Application notes
PS3031-2440-0000
36
Version: 1.0
4.5
Output circuit breakers
Standard circuit breakers (or UL1077 circuit breakers) are generally used for AC supply systems and can
also be used for DC branches.
Circuit breakers are used to protect cables and circuits. If the current value and the characteristics of the
circuit breaker are matched to the cable thickness used, the wiring is considered thermally safe, regardless
of whether the circuit breaker opens or not.
To avoid voltage drops and situations with undervoltage in adjacent 24 V branches fed from the same
source, a fast (magnetic) trip of the circuit breaker is desirable. Fast switch-off within 10 ms is required,
which approximately corresponds to the hold-up time of PLCs. This requires power supplies with high
reserve current and large output capacitors. In addition, the impedance of the faulty branch must be
sufficiently small for the current to actually flow. The following table contains typical test results that show
which circuit breakers with B and C characteristics trip magnetically, depending on the wire cross-section
and the wire length.
Fig. 26: Test circuit
Maximum wire length
*)
for fast (magnetic) tripping:
0.75 mm
2
1.0 mm
2
1.5 mm
2
2.5 mm
2
C-2A
28 m
38 m
54 m
78 m
C-3A
26 m
35 m
50 m
74 m
C-4A
19 m
26 m
38 m
58 m
C-6A
12 m
16 m
24 m
32 m
C-8A
9 m
12 m
17 m
25 m
C-10A
7 m
10 m
15 m
21 m
C-13A
4 m
5 m
7 m
11 m
0.75 mm
2
1.0 mm
2
1.5 mm
2
2.5 mm
2
B-6A
19 m
26 m
35 m
59 m
B-10A
11 m
17 m
26 m
37 m
B-13A
10 m
13 m
21 m
32 m
B-16A
8 m
11 m
14 m
24 m
B-20A
4 m
6 m
8 m
14 m
*)
Don't forget to double the distance to the load (or the cable length) when calculating the total cable length
(plus and minus cable).
