Fuji Electric BW32AAG - Application of Protective Devices

54
3
Selection and application
3-4 Wiring protection
Table 3-10 MCCB protected wiring (up to the rated short-time range)
Wire
size
(mm
2
)
Allowable
current (A)
Allowable
current
considering
MCCB trip
characteristics
(A)
MCCB rating (A)
Open
3-wire
Duct
3-wire
Amb. temp. 30°C
Rise 40K
30°Co100°C
10 15 20 40 50 60 75 100 125 150 175 200 225 250 300 350 400 500 600 700 800
1000
1 14.5 10.2 12
1.5 18.5 13.0 18
2.5 25 18 27
4342440 Unprotected region
6433052
10 60 42 68
16 80 56 98
25 101 71 123
35 126 88 154 Protected region
50 153 107 187
70 196 137 239
95 238 167 290
120 276 193 337
150 319 223 389
185 364 255 444
240 430 301 525
300 556 389 678
400 664 465 810
500 757 530 924
630 856 599 967
(2) Wiring protection by MCCBs
The MCCB to be used for overcurrent protection of wiring
can be selected by observing the principle in item (1). In
the short- time region discussions, remember that the
tripping characteristic curve of an MCCB may cross the
allowable current characteristic curve of wire near the point of
intersection of the inverse time-delay trip and instantaneous
trip characteristics of the MCCB.
Table 3-10 shows the combinations of MCCBs and PVC wires
available for protection in the short-time region. Protection
for wires in the short-circuit region can be determined by
reviewing
the allowable I
2
t value of the wire. Table 3-11 shows the max.
breaking I
2
t values of MCCBs at the time of short-circuit
current breaking and the allowable I
2
t values of wires needed
to accomplish this determination.
3-4-3 Application of protective devices
(1) Principle
When an overcurrent fault occurs, a circuit breaker must be
chosen that can interrupt an overcurrent before the wire is
heated above its allowable temperature. The rated current
of the circuit breaker thus must be lower than the allowable
current of the wire. In the short-time region, the circuit breaker
should exhibit operating characteristics below the allowable
current-time characteristics of the wire as shown in Fig. 3-13.
In the short-circuit region it is necessary to verify, by way of
calculation, that the max. breaking I
2
t value of the overcurrent
protective device is lower than the allowable I
2
t value of the
wire.
Fig. 3-13 Protection coordination for wiring
Current
MCCB operation characteristic
Wire's allowable characteristics
(Short-time region)
I
2
t = Constant
(short-circuit region)
Time