The different types of insulators must undergoes the following tests:
There are three types of flash over tests which are to be done before an insulator can be passed the following Flash over Test.
In this test voltage is applied between the electrode of the insulator which is shown in fig. and the applied voltage is gradually increased upto the certain voltage at which the surrounding air breaks down. And that voltage is called the flash over voltage. The insulator must be capable of sustaining the rated voltage for one minute.
In this test again the insulator is mounted in the same manner as above, and the voltage is applied gradually but in addition to this the insulator is sprayed with water at an angle of 450 in such a manner that its precipitation should not be more than 5.08 mm per minute. The resistance of the water used for spraying must be between 9,000 and 11,000 ohms per centimeter cube at normal atmospheric temperature and pressure. The withstanding the minimum standard voltage for 30 seconds.
This test is similar to that of wet-test, but in this case, the voltage should be noted when the surrounding air breaks down. The voltage must be equal to the minimum specified value.
The insulator used in the field, must also be tested against the high voltage surges caused by lightning etc. For this test the generator developing lighting voltage is employed, it develops a very high voltage at the frequency of several hundred thousand cycles per second. Such a voltage is applied hundred thousand cycles per second. Such a voltage is applied to the insulator and the spark over voltage at 50 cycles per second is called as impulse ratio, i.e.,
This ratio should be approximately 1.4 for pin-type and 1.3 for the suspension-type insulator.
The following are the performance tests:
For this, the insulator is first heated in water at 70⁰C for one hour than the insulator is immediately cooled in water at 7⁰ C for another one hour. After such three temperature cycles, the insulator is dried. It should be noted that after this test the glaze of the insulator should not be damaged and later it should not be damaged and later it should be tested for the routine high voltage test.
For such a test the insulator is suspended in insulating oil a certain minimum voltage is applied. The value of this potential in case of suspension voltage is 1.3 times that of dry flash over voltage. The good insulator should not puncture under this test.
The porosity of the material can well be determined by the fuchsine penetration test. In this test, the insulators are broken into pieces and are immersed in a 0.5 percent alcohol solution of fuchsine dye under the pressure of about 140.7 kg per sq. cm for about 24 hours. After that the same are removed and examined, a slight porosity of the material is indicated by the deep penetration of the dye into it.
In this test, the insulator is mounted on a steel pin and 21/2 times the maximum working load is applied for one minute. This test is also called as the bending test and is applicable to only pin insulators.
This test is applied to suspension insulators only, it consists of the application of tensile stress of 21/2 times the maximum working tensile strength for about one minute. Then after this test the insulator is tested for 75 per cent of the dry spark over voltage.
The following routine tests must be done before it can finally recommend for use in the field.
For this test, all types of insulators are assembled and a tensile load of 20 percent in excess of the specified load is applied for about one minute.
The insulators complete with galvanized or steel fitting etc. must be tested against corrosion. For this test, its fittings are suspended in a copper sulphate solution at 15.6⁰ C for one minute. Then the fittings are removed, wiped, cleaned and again put in copper sulphate solution. This is repeated for four times. Then on examination, there should not be deposition of metal over it.
In this test, the pin insulators are inverted and are put into water up to the neck. The water is also put into the spindle hole. Then high voltage is applied for 5 minutes. Under this, there should be no damage to the insulator.
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