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What elements should be disconnected, inserted, closed, joined together, connected, etc. for making test on installations?
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What elements should be disconnected, inserted, closed, joined together, connected, etc. for making test on installations?
24. Read the text and find in it the answers to the questions that follow it.
Contactors Type 370
These contactors consist of the following main parts:
• one fixed bar
• one magnetic circuit which may be either a. c. or d. c.
• one or more auxiliary contacts (the maximum number of auxiliary contacts is 4)
• one or more poles for use with a. c. or d. c. loads.
These components are to be installed.
Their functions are as follows: fixed bar has the function of supporting all the stationary parts of the contactor. At both ends it is provided with holes for mounting the contactor. Moving shaft is made of steel. It is insulated for the installation of both main pole and auxiliary moving contacts.
Fig. 23. Schematic positioning of the different contactor components:
1 - fixed bar; 2 - magnetic circuit: 3 - auxiliary contacts; 4 - poles
1. What are the main parts of the contactors?
2. Which are the functions of each of these parts?
3. Describe Fig. 23.
25. Read the text and find in it the answers to the questions that follow it.
Disconnecting Switches
Application. Indoor disconnecting switches are devices that are intended to make and break electric circuits rated at 6 to 10 kV, a. c. with no load currents. The single-pole disconnecting switches are controlled manually, by means of an insulated rod. The triple-pole disconnecting switches are controlled by means of manual lever-type operating mechanisms.
Mounting Instructions. 1. Clean the switch from dust and dirt. 2. Inspect it on the outside. 3. When insulating the switch, see that the bolts and switch terminals are reliably protected.
1. What are indoor disconnecting switches intended for?
2. What means are the single-pole switches controlled by?
3. What means are the triple-pole switches controlled by?
4. What recommendations do the mounting instructions include?
26. Think of 8-10 questions covering the contents of the text below. Use them in a talk with your groupmate.
Electric Power
Electric power is generated by converting heat, light, chemical energy, or mechanical energy to electrical energy. Most electrical energy is produced in large power stations by the conversion of mechanical energy or heat. The mechanical energy of falling water is used to drive turbine generators in hydroelectric stations, and the heat derived by burning coal, oil, or other fossil fuels is used to operate steam turbines or internal-combustion engines that drive electric generators. Also, the heat from the fissioning of uranium or plutonium is used to generate steam for the turbine generator in a nuclear power plant.
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Electricity generated by the conversion of light or chemical energy is used mainly for portable power sources. For example, a photoelectric cell converts the energy from light to electrical energy for operating the exposure meter in a camera, and a lead-acid battery converts chemical energy to electrical energy for starting an automobile engine.
Electric power produced in large power stations generally is transmitted by using an alternating current that reverses direction 25, 50, or 60 times per second. The basic unit for measuring electric power is the watt - the rate at which work is being done in an electric circuit in which the current is one ampere and the electromotive force is one volt. Ratings for power plants are expressed in kilowatts (1, 000 watts) or megawatts (1 million watts). Electric energy consumption normally is given in kilowatt-hours - that is, the number of kilowatts used times the number of hours of use. Electricity is clean, inexpensive, and easily transmitted over long distances. Since the 1880’s, electricity has had an ever-increasing role in improving the standard of living. It now is used to operate lights, pumps, elevators, power tools, furnaces, refrigerators, air-conditioners, radios, television sets, industrial machinery, and many other kinds of equipment. It has been counted that in developed countries about 43% of the electric power is generally used for industrial purposes, 32% in homes, and 21% in commercial enterprises.
27. Read the text and find in it the answer to the question that follows it.
Electric Power Interruptions
On November 9, 1965, at 5: 16 p. m., a back-up relay failed at one of the five main transmission lines at No. 2 station near Toronto, Canada. As the load had shifted to the other four lines, they became overloaded, and as a result the relays failed in all four lines. The failure resulted in the load being shifted to the other plants in the system. The plants got overloaded, which caused them to shut down. Within minutes, power plants in Canada, New York, and the New England states got out of service. The blackout affected 30 million people and covered an area of 306, 000 sq. m. In some areas, such as New York City, power was not restored for about 13 hours.
This massive power blackout resulted in the construction of the national Electric Reliability Council in June 1958. This council sets standards for the design, operation, and maintenance of generating and transmission systems. These standards serve to prevent a failure in one power system from spreading to other systems. Yet local system failures cannot be avoided.
Nowadays in some European countries and in the US there are from 60 to 80 power interruptions per year, in which there is a loss of service for customers for more than 15 minutes. Mostly these interruptions are caused by weather conditions - ice, freezing snow, lightning or storms. There can be also failures of equipment - transformers, relays, insulators and so on. However, the reliability of electric service is extremely high.
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