Siemens S7-400 Installation of Programmable Controllers for EMC, Inactive Metal Parts

Assembling and Installing Systems

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	A-9
A5E00850741-01