INSULATION RESISTANCE TEST....

Q.  What is the need of doing this test?

A.   We all know the winding in transformer (either primary or secondary) should be isolated from the ground and primary and secondary winding should be connected magnetically but not electrically. so transformer should maintain some insulation from ground and other winding, and also PT should maintain insulation between cores(in case of multi core PTs).

               always apply 500v megger for secondary to earth and core to core megger test.

Q. Why we should only apply only 500v ?    

A.The secondary circuit of PT was designed to withstand only low voltages up to 500 volts.If we apply high voltages it will destroy(insulation problem).

•        while doing the megger, specify the temperature 

                  why,because  

                                       resistance ∝ 1 / temperature

PROCEDURE :
     Remove all external connections before conducting the test.

1.  Connect the positive lead of megger to primary conductor and negative lead to earth,apply megger voltage for one minute and note down the reading.
2. Connect the positive lead of megger to  secondary winding and negative lead to earth,apply megger voltage for one minute and note down the reading.
3.  Connect the positive lead of megger to primary conductor and negative lead secondary winding,apply 500v megger voltage for one minute and note down the reading.
4.  Connect the positive lead of megger to core 1 and negative lead remaining cores, one by one (in case of multi core PT),apply 500v megger voltage for one minute and note down the reading.

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