Switching device

As far as the user is concerned, the particular type of electronic switch used does not matter greatly, but we should briefly describe the main types used, so that the reader has some understanding of their advantages and disadvantages. Table 10.1 shows the main characteristics of the most commonly used types. 

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By electronic engineering, a system of interconnected switching devices is able to respond in one of only two modes (on or off), and these modes can be controlled at the basic level of a bit. Bits are assembled into bytes, as with an 8-bit device, and through programming of the bytes a computer central processor can be made to follow sets of instructions (programs) written in special languages, either at a direct level (machine code) that can be acted upon immediately by a computer or at a high level that is translated for the user into machine code. 

A schematic diagram of a six-vessel UOP Cyclesorb process is shown in Figure 15. The UOP Cyclesorb process has four external streams feed and desorbent enter the process, and extract and raffinate leave the process. In addition, the process has four internal recycles dilute raffinate, impure raffinate, impure extract, and dilute extract. Feed and desorbent are fed to the top of each column, and the extract and raffinate are withdrawn from the bottom of each column in a predeterrnined sequence estabUshed by a switching device, the UOP rotary valve. The flow of the internal recycle streams is from the bottom of a column to the top of the same column in the case of dilute extract and impure raffinate and to the top of the next column in the case of dilute raffinate and impure extract. 

The development of this hybrid combination in a bipolar transistor has greatly enhanced the application of power transistors in the field of power conversion and variable-speed drives. It possesses the qualities of both the power bipolar transistor (BJT) and the power MOSFET. Like a power MOSFET, it is a voltagc-eontrollcd switching device... 

Note The above switching devices by themselves or in conjunction w ith power diodes can be developed into a variety ol new devices to suit any power conversion and control application. MCTs and IGCTs arc a few such hybrid devices. For more details refer to the literature on the subject in the Further reading. 

Any of the above reasons tnay result in noise and an increase in temperature and must be corrected. Critical installations such as a refinery, a petrochemical plant, a chemical plant or a petroleum pipeline may require special precautions and control to avert any excessive heating of the bearings, which may become fire hazards. For these installations, bearing temperature detectors with a relay and alarm facility may also be installed in the control circuit of the switching device to give warning or trip the motor if the temperature of the bearing exceeds the preset safe value. 

The selection should be such as to provide proper discrimination at the various levels of a multi-distribution network. Our discussions generally take account of the recommendations in lEC 60947-4-1 regarding coordination between the short-circuit protection and the main components such as switching devices [switch, breaker. 

These assemblies are fitted with switching devices (breakers, switches, fuse switches and contactors etc.) and control and measuring instruments, indicating, regtilating and protective devices etc. to transform the assemblies into composite units, called control centres to perform a number of functions in the field of distribution and control of electrical power. Some of these functions may be one or more of the following ... 

The rated momentary peak value of the fault current, /y, will relate to the dynamic rating of an equipment. It is also known as the making current of a switching device and defines its capability to make on fault. 

Between phase to phase and each phase to ground with the switching devices in the closed position... 

Between phase to phase and each phase to ground with the switching devices in the open position. This test is to be conducted on both line and load sides of the switching device,... 

Between the line and the load terminals of each phase, the main switching devices being in the open position. 

Note For details of temperature rise of various parts and materials of an HT switching device refer to lEC 60694. 

For a switching device (which has not been previously tested for a short-circuit test). This should be closed and held in the normal service position. The test voltage (that would generate the required level of fault current) may be applied on one set of terminals, the other terminals being shorted. The test may be continued until the short-circuit device operates to clear the fault, but in no case for less than 10 cycles. In LT assemblies the point where the short-circuit is created should be 2 0.4 m from the nearest point of supply. 

For a switching device having no protection (e.g. an Isolator). The required test cuirenl may be applied for the necessary duration (I or 3 seconds) and the dynamic and thermal strengths should be verified. 

This lest is conducted to establish the satisfactory functioning of mechanical parts, such as switching devices and their interlocks, shutter assembly, draw-out mechanisms and interchangeability between identical draw-out modules, A brief procedure to test these features is as follows. 

The interlocks should be set in the intended position to prevent operation of the switching device and insertion or withdrawal of the removable parts. Fifty attempts must be made to operate the switching device and removable parts inserted and withdrawn 25 times each. 

If the surge is very steep, say, with a front time of 0.2 jjs. or less, and the cable length is short, say, lO m (presumed), it may inflict all its severity to only the entrance coil of the machine and sometimes even only a few entrance turns of this coil. However, with the technological improve-ment of switching devices, the arc prestrikes are now less predominant or non-existent and hence, the switching surge may not be as steep as described here. 

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