Prevention Of Interference Sources - Siemens SIMATIC MOBY D System Manual

Planning the MOBY D system
4.5 EMC Guidelines
4.5.6

Prevention of interference sources

A high level of immunity to interference can be achieved by avoiding interference sources.
All switched inductances are frequent sources of interference in plants.
Suppression of inductance
Relays, contactors, etc. generate interference voltages and must therefore be suppressed
using one of the circuits below.
Even with small relays, interference voltages of up to 800 V occur on 24 V coils, and
interference voltages of several kV occur on 230 V coils when the coil is switched. The use
of freewheeling diodes or RC circuits prevents interference voltages and thus stray
interference on conductors installed parallel to the coil conductor.
Figure 4-27
Note
All coils in the cabinet should be suppressed. The valves and motor brakes are frequently
forgotten. Fluorescent lamps in the control cabinet should be tested in particular.
78
Suppression of inductance
System Manual, 01/2010, J31069-D0147-A6-7618
MOBY D