What is that sugar like thing in the porcelain body of NH2 fuses,315 amp,400v?
Since you don't add a direct link or an image, I can only guess it is an High Rupturing Capacity (HRC) Fuse, a.k.a. High Breaking Capacity (HBC) Fuse.

In those fuses the fusible element (the conductor that gets melted when an overload occurs) is embedded in a sand-like compound used to quench the possible electric arc that can be generated when interrupting a current in a circuit capable of delivering an high level of energy.

Without that powder the fuse may explode and its fragments could damage other parts of the equipment the fuse it's intended to protect, causing more failures, even fires!

See also this link.