Fuses and Potential Transformers

there is also fuse for 15.5 kV and 25 KV.

Because they are provided on the lower voltage side between the load and the transformer stepping down the high voltage (where the fault is likely to occur and where the fuse is really needed). It's cheaper that way and makes more sense discrimination-wise from a protection point of view (no sense in a single HV fuse opening and taking out a large number of customers).

In special cases where high voltage fuses are necessary they are available and are used.

SF6 (Sulfur Hexafloride) is the most popularly used fuse on high tension side beyond 11 Kilo Volts. It is used on 33 as well as 66 KV lines.