We have a customer site where they utilize a 21kv loop type distribution to feed multiple 2500kva 21kv x 277/480v distribution transformers that feed multiple buildings. The 21kv switchgear uses load interrupters with 300E fuses (Eaton RBA Expulsion Type) to protect 4/0 MV105 cable. This cable is looped through the campus (underground duct) and feeds multiple transformers. While coordinating the 480V distribution I added the 21kv circuit and fuses to the coordination for reference purposes. What I noticed was that the TCC's for the 300E were above the cable amp rating. The 4/0 MV105 is rated for about 300A amps. Using 300E fuses seems right, but the 300E's melting curve is at about 700amps @ 4000 seconds (http://www.eaton.com/Eaton/Products...eFuses/Expulsion/RBARDBFuses/index.htm#tabs-2). In doing some research, SKM application guide stated "The fuse curve will always be above the cable ampacity at 1000 seconds." http://www.skm.com/applicationguides15.html. This matches the TCC that I came up with.
I see how the fuse protects the cable for short circuit (below damage curve), but I don't understand how the fuse will protect the cable from overload or if it if is even designed to do so. If not, does the MV cable need overcurrent protection or should the infrastructure be designed so that the cable can't be overloaded?
I see how the fuse protects the cable for short circuit (below damage curve), but I don't understand how the fuse will protect the cable from overload or if it if is even designed to do so. If not, does the MV cable need overcurrent protection or should the infrastructure be designed so that the cable can't be overloaded?
