For students assigned to Figure 6.23: Enclosed [9, 10] are %u2018%u2018melting t
ID: 1813172 • Letter: F
Question
For students assigned to Figure 6.23: Enclosed [9, 10] are %u2018%u2018melting
time%u2019%u2019 and %u2018%u2018total clearing time%u2019%u2019 curves for K rated fuses with continuous
current ratings from 15 to 200 A. Select suitable branch and tap fuses from
these curves for each of the following three locations on your single-line diagram:
bus 2, bus 4, and bus 7. Each fuse you select should have a continuous
current rating that is at least 15% higher but not more than 50% higher than
the normal load current at that bus (normal load currents are computed in
Assignment 2). Assume that cables to the load can withstand 50% continuous
overload currents. Also, branch fuses should be coordinated with tap fuses;
that is, for every fault current, the tap fuse should clear before the branch
fuse melts. For each of the three buses, assume a reasonable X/R ratio and
determine the asymmetrical fault current for a three-phase bolted fault (subtransient
current is computed in Assignment 3). Then for the fuses that you
select from [9, 10], determine the clearing time CT of tap fuses and the melting
time MT of branch fuses. The ratio MT/CT should be less than 0.75 for
good coordination.
Source/ power system analysis and design / J. Duncan Glover, Mulukutla S. Sarma, Thomas J. Overbye / 5th edition / Question page 512-513 / assignment #4
Explanation / Answer
This standard applies to fuses rated 1 kV or less, AC or DC, and with breaking capacity up to 200 kA. These fuses are intended for installations following Canadian Electrical Code, Part I (CEC), or the National Electrical Code, NFPA 70 (NEC). The standard ampere ratings for fuses (and circuit breakers) are considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses are 1, 3, 6, 10, and 601.