I work on equipment that runs off 3 phase 208V but it uses uses a transformer to drop it down to 120V for most of the controls. On this equipment I noticed that there are two fuses on the lines exclusively feeding the 208V side of the transformer and a fuse directly off of the hot side on the 120V side of the transformer.

Isn’t the fuse on the 120V side of the transformer redundant? From my understanding, if there is a current spike on the 120V side of the transformer then that will cause a current spike on the 208V side of the transformer and immediately blow those fuses anyways. Is this just a certification thing where that redundancy is required? I’m in the US but this equipment does also get shipped to various overseas locations. Also, while it isn’t standard, this equipment is capable of passing a TUV inspection if a customer requests it so I’m not sure if the potentially redundant fuse is just a TUV requirement.