Page 197 - Handbook of Electrical Engineering

180 HANDBOOK OF ELECTRICAL ENGINEERING

Figure 8.3 Peak cut-off current versus prospective symmetrical rms current for typical fuses in the range of
100 A to 200 A.

See Table H.1b for X-to-R ratio of 25, the doubling factor is 2.661.
Therefore the peak asymmetrical current is I pkasym ,

I pkasym = 13,122 × 2.661 = 34,944 amps pk

Step 4. Decide upon suitable cut-off current.
Choose the maximum cut-off current to be 45% of the peak asymmetrical fault current

I pkcutoff = 0.45 × 34,944 = 15,724 amps pk
Round this up to 16,000 amps pk

Step 5. Select the largest fuse to suit the cut-off limit.
Use Figure 8.3 even through the prospective current is shown as rms symmetrical. This
example is a special case since the X-to-R ratio is known.
34,944
Prospective current = √ = 24,713 amps rms
2
Hence the ‘largest’ fuse for cut-off limit is 160 amp rating.
Step 6. Compare the I-squared-t characteristic of the fuses with the I-squared-t characteristic of
the cables, in Figure 8.4. A 160 amp fuse will protect the 35 mm sq cable for fault currents beyond