Siemens SIMATIC S7-300 Hardware And Installation Manual page 210

Appendix
Observe for ground connection:
• In the same way as with active elements, exercize 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).
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S7-300 Programmable Controller Hardware and Installation
A5E00105492-02