Jun. 2020 / Rev.0.1 DS-UB40.241-EN All values are typical figures specified at 24Vdc input voltage, 40A output current in
power supply mode at 25°C ambient, no charging and after a 5 minutes run-in time unless otherwise noted.
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23.1. OUTPUT CIRCUIT BREAKERS,
Some applications require branch circuit or branch circuit conductor protection on the 24V supply voltage. Therefore, standard
miniature circuit breakers (MCB’s or UL 1077 circuit breakers) are commonly used on 24V 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 high peak currents to open the circuit breaker in the required time. Furthermore, the impedance of the faulty branch must be
sufficiently small in order for the current to actually flow.
For this discipline, two cases must be differentiated:
A) DC-UPS is operating in power supply mode:
In this case, the supplying source is responsible for the peak current to trip circuit breakers. These values can be found in the
datasheet of the power supply and must be slightly reduced due to losses in the DC-UPS.
B) DC-UPS is operating in battery mode:
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.
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