Primary fuse and xfmr damage curve

[Humble2000]

in one of the S&C publications , it is referred to an article from " J. R. Dunki-Jacobs " The effects of Arcing ground faults on low-voltage system design" which shows the secondary switchboard on 480/277Y volt circuit can have magnitudes as low as 40% of the maximum available L-G fault current.
Then it’s shown for a Z=8% xfrm this can be as low as 290% and this is considered as the minimum safe point for fuse curve to intersect the xfmr short-time curve.
I can not find the article in IEEE website as it was printed in1972.
1) Does anybody know a similar article or literature that I can find values for say 600V switchboard?
2) I have also found that there is no difference between the 1000KVA and 1500KVA or 2000KVA xfmr curves, am I making mistakes somewhere, SKM shows same curve for all three!

 

[jghrist]

The ANSI/IEEE C57.109 transformer damage curves are all in terms of current times normal base current, so transformer size is automatically considered.

 

[rbulsara]

Transformer primary fuses are not intended to, and they generally do not, protect against "arcing" faults. They are only there for bolted fault (short circuit) protection.

It is true that arcing faults are generally considered to be around 40% of available short circuit current. The current standards, IEEE 1584 and NFPA 70E, considers it to be 38% for its calculaiton methods. Do not confuse this with sizing of fuse sizing for transformer protection. This is even true for any type of overcurrent protection.

 

[Zogzog]

"The current standards, IEEE 1584 and NFPA 70E, considers it to be 38% for its calculaiton methods."

That 38% referes to the min current that the arc will be self sustaining, I think that is only for 480V systems.

 

[Borti]

Humble2000, I do not know what you mean by "transformer short time curve". My assumption is that you were dealing with a transformer through-fault protection curve. The reason why your curves are the same is that 1000, 1500 and 2000 KVA transformers (3-phase) are category II transformers and the curve is the same. Your curve should have shifted to the right on your graph as you were increasing the transformer rating.
When it comes to the ground fault protection, you have to compensate for the delta-y arrangement. The primary fuse will see 58% of the secondary winding current(per-unit). You have to shift the transformer through-fault curve by 58% to the left and check your fuse curve against it. Hope this answers some of your questions.