Rockwell Automation 150 MNL/SMC DIALOG+/APP-PROD GUIDE - Manual (Page 127)

SMC Dialog Plus Controller Special Application Considerations 5-3
Current Limit Fuses
(Overcurrent Protection of SCRs) The heat energy of high-level fault currents can quickly damage an
SCR. As illustrated in Figure 5.2, a current limit fuse limits peak let
through current (Ip) to a level which is a fraction of the potential
available short circuit peak current.
The fast response to the buildup of short circuit current and the
quickly decaying short circuit current as the fuse suppresses internal
arcing together can limit the I
2
t let through values substantially lower
than the I
2
t withstand of the semiconductor device. The fuse clearing
time decreases with the higher available short circuit currents.
The SCR fuses are coordinated with the SCRs used in the SMC
Dialog Plus controller. They are sized to clear a short circuit fault in
approximately three milliseconds or less.
These SCR fuses are sized to operate the controller at 60-70% of the
fuse rating to avoid deterioration of the fuse. The suggested fuse fo
the particular current rated device should be used to allow prope
protection of the SCRs. Fuses which are undersized will cause
nuisance trips and incur fuse replacement costs.
Fast acting SCR fuses may not provide branch circuit protection.
Branch circuit protection in accordance with applicable electrical
codes may require additional branch fusing (or a circuit breaker) even
though fast acting current limiting fuses are used.
Figure 5.2 Clearing Time of Fuse
Current
28,000A
2,800A
Total Clearing Time of Fuse
Available Short Circuit Current
lp
Time
tc