Over All Screen and Individual Shield termination in Marshalling

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Usman Nazar Rathore Associate Join Date: Sep 2009 Location: United Arab Emirates, Abu Dhabi Posts: 50	Over All Screen and Individual Shield termination in Marshalling 09/26/2009 1:23 AM
	Gentlemen, I have a query regarding the concept of Instrument Over all and individual shield (i am talking about multipair cables ofcourse). For example if we have a lets say 24 pair cable with OAS and IS both (and the field end is not connected and kept open to avoid ground loop in this case) , the IS will definitely be connected to the Instrument Reference Ground Bar within the Marshalling Cabinet. My confusion is about the overall screen of the cable "WILL THE OVER ALL SCREEN BE CONNECTED TO THE INSTRUMENT REFERENCE GROUND OR THE POWER SAFETY GROUND WITHIN THE CABINET ? "
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Shyam Guru Join Date: Aug 2006 Location: AM-51, Deen Dayal Nagar, Gwalior, Madhya Pradesh, MP 474001, India Posts: 3418 Good Answers: 32	#1 Re: Over All Screen and Individual Shield termination in Marshalling 09/26/2009 11:56 PM
	Signals are never connected to power safety ground. perhaps a shield to transformer can be connected to power safety ground. In case casing is power safety ground connected then all signal ground need to be isolated from instrument case. If signals are differential type, then they can have common ground reference even at higher potential and common mode rejection can remove the elevated DC portion. If you are sure that signal ground and power ground are very near to each other then the power ground can be connected to signal ground through paired diodes to keep a maximum difference of +/-600mV maximum from power ground. For signal levels very small this to be avoided. __________________ Prof. (Dr.) Shyam, Managing Director for Sensors Technology Private Limited. Gwalior, MP474001, India.
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Haajee Guru Join Date: Feb 2006 Posts: 1758 Good Answers: 6	#4 In reply to #1 Re: Over All Screen and Individual Shield termination in Marshalling 09/28/2009 12:15 AM
	Regards. A GA for proer reply. I would like to add that it should only b on one point ONLY not on both ends of cable.
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Anonymous Poster	#2 Re: Over All Screen and Individual Shield termination in Marshalling 09/26/2009 11:57 PM
	Both Overall and Individual screen shall be connected to Instrument Earth bar in the Marshalling cabinet.
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Shyam Guru Join Date: Aug 2006 Location: AM-51, Deen Dayal Nagar, Gwalior, Madhya Pradesh, MP 474001, India Posts: 3418 Good Answers: 32	#3 In reply to #2 Re: Over All Screen and Individual Shield termination in Marshalling 09/27/2009 7:12 AM
	This may be OK in electrical wiring but not in instrumentation where power ground and signal ground are separated by isolation and this comes with price. See how they do it at www.dataforth.com/ __________________ Prof. (Dr.) Shyam, Managing Director for Sensors Technology Private Limited. Gwalior, MP474001, India.
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Anonymous Poster (1); Haajee (1); Shyam (2)

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