74
60Hz
50Hz
Reactance(mW/m)
Load
Supply side
MCCB load terminals in the case of bare line (the line
impedance on the MCCB load side may not be added).
MCCB
Load terminal in the case of
insulated line (the line
impedance on the MCCB
load side can be added.)
Fig. 9.1 Short-Circuit Locations for Current Calculations
Table 9.1 Impedances of 3-Phase Transformers
%R
1.81
1.78
1.73
1.61
1.63
1.50
1.25
1.31
1.17
1.23
1.13
Transformer
capacity (kVA)
50
75
100
150
200
300
500
750
1000
1500
2000
Impedance (%)
%X
1.31
1.73
1.74
1.91
2.60
2.82
4.06
4.92
4.94
5.41
5.89
4. Line and Bus-Duct Impedance (Z
W
, Z
B
)
Table 9.2 gives unit impedances for various configu-
rations of wiring, and Table 9.3 gives values for duct-
ing.
Since the tables give ohmic values, they must be con-
verted, if the %-impedance method is employed.
5. Other Impedances
Other impedances in the path to the short-circuit point
include such items as CTs, MCCBs, control devices,
and so on. Where known, these are taken into con-
sideration, but generally they are small enough to be
ignored.
9.3.2 Equivalent Circuits
1. Three-Phase
Based on the foregoing assumptions for motors, the
equivalent circuits of Fig. 9.2 can be used for calcu-
lating 3-phase short-circuit current. The motor imped-
ance (Z
M
) can be considered as shunting the series
string consisting of the supply (Z
L
) and transformer
(Z
T
) impedances, by busbars of infinite short-circuit
capacity. When the three impedances are summed,
the total impedance and the resistive and reactive
components are given as:
Cable size
(mm
2
)
Resistance
(m
Ω
/m)
2-or 3-core
cables
1-core cables
(close-spaced)
1-core cables
(6cm-spaced)
2-or 3-core
cables
1-core cables
(close-spaced)
1-core cables
(6cm-spaced)
1.5
2.5
4.0
6.0
10.0
16.0
25.0
35.0
50.0
70.0
95.0
120.0
150.0
185.0
240.0
300.0
400.0
500.0
630.0
12.10
7.41
4.61
3.08
1.83
1.15
0.727
0.524
0.387
0.268
0.193
0.153
0.124
0.0991
0.0754
0.0601
0.0470
0.0366
0.0283
0.1076
0.1032
0.0992
0.0935
0.0873
0.0799
0.0793
0.0762
0.0760
0.0737
0.0735
0.0720
0.0721
0.0720
0.0716
0.0712
–
–
–
0.1576
0.1496
0.1390
0.1299
0.1211
0.1043
0.1014
0.0964
0.0924
0.0893
0.0867
0.0838
0.0797
0.0806
0.0818
0.0790
0.0777
0.0702
0.0691
0.2963
0.2803
0.2656
0.2527
0.2369
0.2138
0.2000
0.1879
0.1774
0.1669
0.1573
0.1498
0.1427
0.1356
0.1275
0.1195
0.1116
0.1043
0.0964
0.1292
0.1238
0.1191
0.1122
0.1048
0.0959
0.0952
0.0915
0.0912
0.0884
0.0882
0.0864
0.0865
0.0864
0.0859
0.0854
–
–
–
0.1891
0.1796
0.1668
0.1559
0.1453
0.1251
0.1217
0.1157
0.1109
0.1072
0.1040
0.1006
0.0956
0.0967
0.0982
0.0948
0.0932
0.0843
0.0829
0.3555
0.3363
0.3187
0.3033
0.2843
0.2565
0.2400
0.2254
0.2129
0.2001
0.1888
0.1798
0.1712
0.1627
0.1530
0.1434
0.1339
0.1252
0.1157
Notes: 1. Resistance values per
I
EC 60228
2. Reactance per the equation: L(mH/km) = 0.05 + 0.4605log
10
D/r(D=core separation, r=conductor radius)
3. Close-spaced reactance values are used.
Table 9.2 Wiring Impedance
Table 9.3 Bus-Duct Impedance
Rated
current (A)
400
600
800
1000
1200
1500
2000
2500
3000
Resistance
(m
Ω
/m) at 20
°
C
0.125
0.114
0.0839
0.0637
0.0397
0.0328
0.0244
0.0192
0.0162
Reactance (m
Ω
/m)
50Hz
0.0250
0.0231
0.0179
0.0139
0.0191
0.0158
0.0118
0.0092
0.0077
60Hz
0.0300
0.0278
0.0215
0.0167
0.0230
0.0190
0.0141
0.0110
0.0092