No fixed set of rules but the ministry decides and has to file a detailed report whenever an extension case is put up. Besides the report also has to explain why the person is irreplaceable (in other words why isn't there an alternate?)
In organizational hierarchy, it is expected that alternate to chief of departments be identified that can take charge in case of absence or medical issues. So by that practice it is difficult to prove that there isn't an alternate and that explains why extensions are becoming rarer..
This is very important question in general .
rocket science is indeed a rocket science .
and therefore Technocrats have to be treated in manner commensurate with their value .
One can't have similar rules for scientists working in such a critical area .
It takes decades to develope their expertise . and all that knowledge will be lost once such people are discharged from service . there must be way to harness expertise of such technocrats .
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One size fits for all Planning commission was replaced by a more inclusive & diverse institution called Niti Ayog. Let's wait & see how Niti Ayog does before sending our condolences to Planning commission, which hasn't done a great job despite being in existence for so many decades. So it's not destroying exactly..,.but re-structuring.
Coming back to DRDO.. I'm not really conversant with the functioning of this organisation, but I have full confidence in Mr Parikkar's working, considering the kind of reputation he carries from his previous stint as the chief minister of Goa!
I just quoted planning commission as an example . I do not want to divert the topic .
But let me tell you Planning commission subserved very important function .
and whether it was a good thing to re structure the planning commission or not is the question that remains to be seen .
I just hope Modi is proved correct ..but I have my own fears ..