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Security Power Supplies

MINIATURE CIRCUIT BREAKERS FOR USE IN CONJUNCTION WITH MOTOR STARTERS AND TRANSFORMERS

 
Table 2-1 phase 240V AC DOL starting
Motor starters
In general miniature circuit breakers can give only short circuit protection to motor loads due to high starting currents which may be encountered: typically 3 to 12 times full load current (FLC)

Assumptions
The tables give recommended mcb ratings for motors up to 37kW based on the following assumptions:

Direct on-line starting
starting current = 7 x FLC
run up time =
6 seconds, motors < 3 kW
10 seconds, motors < 22 kW
running currents = average values only
(individual manufacturers figures may vary)
four pole motors i.e. speed approx.
1500 rev/min.

For Higher inertia loads i.e. hoists and fans run up times maybe considerably longer than those assumed above. The rating of the mcb must take account of the greater run-up time and starting current. The required mcb rating can be determined by reference to time/current curves (consult us)

Star/ delta starting
Since, during the changeover from star to delta, a high current surge in the order of DOL values may be met, the mcb rating selected should be the same as that
recommended for DOL starting
KW
Hp
Running
C60H
C60HC
C60HD
NC100C
NC100D
0.12
0.166
0.55
2
1
1
    
0.18
0.25
0.7
2
1
1
    
0.25
0.33
0.87
2
2
1
    
0.37
0.5
1.35
4
2
2
    
0.55
0.75
1.55
4
2
2
    
0.75
1
1.93
6
4
2
    
1.1
1.5
2.5
6
4
4
    
1.5
2
3.5
10
5
6
    
2.2
3
4.8
16
10
10
10
10
3
4
6.4
16
16
10
16
10
3.75
5
7.8
20
20
16
20
16
4
5.5
8.1
25
20
16
20
16
5.5
7.5
11
25
25
16
25
16
7.5
10
14.4
32
25
20
25
20
9.33
12.5
17.3
40
32
20
32
20
11
15
21
50
40
25
40
25
13
17.5
25
63
50
32
50
32
15
20
28
63
50
40
50
40
18.5
25
35
  
63
50
63
50
22
30
40
  
63
50
63
50
30
40
54
    
63
80
63
37
50
65.5
      
100
80
Table 2-1 phase 240V AC DOL starting
KW
Hp
Running
C60H
C60HC
C60HD
NC100C
NC100D
0.12
0.166
0.95
2
2
1
    
0.18
0.25
1.5
4
2
2
    
0.25
0.33
1.7
6
2
2
    
0.37
0.5
3
10
6
4
    
0.55
0.75
4.5
16
10
6
10
  
0.75
1
5.5
16
16
10
16
10
1.1
1.5
8.5
20
20
16
20
16
1.5
2
10.5
25
25
20
25
20
2.2
3
15.5
32
32
25
32
25
3
4
20
40
40
32
40
32
3.75
5
24
50
50
40
50
40
5.5
7.5
34
63
63
50
63
50
6.3
8.5
36.5
  
63
63
63
63
7.5
10
45
    
63
80
63
11
15
66.5
      
100
80

 
VA
Primary
in (A)
C60H
C60HC
C60HD
NC100C
NC100D
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 12500 VA and three phase transformers up to 30000 VA on the following formula.

Mcb rating
15 x normal current of transformer
min instantaneous tripping
cc efficient of mcb
500
0.7
4
2
1
    
750
1.04
6
4
2
    
1000
1.39
6
4
2
    
2000
2.78
10
10
6
10
  
5000
6.95
32
16
10
16
10
10000
13.89
50
32
20
32
20
15000
20.84
  
50
32
50
32
20000
27.78
  
53
40
63
40
25000
34.73
    
50
80
50
30000
41.67
    
63
80
63
Table 4 - 1 phase transformers 240V AC supply
VA
Primary
in (A)
C60H
C60HC
C60HD
NC100C
NC100D
50
0.21
1
        
100
0.42
2
1
1
    
250
1.04
6
4
2
    
500
2.08
10
6
4
    
1000
4.17
20
10
10
10
10
2500
10.42
40
25
16
25
16
5000
20.84
  
50
32
50
32
10000
41.67
    
63
80
63

Ohms Law

IF YOU KNOW NEED TO KNOW
 
VOLTS ÷ RESISTANCE = AMPS
VOLTS ÷ AMPS = RESISTANCE
VOLTS x AMPS = WATTS
WATTS ÷ AMPS = VOLTS
WATTS ÷ VOLTS = AMPS
AMPS x RESISTANCE = VOLTS

CABLE LENGTH
RESISTANCE
x
CURRENT
DRAWN
=
VOLT DROP
------------------
-----------
Ohms
AMPS OR M/AMPS
 
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