You need to find out from the power company what is the maximum fault current that they can deliver to you. That depends on the characteristics of the transformer and the conductors between the source and your disconnect.
Your main breaker must have a current interrupt rating at least as great as they can deliver if your system has a fault.
Your disconnect must be able to safely and quickly disconnect without welding the contacts together.
The process usually involves an computer analysis that includes the source characteristics and the rest of the circuit.