Fuse Rating

[cduranph]

A power transformer has a capacity and ratings of 4.0/5.0MVA OA/FA cooling, 34.5kV-480Y/277V, 6.5% IZ, Oil Filled, 3-phase, 60Hz. The designer specifies 160A size of fuse for the primary Load Break Switch of the above transformer. Our problem right now is, the end user approved an specific brand of LBS with only 100A maximum available at 34.5kV application. This LBS manufacturer do not allow switchgear manufacturers to design their fuses in parallel so that we can choose 2 x 80A in order to follow the 160A specs. To be more specific, the brand of LBS is AREVA (Formerly Alstom), Fuse is SIBA.

Hope you can help us on this. Thanks

 

[charlie b]

Re: Fuse Rating

At 5 MVA, the primary rated current is 83 amps. You can use a fuse rated as high as three times that value, or 250 amps. I think 160 amps is a better choice than 100 amps, because the fuses are less likely to blow during a normal start of a large motor.

That notwithstanding, the NEC would not prohibit you from using 5 amp fuses. The owner might not care for the result, but the NEC does not care whether the owner cares for the result.

I can only think of two choices. One is to use the selected LBS with a set of 100 amp fuses, and let the owner deal with any false trips that may occur. The other is to talk the owner into a different LBS that allows for higher rated fuses.