No objection certificate for central government department

I was working as Junior Engineer in Central government department since 2010 now I have applied for state competitive exam for Junior Engineer and the same was intimated to my department through proper channel, now my department has replied me that minimum 20 year service is required for outside employment as per their internal departmental policy. Can anyone tell me the minimum service required for applying NOC in Central government or what action should be taken to get NOC from department kindly advice..