Transformer Primary Fuse 1

[timm33333]

I have a question about sizing the primary fuse for 12.47kV-480V 3750 kVA transformer. The primary full load current would be 174A, and a 200A fuse would be required. For S&C SMU-20 fuses: 100E has a continuous current rating of about 200A, and 200E has a continuous current rating of about 500A. Should the fuse be sized on E-rating or on continuous current rating? In other words, should we select 100E fuse or 200E? Thanks

 

[RRaghunath]

Firstly, I don't think 200A fuse is correct for 174A of transformer full load current.
The fuse needs to be rated to minimum 125% of transformer full load current and to a max of 250% of full load current.
The criteria is that the fuse should not blow on transformer energisation inrush current, should allow overloading of transformer within permissible limits. Check the NEMA or ANSI codes for more guidance.
100E fuse is expected to blow within 600s when RMS current of 200-240% of rated current is passed. This doesn't mean the fuse can take 200A continuously. You seem to have got definitions wrong.

 

[cuky2000]

Below is an excerpt from S&C Table for E-fuse selection. Notice that optimal protection should consider the Transformer Protection Index and the Loading capability.

<sub>a) The Transformer Protection Index is associated with the total clearing of the TCC curve selecting the primary E-fuse to pass below and to the ANSI Point of the appropriate transformer short-time characteristic curve, and second, the point at which the two curves intersect should be at as low a multiple of the transformer primary full-load current as possible.

b)Loading capability refers to the amount of load the transformer-primary fuse can pick up (even after a momentary loss of source voltage) without operating, and carry on a continuous basis. The values listed in the fuse selection tables were determined to be the minimum of continuous peak-load, hot-load,and cold-load pickup capabilities for each fuse ampere rating.</sub>