Existing System

Are you sure of your numbers ? 95kA ? Well, you seem to be in North America, so it may not be far-fetched, with that very strong grid.

Fast acting fuses may do the job, but don't you need some discrimination ? Or cascading ? With fuses, any short-circuit may render the whole system dead, shutting off everything. Is that acceptable ?

Anyway, i am used to max 65kA systems here, and swithboards are built to that level of withstand, the incomer being an ACB of 65kA.

i look forward to answers from American experts on this subject, which will be some learning for both of us.

Cheers.

I wish I was wrong I have (4) 2500kVA 4160:480Y277V

check here

There are two options here (I am guessing you are not replacing the existing switchgear assembly):

(1) Use current limiting (but time delay) fuse as you indicated

(2) Series inductance to reduce the short circuit current

The option (1) will be the simple and less expensive compared to the option (2).

I am not sure what leads you to select the 1200A fuse. For the case of selecting the current limiting fuse, do the study of the 'Peak Let-Through Curve' of the fuse and make sure it limits the short circuit current from 95kA down to below 30kA (25kA or less is preferred). You also have to be very careful for the device coordination as kvsridhar already indicated. If the current limiting fuse is selected appropriately, it will be blown only when there is a fault and it will occur before the breaker trips, so the system shutdown is not an issue. In the case of such fault, the system needs to be isolated anyway.

Documents in these links may help for the selection and coordination of current limiting fuse:

http://www.bussmann.com/library/docs/EPR_Booklet.pdf (see example in page 16)

http://www.littelfuse.com/data/en/Short_Circuit_Current_Ratings_Literature/PF992UsingFusestoIncreaseYourSCCR.pdf

http://us.ferrazshawmut.com/resources/media/articles/SC1-SelectiveCoordination.pdf

http://us.ferrazshawmut.com/resources/media/articles/TT.AFN2.pdf

In addition to limiting the short circuit current, it also helps to reduce the arc flash severity.

- MS