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Redundancy Power Supply - 24V, 10 A, 240 W, Single-phase Input
28
Output Circuit Breakers
Standard miniature circuit breakers (MCBs or UL 1077 circuit breakers) are
commonly used for AC-supply systems and may also be used on 24V branches.
MCB's are designed to protect wires and circuits. If the ampere value and the
characteristics of the MCB are adapted to the wire size that is used, the wiring
is considered as thermally safe regardless of whether the MCB opens or not.
To avoid voltage dips and under-voltage situations in adjacent 24V branches
which are supplied by the same source, a fast (magnetic) tripping of the MCB is
desired. A quick shutdown within 10ms is necessary corresponding roughly to
the ride-through time of PLCs. This requires power supplies with high current
reserves and large output capacitors.
Furthermore, the impedance of the faulty branch must be sufficiently small in
order for the current to actually flow. The best current reserve in the power
supply does not help if Ohm's law does not permit current flow. The following
table has typical test results showing which B- and C-Characteristic MCBs
magnetically trip depending on the wire cross section and wire length.
Figure 34 - Test Circuit
Power Supply
AC
S1... Fault simulation switch
(1)
Maximal Wire Length
0.75 mm
C-2A
30 m
C-3A
25 m
C-4A
9 m
C-6A
3 m
C-8A
—
B-6A
12 m
B-10A
3 m
B-13A
2 m
(1) Consider twice the distance to the load (or cable length) when calculating the total wire length (+ and – wire).
Rockwell Automation Publication 1606-RM110A-EN-P - July 2020
MCB
Load
+
+
Wire length
S1
-
-
DC
for a Fast (Magnetic) Tripping
2
2
1.0 mm
37 m
30 m
15 m
3 m
—
15 m
3 m
2 m
2
2
1.5 mm
2.5 mm
54 m
84 m
46 m
69 m
25 m
34 m
4 m
7 m
—
—
21 m
34 m
4 m
9 m
3 m
6 m
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