Power Supply PS1000-A6-12.16
Application Notes
20
21
-1
1
39
27.4
Output Circuit Breakers
Standard miniature circuit breakers (MCB’s or UL 1077 circuit breakers) are commonly used
for AC-supply systems and may also be used on 12V 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 12V 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 PLC's. 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 27.4 Test circuit
Maximal wire length
1
for a fast (magnetic) tripping:
1
Don’t forget to consider twice the distance to the load (or cable length) when calculating the total wire length
(+ and – wire).
0.75mm²
1.0mm²
1.5mm²
2.5mm²
C-2A
11m
15m
22m
35m
C-3A
9m
13m
18m
23m
C-4A
5m
8m
12m
17m
C-6A
–
1m
2m
3m
B-6A
6m
11m
15m
23m
B-10A
2m
3m
3m
4m
B-13A
1m
2m
3m
4m
MCB
Power Supply
AC
DC
+
-
+
-
Load
Wire length
S1... Fault simulation switch
S1
