I assume you are refering to distribution transformers (and not instrument VTs).
The transformer must be protected on the primary side (though it need not be on the secondary side)either by a fuse or a circuit breaker. The modern practice is to use a circuit breakers for units 1MVA and above.
Apologies for my misunderstanding of the query in the first response.
It is not mandatory (as per IEC anyway)to have a fuse on the primary side on VT if the VT is of a cast resin type design and located in a fault free deemed zone and some switchgear manufacturers have adopted this practice. The secondary side is fused in the usual manner.
Mnufacturers who have a oil filled (often withdrawable ) design would offer a fused primary design.
Therefore it is more of a question of choice.
Depends. If it is a MV VT, I would. Fused cutouts or power fuses are not real expensive and will isolate a failed VT from the system. If the bus that the VT is connected to has instantaneous protection like bus differential or instantaneous overcurrent, you will trip the bus anyway. The fuse will provide a disconnect means to replace the VT if necessary. If the VT is in metal-enclosed switchgear, I definitely would provide primary protection.
For HV, primary protection gets a lot more expensive and you will almost always have instantaneous bus protection. Generally, I would not provide primary fusing on HV VT's.
