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from the instrumentation department...
signal cable shield grounding
PLCs and related questions. topic
Posted by yogesh on 30 August, 2011 - 12:18 pm
why we connect analog signal cable only DCS or PLC side at ground not at JBs or Instrument side or field side? Because we mostly do wiring of any DCS or PLC we just connect our cable shield only panel side ground not in field instrument side.

Pls, reply me in detail explanation


Posted by bob peterson on 30 August, 2011 - 3:34 pm
Convention mostly. it does not matter a whole lot where the shield t earth connection is made.

--
Bob


Posted by Brian Cervi on 31 August, 2011 - 9:11 am
Ground at one end only to create a "drain" and not a ground loop.

If both ends of the cable were at exactly the same potential, which is almost impossible to create, then this would not be an issue. If there is any difference, then the ground will cause noise and interference that will affect the signal quality of the value you are trying to monitor.

If you ground at the field end, you drain the "noise" away from the control system.


Posted by Benson Varghese on 9 October, 2012 - 12:39 pm
yes this is the absolute answer.............if the other end is connected to instrument then it will lead to "current noise" which may disturb signal


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 31 August, 2011 - 11:18 am
Yogesh... I recommend you search the Control.Com Archive for like threads.

You will find that most posters suggest grounding the shield at the panel-end, rather than the instrument-end, and never both-ends simultaneously!

Regards, Phil Corso


Posted by d- on 31 August, 2011 - 9:43 am
Shield grounding in the field holds the shield at the ground potential at that location. With shield continuity to a central location, i.e. the control room or i/o equipment, you now have a shield that is not at the same potential as the control room instrument ground.

Two issues emerge: (1) the i/o gear has a fixed common mode rejection rating so that with the shielded wires coming in but capacitively coupled to a shield that is above ground potential the input hardware spec can be exceeded.

and (2) should the shield be grounded in the control room, with a significant potential difference between the field ground and the control room ground due to motor starting, defective equipment, etc. a ground loop is formed that can also lead to interference.

my own experience has been to ground the shield only at the control room or remote i/o and float the shields in the field while maintaining shield continuity back to the ground point.

in some extreme cases it is necessary to use signal isolators regardless of the grounding scheme.


Posted by Mk6TA on 9 October, 2012 - 3:50 pm
I would also consider the fact that normally control panels are better designed/checked when it comes to connection to ground than junction boxes or instruments. Moreover, there are older instruments (even new ones, few of them) that are not even provided with terminals for ground connection.

So, other reasons aside, connecting all the grounds in one place, easier and faster to check, having no doubts later whether all my instruments are connected to the ground or not, it always seems to me the better idea.


Posted by Roy Matson on 9 October, 2012 - 6:45 pm
Different areas of the plant will be at different ground potential, if you ground the shield at both spots a current flows through the shield and the associated magnetic field induces noise into the conductors.

If you ever work on hi-fi equipment you will soon appreciate the need for grounding at one end only.

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