Potential Transformer Fuse

1. Because we are supposed to.

2. It's rating is chosen by using the appropriate codes and specifications.

Google, or an electrician can tell you more.

Here's a little more:

Fuse Sizing Guide - Omega Engineering

thank you for your reply
but about fuse i am asking about it's role

It's role is the same here as any fuse's, to protect the upstream circuit from interruption/destruction should there be a fault on the downstream side.

You should do your own homework!

Every transformer has a fuse. For some transformers the wires act as a fuse.

To protect the PT and to provide a way of isolating the PT from the source for maintenance and replacement without de-energizing the bus.

A fuse does not "...protect the PT..." because if the fuse blows there's already a problem with the PT or the circuitry leading to it downstream of the fuse. It will also help minimize damage/interruption to the surrounding equipment and circuitry due to a catastrophically failing PT by limiting the available short circuit current's magnitude and duration.

Sorry but I disagree wholeheartedly.

I have replaced PT fuses many times when the downstream circuit wiring and/or the meter has failed/faulted but the PT has been undamaged.

If the fuse was not there the PT would have suffered major current overload damage to the point of short-circuiting the windings which would then result in the source bus voltage being injected into everything downstream causing extreme catastrophic failure.

A fuse is used to protect whatever is attached downstream of it with the amperage size based on the wiring size or equipment current rating limitations.

The speed at which the fuse opens is based on the sensitivity of the equipment downstream of the fuse and the amount of expected inrush current.

I believe we said the same thing but differently.

Thank you all now it is clear