Device fail passive

In the most common design, a fail-passive arrangement reduces the system to its lowest energy level. The system will not operate again until corrective action is taken. Circuit breakers and fuses for protection of electrical devices are examples of this type of fail-safe device. Solenoid valves (see Figure 11-3), such as this one on a steam control valve which is configured fail close shuts off instrument air, are another example. 

Fail-safe devices may be fail-passive, fail-active, or fail-operational. A fail-passive device, such as electrical circuit breakers or fuses, wdl render a system inoperative or de-energized until corrective action is taken. A fail-active device will keep a system energized but in a safe mode until there are corrective actions. A fail-operational device allows a system to function safely, even when the device fails. 

No active devices are currently marketed, and examples of successful developments are fewer than for passive devices. There are some promising device developments in the late stage but also examples of concepts that failed in development. Also, some less expensive developments are currently being pursued, intended for the local therapy of lung diseases. 

In order for a polyimide to be useful as an interlevel dielectric or protective overcoat (passivant), additional demanding property requirements must be met In the case of the passivant, the material must be an excellent electrical insulator, must adhere well to the substrate, and must provide a barrier for transport of chemical species that could attack the underlying device. It has been demonstrated that polyimide filrns can be excellent bulk barriers to contaminant ion motion (such as sodium) [10], but polyimides do absorb moisture [11,12], and if the absorbed moisture affects adhesion to the substrate, then reliability problems can result at sites where adhesion fails. However, in the absence of adhesion failure, the bulk electrical resistance of the polyimide at ordinary device operating temperatures and voltages appears to be high enough to prevent electrochemical corrosion [13]. 

The skin offers an even less naturally permeable boundary to macromolecules than the gastrointestinal tract. Thus, passive transdermal delivery of proteins and peptides using patch technology has not succeeded. Peptides and proteins can be shot through the skin into the body using high-pressure needle-less injection devices. The devices, which inject proteins like insulin, have been available for years, however they have failed to impress doctors or patients due to the associated discomfort and the potential for splash back to transmit blood-borne diseases such as AIDS or hepatitis. 

The broad functions of the safety systems are common to most reactors. In the event of an abnormal condition they should shut down the reactor, isure a suffici t supply of coolant for the fuel, and contain any fission products which might escape from the fuel elemrats. Such safety features can be active (requiring some action from a control system, involving mechanical devices, and relying on an external power source in order to operate) or passive (built-in physical fail-safe features whose operation is not dq)end t on any control system, mechanical device or external power source). 

Material surface characteristics are important for cell—material surface interactions. Three types of cell—material surface interactions can be defined, as illustrated in Fig. 7.1, which is based on the concept proposed in Ref. 76. The first one is nonfouling interactions, in which case cells fail to interact with the material surface. This type of interaction is preferred for various biomedical applications such as artificial blood vessels and valves, artificial heart devices, catheters and blood preservation bags. The second type of interactions is passive adhesion, in which case interfacial response is controlled by physicochemical interactions between the material surface, adsorbed proteins and adhering cells. Surfaces in this category inhibit cellular metabolic changes. The adherent cells remain intact and are readily detached from these surfaces with little damage. The third is bioactive cell adhesion, in which cells activate... 

An experiment in which all other relevant variables are held constant is an eflective approach when it can he achieved. However, aU relevant factors may not be known. Furthermore, circumstances often preclude the ability to hold some variables constant. For example, there may be unforeseen factors that affect the course of a corrosion experiment, as in the poleirization curves in Fig. 7 for nominally identical samples of copper (UNS C10920). Most of the curves indicate the copper will actively corrode in the solution. However, one curve indicates the copper will passivate over a narrow range of potentials. Had a structure or device been built based on the passive curve it would have failed unexpectedly. Fortunately, enough replicate curves were measured to show that the passive curve was the exception [42]. 

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