A.3 Installation Of Programmable Controllers For Emc - Siemens SIMATIC S7-400 Installation Manual

A.3 Installation of Programmable Controllers for EMC
Introduction
Measures for suppressing interference are often only applied when the control
system is already operational, and the proper reception of a useful signal is found
to be impaired.
The cause of such interference is often due to insufficient reference potentials
which can be attributed to errors during assembly. This section tells you how to
avoid such errors.
Inactive Metal Parts
Inactive parts are all the conductive parts which are electrically isolated from active
parts by basic insulation, and can only develop a potential in the event of a fault.
Grounding of Inactive Metal Parts During Installation
When installing the S7-400, ensure large-area grounding of all inactive metal parts.
Properly implemented chassis grounding creates a standard reference potential for
the control system, and reduces the effects of picked-up interference.
The chassis ground provides the electrical interconnection between all inactive
parts. The entirety of all interconnected inactive parts is known as the chassis
ground.
Even in the event of a fault, the chassis ground must not develop a dangerous
touch potential. It must therefore be connected to the protective ground conductor
via an adequate conductor cross-section. 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:
• Connect the inactive metal parts with the same degree of care as the active
parts.
• Ensure low-impedance connections between metal parts, for example, with
large-area good quality contact.
• With painted or anodized metal parts, the insulating protective layer must be
penetrated or removed at the contact point. Use special contact washers or
scratch the layer off fully at the contact point.
• Protect the connection points from corrosion, for example, with suitable grease.
• Use flexible grounding strips to connect movable grounded parts such as
cabinet doors. 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.
Automation System S7-400 Hardware and Installation
A5E00850741-01
Assembling and Installing Systems
A-9