Siemens S5-135U/155U - Manual (Page 34)

3-8
System Manual
C79000-G8576-C199-06
3.2 Installation of Programmable Controllers for EMC
Measures for suppressing interference voltages are often applied only when
the control system is already operational and proper reception of a useful
signal is impaired. The reason for such interference is usually inadequate
reference potentials caused by mistakes in equipment assembly. Described in
the following sections are:
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Basic rules for grounding the inactive metal parts
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Examples of cabinet assembly for EMC
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Example of rack and wall mounting for EMC
3.2.1 Basic Rules for Assembling and Grounding the Inactive Metal
Parts
Ensure wide-area chassis grounding of the inactive metal parts when
mounting the equipment. Properly implemented grounding creates a uniform
reference potential for the control system, and reduces the effects of
picked-up interference.
Chassis grounding is understood to mean the electrical connection of all
inactive parts. The entirety of all interconnected inactive parts is the chassis
ground.
Inactive parts are conductive parts which are electrically isolated from active
parts by basic insulation, and can only develop a voltage in the event of a
fault.
The chassis ground must not develop a dangerous touch voltage, even in
the event of a fault. The ground must therefore be connected to the protective
ground conductor. To prevent ground loops, locally separated ground
elements such as cabinets, structural and machine parts, must always be
connected to the protective ground system in star configuration.
Ensure the following when chassis grounding:
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Connect the inactive metal parts with the same degree of care as the
active parts.
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Ensure low-impedance metal-to-metal connections, e.g. with large-area
good quality contact.
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When you are incorporating painted or anodized metal parts in the
grounding, these insulating protective layers must be penetrated. Use
special contact washers or remove the insulating layer.
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Protect the connection points from corrosion, e.g. with grease.
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Movable grounded parts such as cabinet doors must be connected via
flexible grounding strips. The grounding strips should be short and have a
large surface because the surface is decisive in providing a path to ground
for high-frequency interference.
Installation Guidelines