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IMPULSE TESTS:

 1. IMPULSE WITHSTAND VOLTAGE TEST: This test is done by applying standard impulse voltage of specified value under dry conditions with both positive and negative polarities of the wave.

  •  If five consecutive waves do not cause a flashover or puncture, the insulator is deemed to have passed the test.
  •  If two applications cause flashover, the object is deemed to have failed.
  • If there is only one failure, additional ten applications of the voltage wave are made.
  •  If the test object has withstood the subsequent applications, it is said to have passed the test.

2. IMPULSE FLASHOVER TEST: The test is done as above with the specified voltage.

  • Probability of failure is determined for 40% and 60% failure values or 20% and 80% failure values. The average value of the upper and the lower limits is taken.
  • The insulator surface should not be damaged by these tests, but slight marking on its surface or chipping off of the cement is allowed.

3. POLLUTION TESTING:

The normal types of pollution are

  • Dust, microorganisms, bird secretions, flies, etc.,
  • Industrial pollution like smoke, petroleum vapours, dust, and other deposits,
  • Coastal pollution in which corrosive and hygroscopic salt layers are deposited on the insulator surfaces
  • Desert pollution in which sand storms cause deposition of sand and dust layers,
  •  Ice and fog deposits at high altitudes and in polar countries.

These pollutions cause corrosion, nonuniform gradients along the insulator strings and surface of insulators and also cause deterioration of the material. It also causes partial discharges and radio interference.

The test that is normally done is the salt fog test.

  • In this test, the maximum normal withstand voltage is applied on the insulator and then artificial salt fog is created around the insulator by jets of salt water and compressed air.
  • If the flashover occurs within one hour, the test is repeated with fog of lower salinity, otherwise, with a fog to higher salinity.
  • The maximum salinity at which the insulator withstands, three out of four tests without flashover, is taken as the representative figure.

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