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pitchaipillairengaraj
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Join Date: Mar 2008
Posts: 3

instrument earthing

04/03/2008 8:49 AM

why instrument cable drain wire(shield) is not connected into the instrument side?
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TVP45
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Join Date: Jul 2007
Posts: 4448
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#1

Re: instrument earthing

04/03/2008 8:58 AM

I assume you are asking why the shield is connected at only one end? If so, that is to prevent a ground loop where a current would flow in the shield due to a potential difference between the two ground points. It is a normal practice to ground a shield at the source side and not at the instrument side.

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uweka
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Join Date: Jan 2007
Location: Berlin (Germany)
Posts: 332
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#2
In reply to #1

Re: instrument earthing

04/04/2008 11:39 AM

The only exception is when using double shielded cablings for extremely sensitive measurement applications. In this case triaxial cables are often used with one of the shields being connected on both sides and one only at one side.

But having trouble with hard external EM-influences has no general rules because they often can only be stopped by experimentally determining the best connections. Sometimes it may be even better to connect the grounding point to the instrument side.

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TVP45
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Join Date: Jul 2007
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#3
In reply to #2

Re: instrument earthing

04/04/2008 11:56 AM

Yes, you're quite right. I've often found that I just had to try various configurations to find which one worked; I didn't mean to imply any hard and fast rule.

T

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strlke
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Join Date: Mar 2013
Posts: 1
#4

Re: instrument earthing

03/05/2013 10:47 PM

The reason of not grounding the drain wire at instrument field device side is to eliminate ground loop current. This article may explain http://instrumentationportal.com/2013/job-seeker/interview-question/instrument-engineer/instrument-cable-screen-grounding/
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strlke (1); TVP45 (2); uweka (1)

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