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Anonymous Poster

Instrument Groundings

03/02/2008 9:38 PM

Anyone knows what are the proper grounding design for instruments ( PLC, DCS etc). Can I connect it to the existing electrical groundings.
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biswanath.das
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Join Date: Oct 2006
Location: Kolkata, India
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#1

Re: Instrument Groundings

03/03/2008 3:44 AM

Connecting to existing electrical grounding? --NO!!!

Make a separate grid for Electrinic grounding and maintain grid resistance to specified value. IEEE doesn't say much about Electrinic grounding. Anyone got an authentic standard/ Recommendation is welcome.

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Techart
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#2

Re: Instrument Groundings

03/03/2008 10:50 PM

It would depend on where you are located, In the USA we must abide by the NEC (National Electrical Code). The NEC is very specific about such things. I'm sure that Europe and Asia have strict codes for instrument design also. I do not know about India or Africa. I assume that wherever you are there will be specific requirements. You might inquire of your peers if any of them know where to get the information.

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biswanath.das
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#3
In reply to #2

Re: Instrument Groundings

03/03/2008 11:11 PM

Can you please send the section no of NEC where electronic grounding has been described? There is no standard on Electronic grounding in India. But there is a practice among practicing engineers and consulting engineering co. I have checked General Electric Co documents, where they said the required electronic ground resistance value not the process of obtaining the same, nor said anything if this should be connected to electrical grounding. But, I got a personal experience, where the Instrument transmitter and controller (MOD 30 of Taylor Instruments) power circuit burnet out, since the instrument ground circuit was connected to electrical grounding. The finding was, due to unbalance load there was neutral voltage which appeared across instruments circuits thereby burning the circuit.

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bubbapebi
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#4
In reply to #3

Re: Instrument Groundings

03/04/2008 1:15 AM

Article 250 of the NEC deals with grounding requirements in general; Section L of this article pertains specifically to instruments. You should read the whole of Article 250 before you connect anything. Since replacing good instruments is costly, you should make their working environment a safe one. Of course, the safety of your personnel is even more important.

Please consider this statement from the Introduction to the NEC: "The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity."

And also, "This Code is not intended as a design specification nor an instruction manual for untrained persons."
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jmueller
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#5
In reply to #3

Re: Instrument Groundings

03/04/2008 1:49 AM

Biswanath.das,

Your questions are good ones. Your post suggests a few possible problems or causes. In some way, your ground conductor became a "current-carrying" conductor. Many people get confused between two definitions in the NEC "grounded" and "grounding" conductors. The "grounded" conductor is often called the neutral (such as in a 2-pole 3-wire single-phase supply or a 3-pole 4-wire three-phase supply); and it is a current-carrying conductor, part of the supply circuit. The "grounding" conductor is for safety, and is supposed to be current-carrying only under fault conditions; when it will act to safely return the fault current so the CB or fuse can act to turn off the power.

1--Since the (grounded) neutral is carrying current, its own resistance will cause a drop in voltage along its length. If it is connected to ground ("bonded" to the "grounding" conductor) at only one point in your facility, then you can measure a voltage difference between it and the ground which will be different and changing, depending on your location and the loads at that time. If, however, it is "bonded" at more than one point, then the "grounded" conductor and even the building, its metal piping, and other stuff can all become part of the circuit. If this happens, you can easily have problems and equipment damage. But there are other possible sources for your problems.

2--Cables often have shields (called "screens" in many countries) to protect their low-level signals from interference or to keep them from causing interference. If this shield/screen is grounded at only one point, good. But if it is grounded at more than one point, then it can carry current because of voltage differences at the different locations, and can cause equipment damage. When working with PLC's and similar equipment, I will often bond all shields to ground at the PLC or DCS and insulate the shield at the other end (or provide a non-bonded terminal for the shield(s) to connect to if the cable continues on).

3--The NEC has one requirement and also one permission for you. In §110.3(B), it requires you to comply with all the manufacturer's instructions as well as all the testing authority's listing requirements when installing the equipment, so all these instructions and requirements apply to you as if their words had been directly in the code. In §250.146(D) it talks about receptacles with an isolated ground contact and permits this isolated ground to be carried through all the upstream boxes and panels until it is grounded at the equipment grounding terminal of the service. Therefore, if the manufacturer requires an isolated ground, you must install one . . . If the manufacturer requires all load circuits to be grounded to a specific terminal in their own equipment, then you must do just that!

4--Electrical and electronic noise can be carried on the circuit conductors and also on the ground conductor. There are ways to filter these out. These can include MOV's, low-pass filters, and others. Isolated grounds were introduced to minimize some of this.

5--Under fault conditions, a large voltage spike can be imposed on the ground (such as with a short circuit). If you have a thunder storm with lightning strikes, these can put large voltage spikes on the circuits as well as the ground. That is why the grounding of lightning protection equipment is usually kept many meters/feet from the grounding of electrical service equipment.

