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Dimensions
Section 12
11
In
θ
I
t
1st peak
10 to 25 In
Transformers
High inrush currents are also produced when transformers are switched on,
typically 10 - 15 times full load current.
Assumptions
The tables give recommended MCB ratings for single phase transformers up to
12500VA and three phase transformers up to 30000VA based on the following
formula.
Inrush currents
When LV/LV transformers are switched on, very high inrush currents are produced
which must be taken into account when choosing overcurrent protection devices.
The peak value of the first current wave often reaches 10 - 15 times the rated rms
current of the transformer and may reach values of 20 - 25 times the rated current
even for transformers rated less than 50kVA. This transient inrush current decays
very quickly (in a few milliseconds).
Table 3 - 3 phase transformers 415Vac supply
VA Primary In (A)
C60HB
C60HC
C60HD
500 0.7
4
2
1
750 1.04
6
4
2
1000 1.39
10
6
4
2000 2.78
16
10
6
5000 6.95
40
25
16
10000 13.89
-
50
25
15000 20.84
-
63
32
20000 27.78
-
-
50
25000 34.73
-
-
63
30000 41.67
-
-
63
Table 4 - 1 phase transformers 240Vac supply
VA Primary In (A)
C60HB
C60HC
C60HD
50
0.21
2
-
-
100 0.42
4
2
1
250 1.04
6
4
2
500 2.08
16
10
4
1000 4.17
25
16
10
2500 10.42
63
32
16
5000 20.84
-
63
32
10000 41.66
-
-
63
12500 52.08
-
-
-
For use in conjunction with
motor starters and transformers (cont.)
Miniature circuit
breakers