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Hasmoz
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Join Date: May 2011
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Shielding and Grounding of 24 VDC Signals

05/09/2011 5:07 AM

Would it be necessary to consider shielding for cables carrying 24 VDC signals from field instruments to a PLC-based process control system? Also, please let me know if there is a need for a Y/G earth wire alongside the cable, to be as part of the cable, carrying a.m. 24 VDC signal?
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PWSlack
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#1

Re: Shielding and grounding of 24 VDC signals

05/09/2011 5:09 AM

Just earth the 0V line at the power supply. Everything will be just fine.

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Noudge79
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#2

Re: Shielding and grounding of 24 VDC signals

05/09/2011 8:21 AM

Yap, surge from EMI or lightning happens anytime, might as well conform to standard protocols for communication system.

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crashcol
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Location: Perth, Western Australia.
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#3

Re: Shielding and Grounding of 24 VDC Signals

05/10/2011 12:14 AM

Only need shielding if you have got other sources that may interfere. As for runnnig and earth wire..Why? it won't do any thing except cost time and money and if you've got money to waste I can always assist in 'looking' after it.

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rhkramer
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#4

Re: Shielding and Grounding of 24 VDC Signals

05/10/2011 6:04 AM

What exactly is the signal being carried? Based on your use of "24 VDC signals" I'm jumping to the possibly wrong conclusion that they are DI and DO signals (digital input and digital output signals), which often don't need to be shielded.

However, if they are carrying something more sensitive (signals to an A/D converter, for example) they do need to be shielded.
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Hasmoz
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#7
In reply to #4

Re: Shielding and Grounding of 24 VDC Signals

05/22/2011 3:52 AM

Sorry for the delay and many thanks for your reply. You jumped to exactly the right conclusion. the cables carrying DI and Do signals to a PLC-based control system. If these need to be shielded, considering the fact that noise may hardly distort the 24 VDC signal, why shielding might be necessary?
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mrxphil
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#5

Re: Shielding and Grounding of 24 VDC Signals

05/10/2011 9:27 AM

Unless you have 1000 miles of this cable, the extra cost of the shielding is negligible and as has been pointed out, EMI is about everywhere today. Lightning alone and stray currents are justification enough. Sensitive signals like 4-20 AI and AO lines do require shielding. DI & DO not so much, but it's still a good idea. If you do use shielded, be certain that it is only grounded at the source/PLC end, though. You will get problems that are hard to find if you have ground loops.

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sureflowmeter
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#6

Re: Shielding and Grounding of 24 VDC Signals

05/11/2011 4:05 AM

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