Page 335 - Industrial Power Engineering and Applications Handbook

12/314 Industrial Power Engineering and Applications Handbook
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10

1
t

Symmetrical prospective current (rms) Amperes -

Figure 12.49 Typical time-current characteristics of 6.6 kV HRC fuses

12.10.4 HRC fuses
The rating of the fuses often decide the rating of the
above components. They are readily available up to 1000
A to switch all sizes of motors recommended on an LT
system. The characteristics of the fuses must match with
the thermal withstand Z2 - t characteristic of the equipment
or the circuit it has to protect, as discussed earlier. Broadly
speaking, they must prevent the switch or the contactor
from breaking currents beyond their thermal capacity
and prevent contact welding. The minimum rating of the
fuses is chosen as 1.6 to 2 times the full-load current of
the motor for a DOL start and about 1.25 to 1 .SO for a
YlA start to ensure that the fuses stay intact without
interrupting the motor during a start. One particular brand
of HRC fuses has been considered here in making the
selection. The ratings may slightly vary with the other
brands of fuses, depending upon their I’ - t characteristics
(Figure 12.59).
12.10.5 Selection of cables
1 .o
1 .o 5 10 T 150 100 This is also based on the full-load current. A cable
30 43.8
Prospective current + generally has a much higher thermal capacity than the
kA (rms symmetrical) cut-off time of the HRC fuses of a corresponding rating.
Also refer to Z2 - t curves for cables in Chapter 16,
lllustra tion
A 100 A fuse will cut-off an I,, of 30 kA (peak value up to Appendix 1, Figures A16.4(a) and (b) and cable selection
2.5 x 30 kA) at less than 6 kA peak criterion as in Figure AI 6.5. It is, however, advisable to
check the fuse let-through energy, which should not be
Figure 12.50 Typical current cut-off characteristics of 6.6 kV more than the short-time rating of the cable. The following
HRC fuses at prospective currents up to 43.8 kA may also be kept in mind while selecting the cables: