Ac 24 V Transformer Sizing - Siemens Desigo TRA Engineering, Mounting And Installation Manual

Communications
Note!
STOP
Operating voltage
Specification
for AC 24 V
transformers
Power consumption
PXC3
Terminals see page 27
Siemens
Building Technologies
If USB communications do not work, this is an indicator that the AC 24 V operating
voltage is incorrectly wired (conductors V~ and
6.3.3

AC 24 V Transformer sizing

The engineering responsibility includes transformer sizing.
The selected transformer rating is based on the total power consumption of the
automation station, the I/O modules and the connected field devices.
The operating voltage is AC 24 V. It must comply with SELV or PELV to
HD 60364-4-41 (2007-01-01) requirements.
NEC: Class 2 transformers or Class 2 circuits.
The acceptable deviation of the AC 24 V nominal voltage connected to the
transformer is +20%/–10%. This means that after taking account of the cable
and contact resistances, a tolerance of +/- 20 % can be guaranteed for the field
devices.
IEC: Double-insulated safety transformers to EN 61558, designed for continuous
operation, to supply SELV or PELV circuits.
NEC: Class 2 transformers.
The rated transformer output must be at least 25 VA. In smaller transformers the
ratio of no-load voltage to full-load voltage is unfavorable (> +20 %).
For reasons of power efficiency the rated transformer output should not exceed
200 % of the maximum load.
Max. permissible input current AC 24 V
(through terminals 5 and 6)
Base load (without loading by modules and field
devices)
Island bus supply *)
DC 24 V / max. 600 mA
KNX PL-Link supply *) **)
DC 29 V / max. 160 mA
DALI supply *)
DC 16 V / max. 128 mA
*) The bus supplies can be switched off via tool if not used.
**) The KNX PL-Link supply MUST be switched off via tool if an external bus
supply is used.
Supply for field devices (Transit power AC 24 V )
TX-IO: AC 24V (island bus)
KNX PL-Link: AC 24V (terminals 3 and 4)
AC 24 V (terminals 7 and 8, for additional
AC 24 V consumers)
Desigo TRA Engineering, mounting and installation
Wiring of power lines
inverted).
Total max. 10 A
(Ext. fusing compulsory: max.
T 10 A melting fuse or
max.C 10 A circuit breaker)
8 VA / 0.33 A
30 VA / 1.25 A
12 VA / 0.50 A
9 VA / 0.37 A
Max. 144 VA / 6 A
Max. 48 VA / 2 A
Max. 144 VA / 6 A
(only if the sum of 10 A at termi-
nals 5 and 6 is not exceeded)
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