Testing of Insulators

       The insulators are tested according to the standards. The following tests are performed while testing an insulator :
1. Mechanical tests
2. Electrical insulation tests
3. Enviromental tests
4 .Temporary cycle tests
5. Corona and radio interference tests.
       These tests are further grouped into these classes according to the standard which are,
1. Type tests : these test are performed to judge the design and specified ratings of the insulator. These tests include mechanical tests and electrical flash over tests.
2. Sample tests : these are performed to judge the material used, its quality and mechanical properties. These are also performance tests. These include some mechanical tests.
3. Routing tests : on every insulator some standard tests are performed called routine tests.
1 Mechanical Tests
      The mechanical tests are performed to judge the ability of an insulator to withstand various mechanical stresses. The mechanical tests include :
1. Mechanical strength test
2. Compression test
3. Torsional test
4. Minumum bending test
5. Mechanical vibration test
       In each of these tests, an insulator is subjected to a particular mechanical stress, for particular specified time. The insulator must be capable of sustaining the mechanical stress as per the specifications associated with it.
2 Electrical Insulation Tests
      These tests are performed to judge the electrical properties of the insulation under test. The main test in this category is the flash over test.
Flash over tests are further classified as,
1. 50 Cycle Dry Flash-Over Test: In this test, the voltage is applied between the electrodes of insulator and is gradually increased. The voltage at which the surrounding air breaks down is called the flash-over voltage. This voltage must be greater than the specified limit. Insulators must sustain the minimum voltage for one minute.
2. 50 Cycle Wet Test: the test is conducted similar to the dry test but in addition to the applied voltage, the water is sprayed over the surface an angle of 45°, resembling the raining condition. The insulator must be capable of withstanding the minimum standard voltage for 30 seconds under wet conditions. The wet test is applicable only to the outdoor insulators.
       Other electrical tests include,
1. Power frequency withstand test : Normal power frequency voltage is continuously applied to the insulator. This causes dust particles to align on the surface causing leakage currents. The flash-over is avoided. The voltage magnitude is twice the specified rated voltage. It is applied for 1 minute. There should not be flash-over or puncture. This is a type test as well as routine test.
2. Impulse voltage withstand test : in this test, standard impulse voltage surge is applied to the insulator. Such surges are caused due to the lightning in practice. The standard lightning impulse wave is of 1.2 μsec wave front and 50 μsec wave tail hence called 1.2/50 impulse wave. For this test, the generator developing the lightning voltage surges is used.
       The impulse voltage at very high frequency of sever hundred thousands Hz is applied to the insulator and the spark over voltage is noted down. The impulse ratio is defined as,

       This ratio should be 1.4 for pin type while 1.3 for suspension type insulators.
3. Puncture voltage test : in this test, the insulator is suspended in the oil and certain minimum voltage is applied . this value in case of suspension insulators is 1.3 times the dry flash over voltage. The insulator should not puncture under this test.
3 Enviromental and Temporary Cycle Tests
       In these tests the insulator the insulator is subjected to the alternate temperature cycles, sudden temperature changes, pollution and some other environmental stresses. Out of these tests, for extra high voltage insulators sudden temperature drop test, extremely low temperature test and pollution test are compulsory tests.
       In temperature cycle test, the insulator is heated in water at 70 °C for one hour and is then immediately cooled in water at 7 °C for another hour. Such three cycles are repeated. Then the insulator is dried. After this test, the glaze of the insulator is should not be damaged.
4 Corona and Radio Interference Test
       When the voltage stress level increases beyond corona inspection level, corona discharge starts. The corona discharge means the violet glow, hissing with neighbouring communication lines. By providing suitable voltage grading rings, smooth surface and higher size of conductors, corona discharge and radio interference can be eliminated for certain voltage range.
5 Other Important Tests
       These tests include,
1. Porosity test : This is also called destructive test. The insulator under test is broken into pieces and immersed in a 1 % solution of dye in alcohol under a pressure of 140 kg/cm2 for about a day. Then the samples are removed and inspected. The porosity is indicated by the deep penetration of the dye into it.
2. Proof load test : all types of the insulators are assembled and a tensile load of 20 % in excess of specified load is applied, for about one minute.
3.Galvanising test : in this test galvanised metal parts of the insulator are tested for the strength of galvanazation
4. Corrosion test : the insulator is tested against the corrosion. In this test, insulator is suspended in a copper sulphate solution at 15.6 °C for one minute. Then it is removed, dried, cleaned and again put in the solution. The cycle is repeated for four times and there should not be any type of deposition on the metal. Out of these tests, proof load and corrosion are routine test. For special type of insulators, some more tests may be performed as per the decision of supplier and the consumer.


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