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DC-UPS - 24V, 20 A, 480 W Reference Manual
Figure 28 - Test Circuit
MCB
+
DC-UPS
Output
Wire length
-
S1... Fault simulation switch
34
External Input Protection
The DC-UPS is tested and approved for branch circuits up to 28 A. An external
protection is only required, if the supplying branch has an ampacity greater
than this. If an external fuse is necessary or utilized, minimum requirements
must be considered to avoid nuisance tripping of the circuit breaker. A
minimum value of 25 A B- or C-Characteristic breaker should be used. Check
also local codes and local requirements. In some countries local regulations
might apply.
Output Circuit Breakers
Some applications require branch circuit or branch circuit conductor
protection on the 24V supply voltage. Therefore standard miniature circuit
breakers (MCBs or UL 1077 circuit breakers) are commonly used on 24V
branches.
MCBs 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 undervoltage situations in adjacent 24V branches
that are supplied by the same source, a fast (magnetic) tripping of the MCB is
desired. A quick shutdown within 10 ms is necessary corresponding roughly to
the ride-through time of PLCs. 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 data sheet of the power supply.
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 (setting for buffer voltage:
22.5V).
Maximal wire length
2 A
Load
C-3A
+
C-4A
C-6A
S1
-
C-10A
C-13A
B-6A
B-10A
B-13A
B-16A
(1) Consider twice the distance to the load (or cable length) when calculating the total wire length (+ and – wire).
Rockwell Automation Publication 1606-RM008A-EN-P - November 2021
(1)
for a fast (magnetic) tripping:
0.75 mm²
1.0 mm²
25 m
34 m
24 m
32 m
18 m
24 m
10 m
12 m
5 m
6 m
1 m
2 m
23 m
26 m
10 m
14 m
9 m
10 m
3 m
4 m
1.5 mm²
2.5 mm²
48 m
72 m
47 m
68 m
33 m
48 m
18 m
28 m
9 m
13 m
2 m
3 m
32 m
46 m
20 m
31 m
18 m
27 m
9 m
13 m
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