I know there is no minimum value for contact resistance measurement but with 10kv bus bars are there any guidelines or rule of thumb values as I have no manufacturers information or just go by 10 micro ohms
Depends on what size bar you're looking at. A 400A distribution switchgear busbar will measure differently to a 20,000A generator main output bus: on the latter I'd be concerned about anything in the 10u[Ω] range, but on the former I'd be quite happy.