measuring resistances of CT
When measuring CT working okay in maintenance, somebody say that the resistance of 0 &amp; (+Vc) and 0 &amp; (-Vc) should have similar values.
  I wonder why these two resistance should be similar to. 


Tell 'somebody' that this is not true with active electronics. The circuits are complex arrangements of integrated and discrete components and when they include transistors and diodes a simple resistance check will tell you very little.


  Also, though using multimeter is simple, is there any other way to check it?


Power it up. Feed it a test current. Measure the output voltage. In your case the device is high current (500 A) so you could pass multiple turns through it to simulate a high current. e.g., Use a 10 A heater as a test load, wind 10 turns through the sensor and it should read 100 A.