Output circuit breakers

Standard miniature circuit breakers (MCB's or UL 1077 circuit breakers) are commonly used on AC power systems and may also be used on 24 V 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 24 V branches which are supplied by the same source, a fast (magnetic) tripping of the MCB is desired. Fast shutdown within 10 ms is required, which approximately corresponds to the hold-up time of PLCs. For this purpose, power supplies with high current reserves and large output capacitors are advantageous. Furthermore, the impedance of the faulty branch must be sufficiently small in order for the current to actually 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.

The following test results indicate the maximum wire length for magnetic (fast) tripping. The wire length is always two times the distance to the load (+ and - wires).

Output circuit breakers 1:
Fig.19: Test circuit for maximum wire length

Test results for maximum wire length:

 

0.75 mm2

1.0 mm2

1.5 mm2

2.5 mm2

C-2A

23 m

25 m

41 m

71 m

C-3A

13 m

15 m

23 m

33 m

C-4A

4 m

6 m

8 m

13 m

C-6A

1 m

2 m

2 m

5 m

 

0.75 mm2

1.0 mm2

1.5 mm2

2.5 mm2

B-6A

8 m

10 m

14 m

23 m

B-10A

1 m

2 m

2 m

3 m

B-13A

 

1 m

2 m

3 m