Siemens SIMATIC S5 Manual page 80
Distributed i/o system
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Routing cables; connecting and installing bus connectors
Installation and
grounding of
inactive metal
components
5-12
When the devices are installed, it is important to ensure that the inactive
metal components are correctly grounded with connectors having large
contact areas. Correct grounding establishes a uniform reference potential for
the control system and reduces the effects on spurious interference.
Grounding means the conductive connection of all inactive metal
components. The entirety of all interconnected inactive components is known
as the machine ground.
Inactive metal components are all conductive parts electrically isolated from
active parts by basic insulation at least and which can become charged only if
a fault occurs.
Even if a fault occurs, the ground may under no circumstances carry
voltage that could cause injury in the event of accidental contact. The
ground must, therefore, be connected to the protective conductor. In order to
avoid ground loops, a star pattern must be adopted when grounded objects
(cabinets, parts of structures and machines) not immediately adjacent to each
other are connected to the protective conductor system.
Note the following:
1. When connecting the inactive metal parts, exercise the same meticulous
care and attention as with active parts.
2. Make sure that metal-to-metal connections are low-ohmic, i.e. use
connectors that are efficiently conductive and which have large contact
areas.
3. If painted or anodized metal parts are included in grounding, these
insulating protective surface coatings must be penetrated. Use special
contact washers or remove the surface coatings down to bare metal.
4. Protect the assembled connections against corrosion, e.g. by applying a
coat of grease.
5. Use flexible grounding straps to connect moving grounded parts (e.g.
doors of cabinets). The grounding straps should be short and have a large
surface area, because the surface area is critical for the removal of
high-frequency interference.
ET 200 Distributed I/O System
EWA 4NEB 780 6000-02a
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