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SIMATIC S7-300 / S7-300F

CPU grounding

Thread Starter: Bofely   Started: 8/29/2012 9:39 PM   Replies: 8

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  8/29/2012, 9:39 PM
Bofely
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Hello,

What kind of problems could I expect when I leave my CPU ungrounded. 
If it matters, it's a S7-312C.
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  8/30/2012, 8:08 AM
Aret
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31xC is grounded always - you can't break connection between CPU and backside metal plate.
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  8/30/2012, 8:43 AM
Bofely
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Well we did. With a tape blush

So my question remain.
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  8/30/2012, 10:06 AM
Aret
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Can you place wiring scheme of CPU, PS and integrated modules?
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  8/30/2012, 11:25 AM
Bofely
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The schematic is not my work, so I don't want to upload it.

I know that the CPU is used in a unearthed DC control circuits (IT systems).
This mean that a CPU 31xC is not suitable, right?

I should use a CPU with ungrounded reference potential (CPU with grounding slide contact).
This way a RC network is in between the protective conductor and the rail. My insulation resistance should then be the R (of the RC network) value then? 

I also want to know what problem I could expect when using a Siemens S7-300 CPU ungrounded. Like in my case. CPU312C-tape(isolation)-rail . 
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  8/30/2012, 12:24 PM
Aret
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Ok,, just to make things clear:

You have grounded cabinet. Inside you have isolated power supply. "-" of power supply is NOT connected to any grounded components. + and - are connected to CPU clams (2 pin connector) as well as to AI module clamps (via 20/40 pin front connector). Sensors you connect to AI are isolated.

This is your situation and you want to know if there anything wrong with this design?

Alos, clarify what for you use RC-chain?
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  8/30/2012, 1:29 PM
Bofely
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The RC-chain is a part of the S7-300 PLC (not CPU 31xC), see attachment. Using the RC-chain may be useful in some case of ground-fault monitoring. My question is: is the R value of the RC-chain then the isulation resistance between the CPU and earth?

Because the chosen CPU312C doesn't have a RC-chain, it's not suitable for the unearthed DC control installation. Our solution was to put a tape (insulation layer) between the cpu ground connection and the rack in the cabinet (earth).

- The cabinet is grounded. 
- I have isolated power supply.
- No analog modules, just digital. (But I'm also curious for the case with analog modules)

I think this design is wrong because the CPU have no way to discharge static charge build-up.

But we decide to keep the current situation as it's. What kind of problems could we expect then?

Attachment: plc_rc.pdf  (18 Downloads)
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  8/30/2012, 3:46 PM
Aret
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To be honest i don't feel confident enough to answer your question generally.blush In your design we use auxiliary relays for digital in/out, so having isolated power supply is OK. We did installations with isolated power supply and analog modules also, but i'm not sure was there 1 PS or different PS for different chains.

Sorry, seems i can't help you further on the topic.
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  8/30/2012, 6:27 PM
Bofely
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I still appreciate your help. Thank you Rnf:lblSmileyTooltip_
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