I am working as an engineer in HAL, Banglore, a PSU under ministry of defence. I want to know that whether i will get age relaxation for govt. employees in exams conducted by UPSC. And if I want to apply for ant exam in UPSC whether i have to produce an NOC or send appl. thru proper channel.

There is no age relaxatin for PSUs in UPSC. You will have to inform your department for applying for a UPSC job and a statment to this effect will be submitted to UPSC.