Page 192 - Handbook of Electrical Engineering

FUSES 175

Table 8.1. Characteristic currents that are related to the X-to-R ratio of a circuit

Ratio to rms symmetrical current
Short Short Max 1-phase Max 1-phase Avg 3-phase
circuit circuit peak current rms current rms current
X-to-R power at 1/2 cycle at 1/2 cycle
ratio factor
Infinity 0.0 2.828 1.732 1.394
100 0.01 2.785 1.696 1.374
49.993 0.02 2.743 1.665 1.355
33.322 0.03 2.702 1.630 1.336
24.979 0.04 2.663 1.598 1.318
19.974 0.05 2.625 1.568 1.301
9.9501 0.1 2.455 1.436 1.229
6.5912 0.15 2.309 1.330 1.171
4.8990 0.2 2.183 1.247 1.127
3.1798 0.3 1.978 1.130 1.066
2.2913 0.4 1.819 1.062 1.031
1.7321 0.5 1.694 1.026 1.013
1.3333 0.6 1.594 1.009 1.004
1.0202 0.7 1.517 1.002 1.001
0.75 0.8 1.460 1.0002 1.00005
0.6198 0.85 1.439 1.00004 1.00002
Zero 1.0 1.414 1.0 1.0

Figure 8.1 Fuse cut-off curves for different X-to-R ratios of the fault circuit. The curves show how the
clearance time varies with the ratio..