Transformer damage curve

[tommom]

I am doing a coordination study, involving a 750 kva 12.47-480/277 volt transformer, with primary current-limiting fuse protection, in this case an EJO-1 fuse.

I have never been clear on "how far up" the transformer damage curve I have to protect with the fuse. Is there something in the standards that covers this?

Thanks.

 

[DanDel]

IEEE 242 10.8.3.2 discusses this issue. It allows the primary device curve to cross the transformer damage curve, but recommends the intersection at as low a current as possible.

 

[dpc]

As far as possible, but you won't make it too far with just a fuse.

We used to only consider the "2-second" point, which relates to the mechanical damage. The top part of the damage curve deals with thermal damage.

A current-limiting fuse is not going to provide full range transformer protection for through-faults, especially a delta-wye transformer. It just isn't possible.

The lower portion of the damage curve is more critical, but there is no specific Code requirement on how much of the curve you need to cover.