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4.44
General Catalogue • Protection Devices • Technical
Transformer/Motor Protection
Transformer Protection
Tables 19 & 20 show the recommended MCB’s for the protection of
single phase (230V) and three phase (400V) transformers.
Single Phase 230V
Recommended MCB
Transformer
Rating (VA)
Primary
Current
NBN
NCN
NDN
50
0.22
-
1
6
100
0.43
-
2
6
200
0.87
-
3
6
250
1.09
6
4
6
300
1.30
10
4
6
400
1.74
10
6
6
500
2.17
16
10
6
750
3.26
16
10
6
1000
4.35
25
16
10
2500
10.87
63
40
20
5000
21.74
-
63
32
7500
32.60
-
-
50
10000
43.48
-
-
63
Three Phase 400V
Recommended MCB
Transformer
Rating (VA)
Primary
Current
NBN
NCN
NDN
500
0.72
-
3
6
750
1.08
6
4
6
1000
1.44
10
6
6
2000
2.88
16
10
6
3000
4.33
25
16
10
4000
5.77
32
20
10
5000
7.21
40
25
16
7500
10.82
63
32
20
10000
14.43
-
50
25
15000
21.64
-
63
32
20000
28.86
-
-
50
25000
36.07
-
-
63
Motor Circuit Protection
Tables 28,29,30 and 31 give general recommendations for the
selection of circuit breakers and HRC fuses for the protection of
motor power circuits and are based on the assumptions shown in
Table 28 for a cage motor running at approximately 1400 Rev/Min.
Motor Rating DOL Starting
Conditions
Assisted Start Conditions
Up to 0.75kW 5 x FLC for 6 secs 2.5 x FLC for 15 secs
1.1 to 7.5kW 6 x FLC for 10 secs 2.5 x FLC for 15 secs
11 to 75kW 7 x FLC for 10 secs 2.5 x FLC for 15 secs
90 to 160kW 6 x FLC for 15 secs 2.5 x FLC for 20 secs
1 Phase 230V DOL Starting
Recommended Circuit Breaker
kW hp
FLC A (A)
NBN
(A)
NCN
(A)
NDN
Fuse
(A)
0.18 0.25 2.8
16
10
10
10
0.25 0.33 3.2
16
10
10
16
0.37 0.5
3.5
16
10
10
16
0.55 0.75 4.8
20
16
16
16
0.75 1.0
6.2
25
20
20
20
1.1
1.5
8.7
40
25
25
25
1.5
2.0
11.8 50
32
32
32
2.2
3.0
17.5 -
50
50
40
3.0
4.0
20
-
63
63
50
3.75 5.0
24
-
-
-
63
5.5
7.5
36
-
-
-
80
7.5
10
47
-
-
-
100
3 Phase 400V Assisted Starting Star-Delta
Recommended Circuit Breaker
kW hp
FLC A (A) NCN (A) NDN HRC Fuse (A)
3
4
6.3
16
10
16
4
5.5
8.2
20
10
16
5.5
7.5
11.2 32
16
20
7.5
10
14.4 40
25
25
11
15
21
50
32
32
15
20
27
-
40
35
18.5 25
32
-
50
40
22
30
38
-
63
50
30
40
51
-
-
63
37
50
63
-
-
80
45
60
76
-
-
80
55
75
91
-
-
100
75
100 124 -
-
160
90
125 154 -
-
200
110 150 183 -
-
200
132 175 219 -
-
250
150 200 240 -
-
315
160 220 257 -
-
315