Need opinion on using a burnt ESC as a BEC , and how safe is it ?

Obviously it is not a good option to use a burnt esc.

Esc is the only electronic which gives power to your Motor and all servos via Rx. If the ESC fails if mid air, you will lose all controls on your plane. And your plane will become a flying Hammer , which can hit anyone and can be Deadly or can do serious damage. Just think of it ........
One mistake can take someones life. So a BIG NO to any unreliable and faulty electronics , specially if you are flying something high in air.

Happy flying 

Did you understand what he was asking ??

He is just going to use it as a bec -

To the OP - try it on the ground and if it works - then go ahead
most esc BEC are based on the ubiquitous 7805 linear VR series - if it
works on the ground  then highly unlikely it is going to fail suddenly.

also keep it ON for long ( 5min ) on ground

touch and see for how hot it is ... if it remains cool for 5 min ... you are good to go.

Yes saikatji, i didn't read carefully, my bad.

It works on ground ... And I think its very unlikely that long operation will heat it up coz what we are drawing is a Max of 3 amps ...

So you  are good to go... BTW heating can not only be cause of current overdraw.....

Any thing else that can cause heating .... ?