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Anonymous Poster #1

Screen Wire

09/21/2013 10:42 PM

Dear All

For the screen wire of the instrument cable was connected both End and also connected to PE earth any issue.

Regards

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Guru

Join Date: Jul 2010
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#1

Re: screen wire

09/22/2013 2:16 AM

It depends.
Field instrument wiring typically grounds the shield at the controller end
Profibus DP grounds both ends of the shield.
I'm an instrument guy so I ground the shield at one end only.

1) Field Instrument Grounding

from Control System Power and Grounding Better Practice, by David Brown, David Harrold, and Roger Hope

Pg 4-13
Shield Grounds
Proper field-wiring shield grounds help ensure proper system operation by
reducing electromagnetic interference in signal wiring. A proper shield ground
path is to ground the shield at one end only. Grounding at both ends can cause
ground loops.
The preferred end is at the carrier shield bar. If field transmitters or
final control elements are well-grounded four-wire units, shield wires can be
grounded to the units instead of the carrier shield bar.

2) Profibus DP
The quote below is directly from the Profibus DP Installation Guide from Profibus.com. It's an RS-485 network.

PROFIBUS Technical Guideline Installation Guideline for PROFIBUS-DP/FMS
Version 1.0, September 1998

3.1 Laying of copper cable
The following cable laying instructions apply to shielded twisted pair cables. The cable shield is used to improve the immunity to electromagnetic interference. PROFIBUS cables should be shielded with a combination of both conductive braid and conductive foil. In the following, "shield" refers to both types of shields (braid and foil). A foil shield should not be used on its own, because it is very thin and can easily be damaged.

The cable shield must be connected to the functional ground at both ends of the cable by making a large-area connection to a grounded conductive surface. When laying the bus cables, particular care should be taken to ensure that the cable shield (braid shield and foil shield, if available) is connected to the shield grounding clamp over a large area.

http://www.profibus.com/download/installation-guide/

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Guru

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#2

Re: Screen Wire

09/22/2013 4:15 PM

Does it keep the bugs out?

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Guru

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#3

Re: Screen Wire

09/22/2013 5:32 PM

If you connect to multiple "safety" earths they may have significant power frequency potential difference, which circulates current through the shield, which induces noise in the signal wires. There could even be safety issues with the amount of circulating current.

Instrument systems often have a common bar which is specifically only earthed at one point.

It depends on the frequency your signal works at and how much power frequency filtering is possible or built-in to the signal receiver.

Multiple earth is a no-no for audio equipment, but if unavoidable, balanced transmission/ reception and transformer isolation helps.

Instrument is a word having many meanings, what does your instrument do??

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