Page 35 - Application Guide Semiconductor Fuse Link

Fuse selection examples

3-phase a.c. controller

Select a suitable fuse from the Ferraz Shawmut square-bodied PSC range, size 30, to protect the a.c.
controller shown in Fig.28. The equipment designer has indicated that the possibility of a 3-phase
fault need not be considered.

System data is as follows :

a Load
Max AC line voltage 660 V
F2
Frequency 50 Hz 660 V 100 A r.m.s
Ambient temperature 55 °C a.c.
Cooling air speed 2 m/s
b Load
Load current 100 A
Short-circuit current 5 kA

Continuous load < 12 stops per day

c Load
Occasional overload 200% for 10s

2
2
Thyristor I t withstand 20,000 A s
Fig.28 3-phase a.c.
(at 8.33 ms)
Data for the PSC size 30 is as follows :

a = 130 For continuous load with < 12 stops
per day
= 30
0o
B1 = 1.25 A’2 = 0.8 (see Table 2)
= 0.85
C1
Cf3 = 0.75

First estimate the required continuous current rating

130-55
-
= 0.866
correction for ambient temperature A1 = 130 - 30

correction for non-standard terminal contacts C1 = 0 .85

correction for forced air cooling B
= 1 + (1.25 - 1)*2/5 = 1.1

correction for load variations A’2 = 0.8

Since the r.m.s. fuse current is the same as the load current of 100A, we require that the adjusted
fuse current rating is greater than 100A, which gives
In’ = In x A1 x B x C1 x A’2 100.0

In x 0.866 x 0.85 x 1.1 x 0.8 100.0

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