Siemens S7-300 Installation Manual page 193

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Quite often it is the case that interference suppression measures are not taken until
corruption of user signals is detected after the controller is actually in operation.
Frequently, the causes of such interference are found in inadequate reference
potentials as a result of faulty installation. This section shows you how to avoid
such errors.
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Inactive parts are referred to as electrically conductive elements, separated from
active elements by a basic insulating and only subject to electrical potential if an
error occurs.
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Bond all inactive metal parts to a large-surface ground when you install the S7-300.
Proper ground bonding ensures a homogeneous reference potential for the
controller and reduces the effect of interference coupling.
The ground connection establishes an electrically conductive interconnection of all
inactive parts. The sum of all interconnected inactive parts is referred to as chassis
ground.
This chassis ground must never develop a hazardous potential even if a fault
occurs. Therefore, chassis ground must be connected to the protective conductor
using cables with an adequate conductor cross-section. To avoid ground loops,
physically separate chassis ground elements (cabinets, parts of the building
construction or machine) must be bonded to the protective conductor system in a
star circuit.
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• In the same way as with active elements, exercise meticulous care to
interconnect inactive metal elements.
• Always make sure that you have a low-impedance interconnection between
metal elements (e.g. large and highly conductive contact surface).
• The protective insulating finish on varnished or anodized metal elements must
be pierced or removed. Use special contact washers or completely remove the
finish on the point of contact.
• Protect your connecting elements against corrosion (e.g. with a suitable grease)
• Interconnect moving chassis ground elements (e.g. cabinet doors) with flexible
ground straps. Always use short ground straps with a large surface (the surface
is decisive for the diversion of high-frequency currents).
S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
A5E00203919-01
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