FAULT DETECTOR

Sp < 9 Provide redundancy for the operation or a positive fault detector... 

Power for the control panel should be provided with a suitable uninterrupted power supply (UPS). The panel will provide a DC current to field detectors. This power will enable the panel to monitor all input circuits, output circuits, and trouble signals within the detectors, such as shorts, ground faults, and detector disconnects. It will also provide an AC powering signal to field output devices. All output circuits should be similarly supervised for trouble. An example alarm and detection control panel is shown in Figure 7-18. 

One problem which arises when a detector array is attached to the face of a multi-layer module is the inability of the detector material to absorb forces generated by a mismatch of coefficient of thermal expansion between the detector array material and the module. Furthermore, it is difficult to isolate a fault that may be attributable to either the detector elements, module wiring or processing elements. 

Similar anomalies over buried ore deposits have been reported by other Chinese workers (see Chapter 13). Using a gold-film detector, workers at the Chinese National Non-ferrous Metals Corporation (CNNC) analysed Hg in soil gas and near-surface atmosphere at the Naking Pb-Zn mine, where stratabound ores occur in fractures in Permo-Carboniferous strata. The region is covered by about 30m of allochthonous overburden. Mercury anomalies of up to three times background occur in both soil gas and near-surface atmosphere above a fault that intersects the ore (Fig. 12-23). These are coincident with higher-contrast Hg soil anomalies, but corresponding Pb and Zn data are not available. 

The high impedance is usually a resistance chosen to limit the earth current to about 20 mA. A current detector is used in conjunction with the resistance to raise an alarm if a line-to-earth fault occurs. A zig-zag transformer, or reactor, is sometimes used with three-wire supplies such as used in drilling rigs and emergency systems. It is specially designed and internally connected to create a very low zero sequence impedance to earth currents. Therefore, the current is limited only by the resistance of the neutral earthing resistor. Some special purpose earth current alarm systems are available that inject a small DC current into the three-phase system, which is used to identify the actual location of the fault. 

Short circuits often develop from faults of a leakage nature. It is therefore advisable to provide each sub-circuit with an earth leakage current relay or alarm unit, which has a sensitivity that adequately coordinates with other devices. Indeed this is a necessary requirement for sub-circuits that feed power to hazardous area equipment. The use of these earth leakage current relays and detectors will greatly increase the confidence that can be placed on the overall performance of the system of protective devices in the UPS. 

Marshall NW (2006) Retrospective analysis of a detector fault for a full field digital mammography system. Phys Med Biol 51(21) 5655-5673... 

Immune algorithm is the detection about selfnonself . It is a round area which was covered by each detector and without overlaping each other. There must be detecting vulnerability between adjacent detectors. In order to reduce the test loophole we can make the radius of detector as variable, and there is a maximum. It can fill the loophole between detectors and improve the fault coverage rate (Liu 2009). 

If the A1 liner in the graphite corrodes thru or breaks, water will leak into the graphite. Water vapor detectors in the helium recirculating system would advise the operators by alarms, as above, of such a failure. Means should be provided for identifying the tube at fault, such as by control rod actuation or... 

Simple devices such as fuses, circuit breakers, and ground fault interrupters are available to cut off equipment when they overload or short out or an inbalance develops between the input and output current from a device or circuit. More sophisticated devices can also be used to determine a problem, such as a redundant heat detector used to deactivate a circuit serving a still, condenser, or heat bath should the temperature become too high. 

Since one seldom observes the energy spectrum from the NaI(Tl) detector in routine x-ray spectroscopy, it is possible for a fault to develop in the spectrometer, resulting in data distortion without the flaw being obvious. Periodic checks of the energy resolution, the energy calibration (at low and high count rate), and the extent of the dark-current noise spectrum should catch any malfunction. These parameters... 

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