23
13.2.4 General information on line routing
– Inside and outside of cabinets –
In order to achieve EMC-compliant routing of the lines, it is advisable to split the lines into the following line groups and to lay
these groups separately.
>
Group A:
• shielded bus and data lines (e. g. for RS-232C and printers etc.)
• shielded analogue lines
• unshielded lines for DC voltages
≥
60 V
• unshielded lines for AC voltage
≥
25 V
• coaxial lines for monitors
>
Group B:
• unshielded lines for DC voltages
≥
60 V and
≥
400 V
• unshielded lines for AC voltage
≥
24 V and
≥
400 V
>
Group C:
• unshielded lines for DC voltages > 400 V
The table below allows you to read off the conditions for laying the line groups on the basis of the combination of the individual
groups.
Group A
Group B
Group C
Group A
1
2
3
Group B
2
1
3
Group C
3
3
1
Table 3: Line laying instructions as a function of the combination of line groups
1) Lines may be laid in common bunches or cable ducts.
2) Lines must be laid in separate bunches or cable ducts (without minimum clearance).
3) Lines must be laid in separate bunches or cable ducts inside cabinets but on separate cable racks with at least 10 cm
clearance outside of cabinets but inside buildings.
13.2.4.1 Shielding of lines
Shielding is intended to weaken (attenuate) magnetic, electrical or electromagnetic interference fields.
Interference currents on cable shields are discharged to earth via the shielding bus which is connected conductively to the chassis
or housing. A low-impedance connection to the PE wire is particularly important in order to prevent these interference currents
themselves becoming an interference source.
Wherever possible, use only lines with braided shield. The coverage density of the shield should exceed 80 %. Avoid lines with foil
shield since the foil can be damaged very easily as the result of tensile and compressive stress on attachment. The consequence
is a reduction in the shielding effect.
In general, you should always connect the shields of cables at both ends. The only way of achieving good interference suppres-
sion in the higher frequency band is by connecting the shields at both ends.
The shield may also be connected at one end only in exceptional cases. However, this then achieves only an attenuation of the
lower frequencies. Connecting the shield at one end may be more favorable if
•
it is not possible to lay an equipotential bonding line
•
analogue signals (a few mV resp. mA) are to be transmitted
•
foil shields (static shields) are used.
In the case of data lines for serial couplings, always use metallic or metallized plugs and connectors. Attach the shield of the data
line to the plug or connector housing.
If there are potential differences between the earthing points, a compensating current may flow via the shield connected at both
ends. In this case, you should lay an additional equipotential bonding line.
Please note the following points when shielding:
•
Use metal cable clips to secure the shield braiding. The clips must surround the shield over a large area and must have good
contact.
•
Downstream of the entry point of the line into the cabinet, connect the shield to a shielding bus. Continue the shield as far as
the module, but do not connect it again at this point!
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