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1. In what part of the power plant does condensation of steam take place?
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1. In what part of the power plant does condensation of steam take place?
2. Why is distilled water used for feeding steam boilers?
22. Arrange the paragraphs of the text below in the logical order.
Testing Motors and Generators
It is of great importance, therefore, to make regular tests of insulation resistance of all machinery so as to detect possible faults. Different conditions may influence the value of the insulation resistance.
It is advisable, therefore, to make the test of the machine as soon after it has been shut down as possible, when the insulation resistance is likely to be lowest. If, after the motor has just been shut down, the insulation resistance is found to be satisfactory, it may be assumed that it will be better at any other time provided that the machine does not stand idle for long in a humid atmosphere.
Faults on electrical machinery must be due to one of two causes. One is the absence of continuity in the conductor which is supposed to be carrying the current. The other is the absence, or partial absence, of insulation. The latter is by far the more common and the more dangerous of the two. A burnt out armature, for example, is usually due to insulation failure.
A drop in insulation resistance may often be accounted for, for example, by damp weather.
As regards the effect of temperature it should be noted that the insulation resistance of motors and generators is generally lower when they are hot than when they are cold as the insulating varnishes used in the building of the machines have a lower resistance when hot than when cold.
23. Read the text and find in it the answer to the question that follows it.
How to Make Tests on Installations
a) Insulation tests to earth.
Disconnect the supply by opening the main switch and withdrawing the main fuses.
Insert all fuses at the distribution board (see Fig. 22).
Insert all lamps.
Close all single-pole switches.
Join together the two contacts on the installation side on the main switch, and connect them to one terminal of the Insulation Tester used.
Connect the other terminal of the Tester to the conduit in which the wiring is run or, if lead-covered cable is used, to the lead sheathing. A second connection should also be made to the consumer’s main earth. This second connection is, however, unnecessary if the continuity and earthing of the conduit had been previously tested.
Turn the handle of the Tester at about 160 r. p. m. and take a reading.
In case the result of the test is considered satisfactory the installation is in proper order so far as resistance to earth is concerned.
If, however, the values obtained are not sufficiently high, withdraw all fuses at the distribution fuse board and test again. This test should include only the portion of the installation between the main switch and the busbars of the fuse board.
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If the fault is not detected, one should proceed to the distribution fuse board and test each branch circuit in turn till the faulty circuit or circuits are discovered. These should be subjected to further tests till the actual fault is detected.
b) Insulation test between conductors.
Remove all lamps.
The main switch should be opened, all fuses inserted at the distribution board, and all single-pole switches in the closed or «on» position.
Connect one terminal of the Insulation Tester to fuse contact and the other to another contact and make a test.
Two readings should be taken on an insulation containing two-way switches, one with both switches on the «on» position and the other with both switches in the «off» position.
If the result of the test between conductors is also satisfactory, no further insulation tests are necessary and the insulation may be considered to be in order.
If, however, the results of the tests are unsatisfactory, proceed to the distribution board, withdraw all fuses and test each branch circuit individually between conductors until the faulty circuit or circuits are located.
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