occur if a characteristic is deviate from the specified range. How such faults propagate into the upper levels up-to a possible violation of safety goals can be assessed from the analyses and the architectures of the higher levels. 

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