I need to size a fuse (in air swicht) to protect underground line 34.5kv 4/0 cu xlpe, which is connected of air line. This underground line feed a transformer 1000 kva 34.5kv

this matter better have POCO help you with this one because there are few reguations you have to deal with this high voltage there

Air switches don't use fuses, if your talking about 3-gang operated.

What style of fused switch are you using?