Siemens Desigo TRA Engineering, Mounting And Installation Manual page 11

Grounding of
(System neutral AC 24 V)
Grounding of
AC 230 V
Mains and operating voltage
Operating voltage AC 24 V It must meet requirements for SELV or PELV. Permitted deviation for nominal
Transformer specification
AC 24 V
Operational voltage fuse
AC 24 V
Mains fuse
Operating voltage
AC 230 V
Caution with
regard to foreign
voltages!
Siemens
Building Technologies
Observe the following points when grounding
As a principle, operating voltage of AC 24 V is permitted in principle for both
grounded as well as non-grounded system neutral. Local regulations and
customers apply accordingly.
Grounding may be required or not allowed for functional reasons.
Recommendation: AC 24 V systems are grounded unless disadvised by the
manufacturer.
In order to avoid ground loops, connect systems with PELV to the ground at one
location only (especially for transformers), if no other indication exists.
AC 230 V devices and AC 230 V field devices must not be grounded.
voltage
At the transformer:
At the room automation station:
At the field devices:
Do not interchange the conductors V~ and
bus and USB communications will not work properly.
IEC: Use safety insulating transformers as per IEC 61558 with double
insulation designed for 100% duty to supply SELV or PELV circuits.
USA: Class 2 circuits to UL 5085-3.
Power taken from the transformer should be at least 50% of nominal load for
efficiency reasons (effectiveness).
Transformer nominal power should be at least 25 VA. For smaller transformers,
the ratio of open circuit voltage to full load is unfavorable
(> + 20%).
Transformers on the secondary side correspond to the actual load of all
connected devices as per transformer sizing:
AC 24 V line (system potential) must always be fused.
Where required, also line
Transformer, primary side:
Installation box fuse (control circuit fuse).
For devices with AC 230 V supply:
The sizing and fuse protection of the supply cables depends on the total load
and on local regulations.
Supply cables must be secured with cable restraints on the room automation
station.
Any insertion or drawing of dangerous voltages to the system's low-voltage circuit,
e.g. caused by improper wiring directly places people at risk and may result in the
partial or complete destruction of the building automation and control system!
Desigo TRA Engineering, mounting and installation
Important information on safety
AC 24 V (system neutral):
AC 24 V -10 - + 20%.
AC 24 V -12 - + 20%.
AC 24 V -20 - + 20%.
(system neutral), otherwise island
(system neutral).
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CM111043en_09
2016-02-22