I am designing a ultralow noise 5V/3A smps.

On the DC side , I see the ground (DC) is earthed (chassis ground) using a direct connection 0ohm or using a Y capacitor . What are the differences between them and whats best for low noise on DC output ?

 On the positive side (DC) I also see that sometimes its earthed (chassis ground) using a Y capacitor. Is it recommended to make this connection (if our goal is low noise on DC output) ? 330pf is enough as a value for the Y cap ?