6--§250.56 requires the true resistance to ground of a single rod, pipe, or plate electrode to be less than 25-ohms; and if it is over, then one additional electrode must be installed. Unfortunately, it does not require the pair to be less than 25-ohms. However, many types of ground electrodes (particularly reinforcing steel in a footing and ground rings) have much lower resistance.

Best approaches? Bond all exposed metal parts of your building, machines, fixed equipment, etc. together (such as through metal electrical conduits and raceways), so they are all at about the same voltage. Ground your electronic equipment and wiring as the manufacturer requires. If in doubt about low-voltage signals, install signal isolators between pieces of equipment--they can provide up to 1500 volts of isolation. Make sure that your neutral conductor is grounded only once in your facility--everywhere else, the ground and neutral are kept separate. Install good surge suppression (and possibly also a means of detecting when it has failed).

I hope this helps--JMM
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NestorLeal
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#6

Re: Instrument Groundings

03/04/2008 12:19 PM

This is very long. It is part of an Instrumentation Electrical Requirements spec we developed for the company I used to work for.

10 GROUNDING AND SURGE PROTECTION (MODIFY)

(Modify whole chapter 10 [of PIP PCCEL001 "Instrumentation Electrical Requirements"])

10.1 Grounding of instrumentation systems shall adhere strictly to recommended

practices in IEEE 1100, 1999, IEEE Recommended Practice for Powering and

Grounding Electronic Equipment. See Appendix "B" for guidelines on grounding

design.

10.2 For field instruments located in remote areas of high risk of lightning strikes, SPD's (Surge Proitecction Devices) may be installed individually at field instrument terminations (positive, negative and shield wires), after a cost–risk–benefit analysis is performed.

10.3 For SIS (Safety Instrumented Systems) applications and critical control loops, SPD's may be installed individually at field instrument terminations and marshalling cabinet terminations (positive, negative and shield wires), after a cost–risk–benefit analysis is performed.

10.4 For intrinsically safe circuits, installation of SPD's shall follow specific SPD

manufacturer instructions for I.S. applications.

10.5 SPD's shall be applied to power distribution panels which support instrumentation equipment, as per IEEE 1100, 1999, 8.4.2.5 and 8.6.

ANNEX TO SPEC

GUIDELINES FOR GROUNDING OF ELECTRONIC EQUIPMENT

I have a very long annex with specific recommendations,based on specific IEEE 1100 Guidelines. If you are interested, please send me your e-mail and I will write directly to you. In any case, you can get the guidelines directly from the IEEE standard.

Below, you may find the beginning paragraphs of the Annex:

The following guidelines may be applied in the design of instrumentation

grounding systems. The standard of reference is IEEE 1100, 1999,

Recommended Practice for Powering and Grounding Electronic Equipment

1. Grounding system consists of four (4) distinct, solidly (e.g.: galvanically)

interconnected functional subsystems. They are as follows:

a. NEC 250 described fault and personnel protection grounding subsystems,

including the earth–grounding conductors (green wires), earth grounding

electrodes (generally in the form of a plant–wide grounding grid) and related

grounding electrode conductors. This subsystem is also called the power

system ground or the AC safety ground, and is installed following the

guidelines of NEC and IEEE. Outside the scope of this specification.

b. Signal reference structure (SRS), consisting either of a signal reference

grounding plane (SRGP) or a signal reference grounding grid (SRGG), and

installed following the original equipment manufacturers instructions and the

recommendations made in chapter 8 of IEEE 1100, 1999. The function of the

SRS is to provide an effective high frequency grounding system for

protection of electronic equipment against damage or interference caused

by power system faults or lighting strikes.

c. Lightning protection subsystem, installed per NEC and IEEE electrical guidelines. This subsystem is outside the scope of this specification.

d. Telecommunications, data transmission and signaling circuit surge

protection grounding subsystem, as installed per the original equipment

manufacturer's instructions, the requirements of the NEC and

recommendations made in chapters 8 and 9 of IEEE 1100, 1999. Outside

the scope of this specification.

2. SRS shall be installed as per IEEE, 1100, chapter 8.5.4. See below a summary list of recommended practices for installing a Signal Reference Ground Grid, extracted from IEEE 1100, 1999, par. 8.5.4.8 .........

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biswanath.das
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#7
In reply to #6

Re: Instrument Groundings

03/04/2008 11:17 PM

NestorLeal,

Thank you very much for the information. I will surely keep in touch on this topic.

Regards

BNDas

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kslimll
Participant

Join Date: Jun 2008
Posts: 1
#8
In reply to #6

Re: Instrument Groundings

06/14/2008 10:38 AM

What is the requirement of the conductor size for the instrument grounding.
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yousef harb
Member

Join Date: Mar 2010
Posts: 7
#9
In reply to #6

Re: Instrument Groundings

03/05/2010 11:42 PM

Yes i'm interesting and i need more informations, how to apply grounding system in a commircial project with high rise building.(calculation debendce on what).

Thank you very much for the information. I will surely keep in touch on this topic
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