Output circuit breakers

Standard miniature circuit breakers (MCB’s or UL1077 circuit breakers) are commonly used for AC-supply systems and may also be used on DC 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 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.

Output circuit breakers 1:
Test circuit

Maximal wire length*) for a fast (magnetic) tripping:

 

0.75mm2

1.0mm2

1.5mm2

2.5mm2

C-2A

31 m

42 m

62 m

92 m

C-3A

28 m

36 m

51 m

77 m

C-4A

18 m

23 m

36 m

55 m

C-6A

11 m

14 m

21 m

33 m

C-8A

8 m

12 m

18 m

27 m

C-10A

7 m

10 m

15 m

22 m

C-13A

4 m

6 m

8 m

13 m

C-16A

2 m

3 m

4 m

7 m

C-20A

1 m

2 m

3 m

5 m

 

0.75mm2

1.0mm2

1.5mm2

2.5mm2

B-6A

23 m

29 m

40 m

64 m

B-10A

13 m

16 m

24 m

38 m

B-13A

11 m

13 m

20 m

32 m

B-16A

8 m

12 m

18 m

26 m

B-20A

4 m

6 m

9 m

15 m

B-25A

3 m

4 m

7 m

9 m

B-32A

3 m

4 m

5 m

7 m

*) Don’t forget to consider twice the distance to the load (or cable length) when calculating the total wire length (+ and – wire).