User Manual

15-6
Siemens Switzerland Ltd www.siemens.com/gamma 2019
Building Technologies Division
Technical Information and Application Examples
Technical Information
System overview
Fitting power supplies for every KNX system
Each bus line needs its own power supply unit. The power supply unit
provides the system power necessary for the instabus KNX.
The KNX system provides for decentralized and central power supply
units. Central power supply units are installed as DIN rail mounted
devices in distribution boards and control cabinets, while decentralized
power supply units are designed for installation in junction boxes, in
parapet channels or in room control boxes.
Central power supply units provide 160 mA, 320 mA or 640 mA bus
current. Maximum up to two central power supply units may be
attached to a single bus line. A second unit is not required unless the
supply voltage at a bus device is less than 21 V.
When more than 30 bus devices are installed in short bus cable
distance (e.g. 10 m), e.g. in distribution boards, the power supply unit
should be arranged near these bus devices. The distance between
power supply unit and any of its bus devices must not exceed 350 m.
A decentralized power supply provides 80 mA bus current. This allows
for decentralized solutions for self-sufficient control of a single room
or, by integration of several room control islands, of a floor or even a
complete building. Up to eight decentralized power supply units may
be operated in parallel, such that a complete KNX bus line can be setup
with e.g. eight room control boxes.
When several bus devices are installed in short bus cable distance (e.g.
10 m), e.g. in distribution boards, or in a room control box AP 641,
the power supply units shall be arranged near these bus devices. The
distance along the bus wire between any bus device and the closest
power supply unit must not exceed 350 m. If only the decentralized
power supply RL 125/23 is used, then the maximum KNX cable length
in a bus line is 350 m for one, 700 m for two, and 1000 m for 3 or
more decentralized power supplies RL 125/23.
In principle, central and decentralized power supply units can be
operated in parallel with each other. Consideration must be taken re-
garding the sum of the short circuit currents of the power supply units,
which must be lower than 3 amperes.
The following table shows the respective short circuit current:
Material number Type
Short circuit
current
Bus
current
5WG1125-4AB23 RL 125/23 < 0.2 A 80 mA
5WG1125-1AB02 N 125/02 < 1.0 A 160 mA
5WG1125-1AB12 N 125/12 < 1.0 A 320 mA
5WG1125-1AB22 N 125/22 < 1.5 A 640 mA
With eight decentralized power supply units RL 125/23 operated in
parallel the maximum short circuit current is 1.6 A.
Additionally, it is possible to operate a power supply unit N 125/02 or
N 125/12 in parallel to eight RL 125/23. Only with the power supply
unit N 125/22 observe that it has a short circuit current of 1.5 A, which
is why only seven decentralized power supply units can be operated in
parallel.
To ensure an uninterrupted power supply a separate circuit with safety
separation should be used for the power supply unit N 125/x2 power
supply line.
The power supply units N 125/x2 can supply DC 24 V power from an
additional pair of terminals (yellow-white). This DC 24 V output volta-
ge can be used to power e.g. an additional line via a separate choke
N 120.
All power supply units N 125/x2, RL 125/23 and JB 125C23 can be
powered by AC 120…230 V or by DC 220 V.
A minimum cable length is not required between these power supply
units from Siemens.
230 V AC
Or
230 V AC
KNX
I201_19146b
80 mA
80 mA
80 mA
80 mA
80 mA
80 mA
80 mA
80 mA
KNX
80 mA
80 mA
80 mA
80 mA
80 mA
80 mA
80 mA
160 ...
640 mA
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