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 24V branches.

MCBs are designed to protect wires and circuits. If the amperage and characteristics of the MCB are matched to the wire size being used, the wiring is considered thermally safe whether or not the MCB opens.

To avoid voltage dips and undervoltage situations in adjacent 24V branches supplied by the same source, fast (magnetic) tripping of the MCB is desired. A fast shutdown within 10ms is required, which is approximately the cycle time of PLCs. This requires power supplies with high current reserves and large output capacitors. In addition, the impedance of the faulty branch must be low enough for the current to actually flow. The best current reserve in the power supply will not help if Ohm's law does not allow current to flow. The following table provides typical test results showing which MCBs with B and C characteristics magnetically trip depending on wire size and 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).

Test results for maximum wire length: