Transformer grounding

Hello,

I'm installing a dry-type transformer, taking 480V, 3-phase from the overhead buss rail and changing it to 120/240V single phase. I planned on running the neutral from the buss disconnect down and through the transformer to the 200A main breaker panel. The ground would go from the buss disconnect down to the transformer, make a ground connection, and then go on to the panel as well.
Do I also need a ground rod to act as an equipment ground? In addition, I know that the ground and neutral are supposed to be separated after the first means of disconnect, but does the buss disconnect count as that? Meaning that in the 200A main breaker panel, the ground and neutral bars need to be separate? Or can they stay connected and that actually serves as the first disconnect?

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