Siemens S5-135U/155U System Manual page 33

Employ special EMC measures for particular applications (see
Section 3.3.4).
Create a standard reference potential; ground all electrical apparatus if
possible (see Sections 3.4 and 3.5).
System Manual
C79000-G8576-C199-06
Fit quenching elements to all inductances which are not controlled by
SIMATIC S5 modules.
Use incandescent bulbs for illuminating cabinets, and avoid fluorescent
lamps.
Use specific grounding measures. Grounding of the control system is a
protective and functional measure.
System parts and cabinets with central controllers and expansion units
should be connected to the ground/protective conductor system in star
configuration. This serves to avoid the creation of ground loops.
In the case of potential differences between system parts and cabinets,
install equipotential bonding conductors of sufficient rating.
Installation Guidelines
3-7