On one side the probability of "loss" of a complete bus system, especially the ones adopted for 220 and 380 kV level, is very low.
Should particular busbas system be adopted, it is necessary to compute the probabilty of loss of the busbars, starting by the forced outages of he various component involved (breakers, switching devices ...).
The utilization of such probabilities depend on the method adopted to simulate the probabilstic behaviour of the generation-transmission system in order to evaluate its adequacy .
I am not interested about the effect of loss of bus bar. Suppose at a bus bar the probability distribution of the load power is discrete type, then how the number and power share of the discrete load power affects the probability distributions of line power flows in load flow studies.