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Siemens TX-I/O Engineering and installation guide CM110562en_13
Building Technologies Safety guidelines 2019-05-15
2.3 System-specific regulations
Electrical safety in the building automation and control systems supplied by Siemens
Building Technologies is essentially based on the use of extra low voltage which is
strictly segregated from the mains voltage.
Depending on the earthing arrangements, this low voltage is either SELV or PELV in
accordance with HD 384 "Electrical installations in buildings":
Unearthed = SELV (Safety Extra Low Voltage)
Earthed = PELV (Protection by Extra Low Voltage)
Mixing of SELV and PELV in a system is not permitted.
Equipment safety is ensured by various strategies, including:
– Low voltage AC 24 V supply, either SELV or PELV
– Double insulation between AC 230 V mains voltage and SELV/PELV circuits
– Fine-wire fuse in the power-supplying units (P-Bus BIM, power supply module, bus
connection module)
– Back-up fuses for the power-supplying units (P-Bus BIM, power supply module, bus
connection module).
Always observe the specific regulations for the electrical wiring of the TX-I/O modules,
as described in the following sections.
Note the following in relation to the earthing of the reference point (system neutral
(symbol
⊥ ) :
• In principle, it is permissible for the reference point, ⊥ of the AC 24 V operating
voltage to be either earthed (PELV) or unearthed (SELV). This decision is
determined by local regulations and conventions.
• There may also be practical reasons which make earthing either compulsory or
inadmissible.
• AC 24 V systems should generally be earthed (PELV) unless otherwise specified.
• To avoid earth loops, ensure that systems with PELV are earthed at one point in
the system only, usually at the transformer, unless otherwise specified.
The following regulations apply to mains and operating voltage:
Item
Regulation
AC 230 V
mains voltage
• Installation only by skilled professionals
• Installation only when mains supply is disconnected
AC 230 V
mains voltage
protection
• Transformers, primary winding:
Control panel fuse (control fuse)
• The mains voltage routed to the TX-I/O modules (supply
cable for relay contacts) must be fused:
– Max. 10 A (slow blow)
– Max. 13 A (miniature circuit breaker)
Operating voltage
The operating voltage is AC 24 V.
It must comply with SELV or PELV requirements to HD 384.
Specification
for AC 24 V
transformers
Double-insulated safety transformers to EN 61558, designed
for continuous operation to supply SELV or PELV circuits.
Safety
SELV and PELV
Equipment safety
Earthing of
system neutral
Recommendation: PELV
Mains and
operating voltage