I am not sure if understood well your question but in general, the continuous current rating of a breaker should be determined for the realistic worst case scenario.
In ring bus configuration, this could occur for the ring open condition. For breaker 1 1/2 substation, this could happen with one of the main bus out of service.
The momentarily short circuit rating should be determined for the larger SC condition. Insulation rating should be based on the larger value of either for lighting impulse (BIL) or switching surge level (BSL).
Sorry for not being clear here. Let see if I can explain to you my question.
When I run contingency analysis, if there is an overload on a breaker in a ring bus or breaker and half station, the real rating of a breaker in a ring bus station should be almost double of its thermal rating I assumed or that what I have been told because the flow on a line is divided in half in a ring bus or a beaker and half station. The real rating of a breaker in a ring/beaker and half station can be calculated as breaker open or close (open breaker rating or close breaker rating). Is that true and how can I do these calculation?
I would not approach the problem as considering the real rating of the breaker different from its thermal rating if it is a ring bus or breaker-and-one-half station. You need to look at all possible cases, as indicated by cuky, to determine what the maximum current through the breaker will be and compare that to the thermal rating.
I'm not sure what you would consider the current rating of an open breaker to be. I'd say zero because if there is any current across the open contacts, you have breaker failure.
Is there confusion between the maximum loading of the line or transformer conencted between the brekaers and the breaker rating?
With both breakers closed the current splits in two directions, so the maximum current is twice the allowable if one breaker is open. (Assuming equal current distribution and assuming that the bus and other taps or feeders can take the resulting load.)
