Page 244 - Handbook of Electrical Engineering

228 HANDBOOK OF ELECTRICAL ENGINEERING

Figure 9.5 Cut-off and prospective current of fuses.

For the selection of fuses the prospective symmetrical rms value of the off-set fault current
I frms is calculated as,

147,654
I fpka
I frms = √ = √
2 2 2 2
= 52,204 amps

From the graphical characteristics given by the manufacturer the maximum fuse rating can be
selected, see Figure 9.5.
The maximum fuse rating is 315 A and so the choice of 200 A is satisfactory.
Select a cable and check that its I-squared-t characteristics are adequate.
From Table 9.17 the derating for ambient temperature K air is 0.92 and assume no derating for
2
◦
the grouping of cables. Hence the nearest cable rating to carry 200 amps at 35 Chas a 70 mm cross-
sectional area. This cable has a thermal energy constant K of 143. Hence its time-current characteristic
is given by two or more points on a straight line on a log-log graph. From the following,
√ KA 143 × 70 10010.0
t = = =
I I I

Several suitable points on the graph are given in Table 9.27.