Why does it need to do an insulation resistance test?
To determine whether the equipment is in the proper condition to put in service for which it was designed and to give some basis for predicting whether or not a healthy condition will remain or if deterioration is underway which can result in an abnormally short life. These results can be kept as a record to compare in the future and to understand the trend of insulation during maintenance.
To do the test, we need the insulation resistance tester. :
The following precautions should be taken care, of before starting the testing.
- A visual inspection to be made to ensure the surface dust and moisture have been removed from the component under test.
- Ensure the component is isolated from other connected systems, which may feed back to other components or circuits not under test.
- A check shall be made to verify the ground for the component under test and test equipment being connected to system ground and equipment
like Lightning arrestor, capacitor and VT/ control transformer to be isolated.
Insulation test:
Insulation tester leads shall be connected between one phase conductor and the earth. Test voltage shall be selected according to table 4.1. The duration
could be 1min to 10 min, and the reading shall be taken after this duration. Before disconnecting the test leads test object shall be discharged through
ground. The same procedure shall be followed for other phases.
Influencing factors:
Various factors like temperature,
humidity, and moisture influence insulation resistance value. The ambient temperature value shall be noted during the test. The relation between temperature and insulation resistance is inversely proportional.
Test methods:
- Short time or spot reading:
In this method the megger instrument is connected insulation to be tested and is applied for 60sec. The reading is recorded at the end of that time.
- Time resistance method:
In this method, the testing is fairly independent of temperature providing considerably more information about the condition of insulation than the spot
measurement. The test voltage shall be applied for 10 minutes and readings are taken every 15sec for the first minute and every minute for 10 minutes.
Dielectric absorption factor, polarization index could be calculated as below, Dielectric absorption factor = 60sec reading/ 30sec reading. Polarization index = 10min reading / 1min reading. The insulation condition could be categorized as shown in the table 4.2.
- Step or multi- voltage method:
This method requires a multi voltage megger instrument, preferably with 1:5 voltage ratio ranges. Any reduction of insulation resistance at higher voltage is a sign of an insulation weakness.