User Manual

42/118
Siemens TX-I/O Engineering and installation guide CM110562en_13
Building Technologies Wiring 2019-05-15
6.4.1 Wiring for AC 24 V
Cable materials
The requirements for the 24 V~ and ⊥ conductors of the AC 24 V supply are as
follows:
– Stranded or solid wire (copper)
– Singly or in two-core cable
Maximum cable lengths
For details refer to Section 10.2.
Separate transformer for
each I/O island
Recommendation: A separate transformer is recommended for each I/O island.
However, if the devices are installed in the same control panel, then the bus masters
and associated I/O rows may be supplied from the same transformer.
Separate supply cables
for AC 24 V
("star" configuration)
Separate wiring to the bus master and each I/O row from the connection terminal strip
(⊥, 24V~) in order to keep voltage drop small. This terminal strip should be installed
close to the transformer (see "Wiring examples").
Connecting
⊥
of separate
systems
The system neutral conductors (⊥) of separate systems must not be interconnected.
This is to prevent earth loops.
Examples:
– 0 … 10 V converters or current loops (do not earth the local supply transformers!)
– PCs (PCs are generally earthed  use a notebook PC for tools! )
Separate power supply
for bus masters and I/O
islands
and
Earthing of
⊥
PELV systems must be connected to earth at one point only for each I/O island.
Usually they are earthed at the transformer.
A PXC3... room automation station and the island bus are electrically connected
(⊥). Within one I/O island, it is permissible to supply power separately to the PXC3...
and the associated I/O rows by use of one transformer each.
However, the system neutral terminals (⊥) of the two transformers must be inter-
connected, because ⊥ is used as the common return.
An automation station with island bus connectors and the I/O island are
electrically isolated (the island driver of the automation station is supplied by the I/O
island ).
Separate supply of the automation station and the adjacent I/O row by separate
transformers is admissible. System neutral (⊥) of the two transformers may be
connected or isolated.
P-bus and island bus are electrically connected (G0 and ⊥).
Within one I/O island, it is permissible to supply power separately to the P-bus BIM
and the associated I/O rows by use of one transformer each.
However, the system neutral conductors (⊥) of the two transformers must be
interconnected, because ⊥ is used as the common return.
PROFINET and island bus are electrically connected.
Within one I/O island, it is permissible to supply power separately to the BIM and the
associated I/O rows by use of one transformer each.
However, the system neutral conductors (⊥) of the two transformers must be
interconnected, because ⊥ is used as the common return.
Island bus expansion:
The island bus and island bus expansion are electrically isolated (via a PTC
protective resistor).
For most cases, it makes sense to separate power to local sub-island and remote
sub-islands.
You can also power the remote sub-island from the local sub-island when it requires
very little electricity (does not power field devices).