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Why cannot silver be extensively used?
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Why cannot silver be extensively used?
14. Read the text and find in it the answers to the questions that follow it.
Switchboard
Switchboard is an assemblage of switches, controlling or indicating devices mounted upon a frame for the purpose of control or an inspection of an electric path, circuit or system of circuits. Usually it is a metal frame carrying vertical slabs with switches, controlling handles and indicating or controlling instruments mounted thereon in an electric central station or distributing centre.
1. What is described in the text?
2. Where is the device used?
15. Read the text and find in it the answers to the questions that follow it.
Dynamo
Dynamo is a common device for converting mechanical energy into electric energy. This process depends on the fact that if an electrical conductor moves across a magnetic field, an electric current flows in the conductor.
Usually a dynamo includes an electromagnet, called the field mag net, between the poles of which a suitable conductor, usually in the form of a coil, called the armature, is rotated. The mechanical energy of the rotation, in the form of a current in the armature, is thus converted into electric energy.
1. What device is described in the text?
2. What types of the device do you know?
16. Read the text and find in it the answers to the questions that follow it.
Test Blocks Types B-4 and B-6
The types B-4 and B-6 test blocks with test plugs are designed as multipole connectors in the circuits of protective relays and measuring instruments.
The test blocks provide an easy and safe checking and replacement of relays and instruments during operation without breaking connections in wiring and on terminal blocks.
The test blocks are made in several versions which differ in the number of poles and in the way of wire connection.
Design. The test blocks consist of two units: the base and the removable cover.
While testing, the cover is removed and is replaced by the test plug which is inserted into the base and is electrically connected to the circuit for testing purposes.
Technical data. Rated voltage: 250 V; rated current: 5 A. Test voltage: 2 kV, a. c. 50 c. p. s. Transient stability: 300 A. The insulation resistance at ambient temperatures of 20±5°C should be not less than 2 megohms at relative humidity up to 80%, and not less than 2 megohms at relative humidity of 95%.
Mounting. Test blocks are mounted on the front panel. Before mounting, blocks should be provided with current-carrying pins.
Operating conditions. The blocks are designed for indoor use in stationary installations at ambient temperature and relative air humidity which are indicated. The blocks are produced in three types of designs: 1. conventional; 2. export; 3. tropicalized. Ambient temperatures for the blocks of the first type of design are from -20 up to +35; for blocks of the second type - from -10 up to +35, and for the third type - from -10 up to +55. As to relative air humidity, it is also different for different types of devices. For the first type it is 80% at 20±5°C; for the second type it is also up to 80% in the same range of temperatures, and for the third type it is 95% at 40°C.
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The test blocks are not to be used a) in an atmosphere containing current-conducting dust or gases which damage metals and insulation; b) where shock and vibration can take place; c) in an explosion-hazardous atmosphere.
The delivery set includes a) base; b) cover; c) fastenings for mounting blocks and connections of wires.
Order form. When ordering, state the type of the block, kind of wire connection and number of units. For example, ORDER: Test block type B-6 for back connection - 3 pcs (= pieces).
1. What are the main characteristics of test blocks types B-4 and B-6?
2. When are the test blocks described not to be used?
17. Read the text and find in it the answers to the questions that follow it.
Compressed-gas-insulated Transmission
Transmission lines in which compressed gas is used as insulator have a number of advantages. The main advantages are simplicity of construction and low cost.
What is the construction of compressed-gas-insulated transmission lines? It is rather simple. They comprise a number of phase conductors; each phase conductor is placed inside a tube and centered by means of circular spacers. The tube space is filled with compressed gas - usually sulphur hexafluoride. Each tube in a 345-kV line has a diameter about 50 cm.
The system including compressed-gas-insulated transmission has the following advantages: its losses are rather low, they are considerably lower than those of cable transmission. Unlike cables, compressed-gas- insulated transmission system can be designed for ultra high frequencies. No external electric field appears in the system. The shunt capacitance is considerably less for a gas-insulated line than for a cable. A gas-insulated line can thus transmit power over larger distances than cable lines.
The system should be protected against metallic particles. In case metallic particles get into the system, they cause a fault - a dielectric breakdown.
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