IT112 Video Terminal

3.
Laying the
cables
Cable
shielding
Earthing of
shields and
electronic
circuits.
Switch-over of
the capacitive
loads
Chapter 3
General information regarding
electromagnetic compatibility
General information regarding
electromagnetic compatibility
Electronic appliances are more frequently used in the
command and control systems. The programmable controllers
(e.g. the PLC s), the man/machine interface systems (e.g.
terminals), control systems (e.g. diagnostic terminals),
interfacing elements (e.g. interfacing boards) and drives (e.g.
the inverters) all belong the this category. Classic
electro-mechanical appliances such as contactors, solenoid
valves and motors etc. are also mounted with these types of
electronic appliances.
Electric interference caused by the functioning of these
appliances can jeopardise the correct functioning and the life
span of the electronic appliances present in the control board
or in the plant. To allow the correct functioning of the electric
and electronic appliances the presence of interference must be
reduced.
Remember to separate the measurement, control and
communication cables from the power cables. Power cables
laid near to and parallel to the communication cables cause
coupling voltages that interfere with or destroy the electronic
components.
For connection of the communication signals, it is necessary to
use suitable shielded cables (total shielding is recommended).
The shielding must be connected to the earth potential.
In many appliances the "0V" is connected to bulk. The bulk
must be connected to earth, but it is a good idea to separate
the bulk of the shields and electronic circuits from that of the
power. Remember that the earth can only perform its function
if the "Resistance of the earth circuit" is within the maximum
limits set by the provisions.
The current peaks that occur on insertion of the capacitive loads
can damage or destroy the control elements. Moreover, the high
frequency component of the current peak can lead to serious
interference of the electronic appliances, caused by inductive
coupling of the connection cables.
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ESAPOLYMATH -HardwareManual- rev. 2.2