INSULATION RESISTANCE...

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 CT should maintain insulation between cores(in case of multi core CTs).

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

Q. Why we should only apply only 500v ?    

A.The secondary circuit of CT 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 500 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 CT),apply 500 voltage for one minute and note down the reading.