Class and Ratio of CBCT

These would all be specific to your application...and the manufacturer would be the best person to ask...

Please understand that there is a precise difference between CBCTs and conventional protection CTs like 5P10. If you connect an earth fault relay in the residual path of 3 phase protection CTs like 5P10, there are 3 CT ratio errors which would compound at the relay terminals (i.e.) there are 3 differect cores that can have their own errors and thus the net error at the CTs' output would be some of the individual errors. Whereas in a CBCT, there is only one core, on which the three or four windings are wound. Now, you have to manage only one core error. That's why CBCTs are preferred for Sensitive Earth Fault Protection. If you opt for conventional protectioin CTs like 5P10 for sensitive earth fault protection applications, then the realy could even for no earth fault, because the current thro the realy could be more than the sensitive setting made in the relay, even under perfectly healthy conditions, due to the CT errors as told above.

Now, if you still opt for conventional protection class CTs for your earth fault protection, you have asked how to select between various Protection CT classes available. It depends upon the accuracy you require (CT 5P is more accurate than CT 10P or 15P) , the magnitude and duration of the worst case earth fault current anticipated in your system at the location of the CTs, the short-time withstand current capacities of equipments connected downstream and upstream.

But what about the diffrerence between 100/1A CBCT & 1000/1A CBCT?

Is this rated current capacity through the core?