How clos is inrush point to the curve

[Humble2000]

Attached is a result of coordination study of a fault fiter fuse( S&C ) and low voltage ckb. What margine need to exist between the inrush point and the fuse curve to safely order the fuses?

 

[tem1234]

Hum, that's a good question

I assume that the point is draw at 12x nominal current of the transformer. This is a conservative assumption, since every transformer is different and the utility impedance will limit the inrush current.

So if the curve doesn't touch the point, this is probably good. I saw a transformer with an instantaneous at 8x is nominal current, but it work fine, so why fix it.

The 12x point is to be sure, when the coordination is tight, I assume that it is less than that.

 

[Humble2000]

Well, Tx is 3000/4000KVA- 27.6KV/.6KV 10.8 % impedance, calculation shows 8X inrush point, so as manufacurer recomendation.
How would I be certain to tell the customer to change his existing fuse to fault fiter, spend alot of money and then , it trips for inrush.....
even if we measure the inrush..it might vary right?

 

[stevenal]

I cannot read your time scale. 12X is generally plotted at 0.1s for hot-load inrush. Is your 8X at 0.1s? If you expect to pick up motor loads or lose diversity following an extensive outage, consider adding cold-load inrush points at 6X for 1s, 3X for 10s, and 2X for 100s. Join all points with straight line segments to produce an inrush plot on your plot.