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GUIDE TO LOW VOLTAGE FAULT LEVEL
kVA
mVA
kVA/1000
Current(I)
@ 11kV
Current (I)
@ 433V
Typical
% Z
Z1 @ 433V
in Ohms
PSC @ 433V
in kA
150.0150.79204.750.59370.421
250.0251.3133.334.50.33750.741
500.052.6266.674.50.16871.482
1000.15.25133.344.750.08912.807
1500.157.87200.014.750.05944.211
2000.210.5266.674.750.04455.614
3000.315.75400.014.750.02978.421
3150.31516.53420.0150.02988.4
4000.420.99500.354.750.022311.228
5000.526.24666.694.750.017814.035
6000.626.24666.694.750.014816.843
7500.7539.361000.034.750.011921.053
7500.7539.361000.036.250.015616
8000.841.991066.74.750.011122.457
1000152.491333.374.750.008928.071
12001.262.981600.0550.007832.001
13501.3570.861800.055.250.007334.287
15001.578.732000.065.50.006936.365
20002104.972666.745.50.005248.486
25002.5131.223333.4360.004555.557
Note:
The attached calculations are based on zero source impedance and are intended as a guide only.
Example: fault level for 400kA Sub:
Ohms =
KV Line Voltage2
MVA
 x %Z
(433)2
0.4x106
 x
(4.75)
100
 = 0.02226 Ω
KA =
KV Line Voltage2
√3 x Ohms
 x %Z
433
√3 x 0.02226
 = 11.228 kA
Transformer Primary Breaker Selection
1ph kVA
240V ln
3ph kVA
400/415V ln
Type
Trip unit type and rating*
lst
Add To Table
 
2.5106.39FD160N, FD160H or LLTMD-25250Add
411812FD160N, FD160H or LLTMD-25250Add
5171014FD160N, FD160H or LLTMD-32320Add
--12.518FD160N, FD160H or LLTMD-32320Add
6.32712.518FD160N, FD160H or LLTMD-40400Add
8342028FD160N, FD160H or LLTMD-50500Add
10422535FD160N, FD160H or LLTMD-63630Add
12.55331.544FD160N, FD160H or LLTMD-80800Add
--4056FD160N, FD160H or LLTMD-1001000Add
16685070FD160N, FD160H or LLTMD-1251250Add
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Transformer Primary Fuse Selection (3-phase 415v)
kVA
Primary FLC(A)
T 'gG'
'gL' 'gG'
Add to Table
 
11.564Add
23.1106Add
34.61610Add
46.11610Add
57.71616Add
69.22016Add
710.72016Add
812.22016Add
10152520Add
12183225Add
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Transformer
1st Period
2nd Period
3rd Period
value5 ms.25 ms.45 ms.
< 50 kvA25 x ln12 x ln5 x ln
> = 50 kvA15 x ln8 x ln3.5 x ln 

    Notes: From bottom of table P 384 
    notes:
    1. FLC values are typical only. Possible deviation of FLC values +-5%
    depending on the manufacturer.
    2. Recommended fuse ratings allow for transformer magnetizing inrush 
    current of 12 times FLC for 0.1 seconds.
    3. Suitable adjustments to the recommended ratings may be necessary for
    high enclosure (fluid environment) temperatures, refer to de-rating 
    information indicated on page 260.
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