60
for 2 minutes. Thus:
E
1.2 x P x 10
3
I
MCCB
≥
where 1.2: Allowance for random variations in
arc-welder current, and supply-volt-
age fluctuations
P: Welder rated capacity (kVA)
E: Supply voltage (V)
The switching transient in the arc welder is mea-
sured as 8~9 times the primary current. Consequently,
using 1.2 allowance, it is necessary to select instan-
taneous-trip characteristics such that the MCCB does
not trip with a current of 11 times the primary current.
7.5 MCCBs for Transformer-Primary Use
Transformer excitation surge current may possibly
exceed 10 times rated current, with a danger of nui-
sance tripping of the MCCB. The excitation surge
current will vary depending upon the supply phase
angle at the time of switching, and also on the level of
core residual magnetism. The maximum is as shown
for switching-point P in Fig. 7.7. During the half cycle
following switch-on the core flux will reach the sum of
the residual flux f
r
, plus the switching-surge flux 2f
m
.
The total, 2f
m
+f
r
, represents an excitation current in
excess of the saturation value. The decay-time con-
stant of this tends to be larger for larger transformer
capacities. Table 7.2 shows typical values of excita-
tion surge current, but as these do not take circuit
impedance into account, the actual values will be
larger. If both the primary leakage impedance and cir-
cuit impedance are known, the surge current may be
derived by considering the transformer as an air core
reactor; otherwise the values in Table 7.2 should be
used. This table gives maximum values, however, that
are based on the application of rated voltages to rated
taps; it should be noted that supply overvoltage will
result in even larger surges.
Since it is the instantaneous-trip function of the
MCCB that responds to the transient current, ther-
mal-magnetic MCCBs, which can more easily be
manufactured to handle high instantaneous-trip cur-
rents, are advantageous over completely electromag-
netic types, where the instantaneous-trip current is a
relatively small multiple of the rated current.
Table 7.2 Transformer Excitation Surge Currents
Capacity
(kVA)
5
10
15
20
30
50
75
100
150
200
300
500
Decay time constant
(Hz)
4
4
4
4
4
5
5
5
6
6
8
9
First 1/2-cycle peak
(multiple)
1
37
37
35
35
34
34
29
28
24
22
18
17
Decay time constant
(Hz)
4
4
5
5
6
6
6
6
8
8
9
12
First 1/2-cycle peak
(multiple)
1
26
26
26
26
26
23
18
17
14
13
13
11
1ph transformer 3ph transformer
Table 7.3 Transformer Capacities and Primary-Side MCCBs
Tran.
kVA
5
7.5
10
15
20
30
50
75
100
150
200
300
500
3 phase 400V
NF30-SP ( 30)
NF50-HP ( 40)
NF50-HP ( 50)
NF100-SP ( 50)
NF100-SP ( 60)
NF100-SP ( 100)
NF250-SP ( 150)
NF250-SP ( 175)
NF250-SP ( 225)
NF400-SP ( 300)
NF400-SP ( 350)
NF630-SP ( 600)
NF1000-SS ( 900)
MCCB Type (rated current (A))
1 phase 230V
NF100-SP ( 75)
NF100-SP ( 100)
NF250-SP ( 150)
NF250-SP ( 200)
NF400-SP ( 300)
NF400-SP ( 400)
NF630-SEP ( 600)
NF1000-SS ( 500)
NF1000-SS ( 500)
NF1000-SS ( 800)
NFE2000-S (1200)
NFE2000-S (1500)
—
1 phase 400V
NF100-SP ( 40)
NF100-SP ( 60)
NF100-SP ( 75)
NF250-SP ( 125)
NF250-SP ( 150)
NF250-SP ( 225)
NF400-SP ( 400)
NF630-SP ( 500)
NF630-SP ( 630)
NF1000-SS ( 500)
NF1000-SS ( 600)
NF1000-SS ( 900)
NFE2000-S (1400)
3 phase 230V
NF50-HP ( 50)
NF100-SP ( 40)
NF100-SP ( 60)
NF100-SP ( 100)
NF250-SP ( 125)
NF250-SP ( 175)
NF400-SP ( 250)
NF400-SP ( 300)
NF400-SP ( 400)
NF630-SP ( 500)
NF630-SP ( 600)
NF1000-SS ( 900)
NF1600-SS (1400)
Note: 1 “Multiple” means the first 1/2-cycle peak as a multiple of the rated-current peak.
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