Output circuit breakers

Standard miniature circuit breakers (MCBs) are commonly used for AC supply systems but can also be used in DC branches if specified.

Such MCBs are designed to protect wires and circuits. When the ampere rating and characteristics of the MCB are matched to the wire size used, the wiring is considered thermally safe whether the MCB opens or not.

To avoid voltage dips and undervoltage situations in adjacent DC branches fed by the same source, fast (magnetic) tripping of the MCB is desirable. For example, a fast trip within 10ms avoids a shutdown and restart of the PLC. For this purpose, power supplies with high current reserves and large output capacitors are advantageous. In addition, the impedance of the faulty circuit must be sufficiently small so that the current can actually flow. The best current reserves of a power supply are of no help if Ohm's law does not allow the current to flow.

The following table provides typical test results showing which MCBs magnetically trip as a function of wire size and length. The lengths given correspond to the total length, which is the sum of the outgoing (+) and return (-) wire. The tests were performed without additional load.

Test results for maximum wire length: