Megger Value

Remove the spider's web hanging between the R and the Y busbars.

It is new installation hence no spider web sorry...

"can you suggest me what will be the problem": Imbalance in the system

"and how to rectify it to get nearby reading in all three phases." Call an electrician out to fix it.

70 megohms on 415V electrical insulation is acceptable. There are always some variations in resistance between phases - you will NEVER get them exactly the same. All it takes is a little dust on one of the insulators, a little moisture in an area that gets less ventilation, or something minor like that to give you the difference.

What is your insulation resistance to ground? That is also important.

we'll never know but....if he warms the general area and takes his readings again they'll probably change...due to moisture

I Used Space Heater for a day....Reading did change marginally(now 82 mega ohm) but no where near 300 or so.....Damn!

With everything disconnected you would expect almost infinity. I go with the looking for things like spiders and similar debris.

Checking things for tightness will not increase the insulation value.

While it is in bits I suggest a ductor test. Now on this test clean and tight joints really will make the difference. If might be this test will unearth something. You are looking for a really low value.

A flash test might clear the spiders but its a little harsh .....

Is it Delta or Wye wiring?

it is Wye

I hope there is a control transformer for that bus bar system. If it is so, the primary of that transformer must have been fed from R and Y phases. It might be because of the insulation level of that particular MPCB which is feeding that trafo that the megger value between R and Y phase is comparitively low.