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Failure of insulator

Main causes of an insulator failure are abnormal stress, ageing, cracks, flash over etc. Flow of leakage current through the insulator, also may cause of its failure.
Insulator failure due to flashover, which creates an arc through the surrounding air of an insulator, ionizes the surrounding medium, causing insulator failure and subsequent short circuit of the conductor. Lightning surge and switching are the cause of insulator flashover. Using arcing horn, we can minimize the flash over problem. Raindrop, dirt, pollution damage the insulating material which reduces the dielectric strength and increases the leakage current as well as insulation failure.
But the insulation failure rarely occurs as the insulators are chosen by best electrical characteristic.

Testing of Insulators:

For standardize the insulator, there are several types of tests are available. Tests are 1) mechanical tests, 2) Electrical insulation test, 3) Environmental test, 4) Corona test, 5) Radio-interference test, 6) Temporary cycle test, 7) Porosity test, 8) Power frequency puncture test, 9) Galvanizing test.

Mechanical tests are performed to confirm the withstand ability of mechanical stresses. The mechanical tests are tensile strength, compression test, torsional test, bending minimum test, mechanical vibration test.
Electrical insulation tests are performed by power frequency withstand test, in which voltage of the order of twice than rated voltage is applied for one minute in dry as well as wet condition. There should be no flash over or puncture.
Impulse voltage withstand test, in which, a standard impulse voltage wave is applied to the insulator.
▵In case of environmental and temporary cycle test: – thermal shock test, extremely low temperature test, pollution tests are the main tests.
▵By corona and radio interference tests, suitable voltage grading rings and smooth surfaces are provide to minimize the corona discharge and radio interference.
▵In porosity tests, we find the degree of porosity.
▵By power frequency puncture tests, we can find the puncture voltage value at which conduction starts.
▵By galvanizing tests, the galvanized metal parts of an insulator are tested to know the strength of galvanization.

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