Siemens SIMATIC S5 Manual page 72

Routing cables; connecting and installing bus connectors
24 V DC supply
Protection against
external electrical
effects
5-4
The points to bear in mind with regard to the 24 V supply are as follows:
With regard to ...
Buildings
24 V DC supply lines,
signal lines
24 V supply
The points of importance with regard to protection against external electrical
effects and faults are shown below.
With regard to ...
all facilities or systems
incorporating the ET 200
connecting lines and signal lines
signal lines
it is important to ensure ...
exterior lightning
protection
interior lightning
protection
dependable electrical isolation of low voltage
it is important to ensure ...
that the facility or system is connected to
the protective conductor so that
destructive influences are diverted.
that routing and installation are correct.
that line or conductor breakage cannot
cause the facility or system to assume an
undefined state.
ET 200 Distributed I/O System
provide adequate
lightning protection
(see Appendix D) ( A di D)
EWA 4NEB 780 6000-02a