Siemens Simatic S7-300 CPU 31xC Series Installation And Operating Instructions Manual page 212

Appendix
A.1 General rules and regulations for S7-300 operation
Mains voltage
The table below shows what you have to watch with respect to the mains voltage.
Table A-2
In the case of ...
Stationary systems or systems without all-pole
mains disconnect switch
Load power supplies, power supply modules
All circuits of the S7-300
24 VDC power supply
The table below shows what you must observe for the 24 VDC power supply.
Table A-3
In the case of ...
Buildings
24 VDC power supply cables, signal
cables
24 VDC power supply
Protection against external electrical interference
The table below shows how you must protect your system against electrical interference or
faults.
Table A-4
In the case of ...
All plants or system in which the S7-300 is
installed
Supply / signal / bus cables
Signal and bus cables
A-2
Mains voltage
Protection against external electrical interference
Protection against external electrical interference
the ...
building installation must contain a mains
disconnect switch or a fuse.
set rated voltage range must correspond to local
mains voltage.
rated mains voltage fluctuation / deviation must
lie within the permitted tolerance (refer to
Technical Data of S7-300 modules).
you need to observe ...
external lightning protection
internal lightning protection
safe (electrical) extra-low voltage isolation
Make sure that ...
the plant or system is connected to a protective
conductor for the discharge of electromagnetic
interference.
the cable routing and installation is correct.
a cable/conductor break does not cause
undefined plant or system states.
S7-300, CPU 31xC and CPU 31x: Installation
Operating Instructions, Edition 08/2004, A5E00105492-05
Install lightning protection
(e.g. lightning conductors).