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CPU and CP power supply problem

Created by: Reymond at: 6/25/2010 9:57 PM (3 Replies)

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6/25/2010 9:57 PM	Rate (0)
Reymond Posts: 112 Rating: (0)	Dear Colleagues, Good day, I just want to ask if there is a proven or theoritical explanation if there will be problem or not if the CPU and CP that were connected on the same rack gets power supply on different 24Vdc power supply? Thank you very much in advance.... Best regards, Rey
Reymond Posts: 112 Rating: (0)
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6/25/2010 10:46 PM	Rate (1)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 9/13/2024 Posts: 12282 Rating: (2683)	Hwllo Begginer; The main issue here would be ground loops, and the proposed solution from Siemens is "equipotential bonding" (also known as "clean Earthing"), See the recommendations in the Installation Manual for S7-300 hardware, and follow the rules: note that equipotential bonding is very different than signal shielding. http://support.automation.siemens.com/WW/view/en/13008499 Manual: S7-300, CPU 31xC and CPU 31x: Installation A.2 Protection against electromagnetic interference A.2.7 Equipotential bonding Potential differences Potential differences can occur between separate system elements. This can result in high equipotential currents, e.g. if the cable shielding is terminated at both ends and grounded todifferent system components.The cause of potential difference can be differences in the power supplies. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 6/25/2010 10:49 PM
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6/28/2010 8:10 PM	Rate (0)
Reymond Posts: 112 Rating: (0)	Dear Danielle, Thank you very much, Is connecting the common or negative of two cpu enough? I also have additional question because the set-up is we have also local and remote IO, the power supply of et-200M and signal modules are connected also to different power supply from where the CPU is connected. Is this set-up can cause damaged to modules? Thank you very much in advance Best regards, Rey
Reymond Posts: 112 Rating: (0)
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6/28/2010 8:37 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 9/13/2024 Posts: 12282 Rating: (2683)	Hello Rey; Connecting the negative pole of two CPUs is not enough, by itself, for equipotential bonding. In my case (in my latest projects), we were lucky, as we were building the plants from the ground up. So we could implement a clean Earth wiring system, or instrumentation ground, with spikes driven into the ground at regular intervals and low-resistance copper wire.We could then connect all our PLC chassis and plantinstruments to a common clean Earth. This is of course a lot harder to do in an existing plant, especially old ones. Please study the document proposed earlier, especially appendix 2, for specfications on bonding CPus and remote I/O stations, it has very good recommendations on EMC and grounding issues. Also do a search on "equipotential bonding" in the product support" link at the top of this page. Tthere are many other atricles that should help you design your protective wiring. Hope this helps, Daniel Chartier

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