Short Circuit Level 6

[SilverArc]

Hi Jghrist,
I could not find my answer so as per your suggestion, I am starting this new thread:

If I have a 10 MVA transformer with 7 % impedance at 69/13.8 KV...
The short circuit level at secondary is 5981 A and primary is 1196.2 A. When I know this is the possible primary short circuit in primary as well as seconday, then why do I need the short circuit MVA from Utility.

As you advised to have the utility impedance, Utility is an infinite source: I guess the main applications of the SC MVA is to decide the rating of the Primay protective device like a circuit switcher or a breaker and has nothing to do with the secondary rating:
But technically, if my primary short circuit utility short circuit is 40000A then secondary short circuit as per my trasnformer configuration should be 40000x(13800/69000)=8000 A, which is not possible as the transformer can only deliver 5981 A.

Do u think, if I am making sense ?
Thanks

 

[SilverArc]

Thanks apowerenger.

Few weeks ago, I had a discussion with an application engineer(P.Eng) from S & C electric and he advised me that normally for fused cutouts,.. commercial facilities use E-type and utilities use K type. Both fuses are expulsion type.

This morning I spoke 2 an application engineer from cooper power and he told me that all E rated fuses are current limiting. He said K type fuses are expulsion type.

Could you advise me, who is right here.

Thanks

 

[davidbeach]

E rated fuses may or may not be current limiting. You need to look at the fuse curves to see whether a particular fuse product line is current limiting or not. The definition of "E Rated" does not include current limiting, but doesn't preclude it either.

 

[apowerengr]

SilverArc - I have utility customers using K, T, QA, N(200), S&C Standard speed, D and KS fuses in cutouts. Around here it pretty much covers the gamut and depends on application with more utilities using faster (K & KS) fuses for capacitor protection and D (surge-durable) for surge let-through for tank mounted arresters on overhead transformers. I see more utilities using E speed (S&C SMU or SM for example) for either underground or high-fault current applications (above the cutout interrupting rating) even though you can get these interrupting ratings in other speeds (such as K). Some utiliities add partial-range current-limiting fuses in series with cutouts near substations where the fault currents are higher than the cutout interrupting ratings provided the tap fuse is small enough to allow a partial-range CL fuse to handle the load and coordinate. As davidbeach stated - E does not mean current-limiting.